Rapid evolution of enormous, multichromosomal genomes in flowering plant mitochondria with exceptionally high mutation rates

Mitochondrial genome evolution in the orchid subfamily Cypripedioideae (Orchidaceae)

In this study, the mitogenomes of nine species in the subfamily Cypripedioideae were newly sequenced and assembled using both short and long reads for evolutionary analyses. Complete multi-chromosomal mitogenomes were obtained for Cypripedium subtropicum, C. henryi, Phragmipedium humboldtii, Phr. kovachii, and Paphiopedilum micranthum, and draft assemblies were obtained for four additional Paphiopedilum species. Thirty-nine protein-coding genes were annotated and shared in nine sampled species. sdh4 was discovered in all species of Cypripedioideae, and rpl10 was detected in four species of Paphiopedilum. These two genes might have been horizontally transferred from non-orchid plants at different times. Approximately 101 to 998 repeat sequences were identified with total lengths of 417,136 to 785,960 bp in the mitogenomes of Cypripedioideae. There were 634 and 662 RNA editing sites in C. subtropicum and Pa. gratrixianum, respectively, and C-to-U editing was dominant. The nad and ccm genes exhibited high frequencies of RNA editing. Our study revealed the complexity of orchid mitogenomes, including evidence for the horizontal transfer of rpl10 and sdh4.

Analysis of the complete mitogenomes of three high economic value tea plants (Tea-oil Camellia) provide insights into evolution and phylogeny relationship

Introduction

Tea-oil Camellia species play a crucial economic and ecological role worldwide, yet their mitochondrial genomes remain largely unexplored.

Methods

In this study, we assembled and analyzed the complete mitochondrial genomes of Camellia oleifera and C. meiocarpa, revealing multi-branch structures that deviate from the typical circular mitochondrial genome observed in most plants. The assembled mitogenomes span 953,690 bp (C. oleifera) and 923,117 bp (C. meiocarpa), containing 74 and 76 annotated mitochondrial genes, respectively.

Results

Comparative genomic analyses indicated that C. oleifera and C. meiocarpa share a closer genetic relationship, whereas C. drupifera is more distantly related. Codon usage analysis revealed that natural selection plays a dominant role in shaping codon bias in these mitochondrial genomes. Additionally, extensive gene transfer events were detected among the three species, highlighting the dynamic nature of mitochondrial genome evolution in Tea-oil Camellia. Phylogenetic reconstruction based on mitochondrial genes exhibited incongruence with chloroplast phylogenies, suggesting potential discordance due to hybridization events, incomplete lineage sorting (ILS), or horizontal gene transfer (HGT). Furthermore, we identified species-specific mitochondrial markers, which provide valuable molecular tools for distinguishing Tea-oil Camellia species.

Discussion

Our findings enhance the understanding of mitochondrial genome evolution and genetic diversity in Tea-oil Camellia, offering essential genomic resources for phylogenetics, species identification, and evolutionary research in woody plants.

Assembly and Comparative Analysis of the Complete Mitochondrial Genome of Hippophae salicifolia

This study reports the first complete mitochondrial genome assembly of Hippophae salicifolia, an ecologically and economically important plant endemic to the Himalayas. The 475,105 bp genome has a 44.80% GC content and an overall AT bias, comprising 74 genes (37 protein-coding, 31 tRNA, three rRNA, and three pseudogenes). We identified extensive repetitive elements, including 188 SSRs, 20 tandem repeats, and 455 dispersed repeats, and explored their potential roles in genome evolution. Codon usage analysis showed a bias for codons ending in A or U, while RNA editing analysis revealed 415 sites that mostly convert hydrophilic to hydrophobic amino acids. Phylogenetic and collinearity analyses clarified evolutionary relationships within Hippophae and uncovered genome rearrangements. In addition, extensive gene transfer was detected between the mitochondrial and chloroplast genomes. Ka/Ks and nucleotide diversity analyses indicate that most genes are under purifying selection, with some possibly undergoing positive selection. Overall, these findings enhance our understanding of the structural and evolutionary features of the H. salicifolia mitochondrial genome and provide valuable insights for the genetic improvement and conservation of Hippophae species.

Disruption of recombination machinery alters the mutational landscape in plant organellar genomes

Land plant organellar genomes have extremely low rates of point mutation yet also experience high rates of recombination and genome instability. Characterizing the molecular machinery responsible for these patterns is critical for understanding the evolution of these genomes. While much progress has been made toward understanding recombination activity in land plant organellar genomes, the relationship between recombination pathways and point mutation rates remains uncertain. The organellar-targeted mutS homolog MSH1 has previously been shown to suppress point mutations as well as non-allelic recombination between short repeats in Arabidopsis thaliana. We therefore implemented high-fidelity Duplex Sequencing to test if other genes that function in recombination and maintenance of genome stability also affect point mutation rates. We found small to moderate increases in the frequency of single nucleotide variants (SNVs) and indels in mitochondrial and/or plastid genomes of A. thaliana mutant lines lacking radA, recA1, or recA3. In contrast, osb2 and why2 mutants did not exhibit an increase in point mutations compared to wild-type (WT) controls. In addition, we analyzed the distribution of SNVs in previously generated Duplex Sequencing data from A. thaliana organellar genomes and found unexpected strand asymmetries and large effects of flanking nucleotides on mutation rates in WT plants and msh1 mutants. Finally, using long-read Oxford Nanopore sequencing, we characterized structural variants in organellar genomes of the mutant lines and show that different short repeat sequences become recombinationally active in different mutant backgrounds. Together, these complementary sequencing approaches shed light on how recombination may impact the extraordinarily low point mutation rates in plant organellar genomes.

Complete analysis and phylogenetic analysis of Polygonatum sibiricum mitochondria

In this project, we studied the complete mitogenome of the liliaceae medicinal plant Polygonatum sibiricum. The genome is represented by a circular ring molecule with a length of 691,910 bp and a GC content of 46.33%. Mitochondrial genome composition is slightly biased towards A+T, with AT accounting for 53.67%, and AT skewness slightly positive (0.092%). The complete mitogenome has a total of sixty-three unique genes, including thirty-nine protein-coding genes, twenty-one transfer RNAs (tRNAs) and three ribosomal RNAs (rRNAs). We examined codon use, repeat sequence, RNA editing in the mitogenome of P. sibiricum, and elucidated species classification based on phylogenetic trees of mitogenome of twenty-three species. Our results provide comprehensive information on the mitogenome of P. sibiricum and show for the first time the evolutionary relationship between the mitogenome of P. sibiricum and Chlorophytum comosum in the Asparagales family.

Assembly and Comparative Analysis of the Complete Mitochondrial Genomes of Smilax glabra and Smilax zeylanica

BACKGROUND

Smilax glabra (S. glabra) and Smilax zeylanica (S. zeylanica), two medicinally important species within the genus Smilax, have been widely used in Traditional Chinese Medicine (TCM) for the treatment of rheumatism, traumatic injuries, and related ailments. Despite their medicinal significance, research on the mitochondrial DNA (mtDNA) of Smilax species remains limited.

METHODS

We utilized NovaSeq 6000 and PromethION sequencing platforms to assemble the complete mitochondrial genomes of Smilax glabra and Smilax zeylanica, and conducted in-depth comparative genomic and evolutionary analyses.

RESULTS

The complete mitochondrial genomes of S. glabra and S. zeylanica were assembled and annotated, with total lengths of 535,215 bp and 471,049 bp, respectively. Both genomes encode 40 unique protein-coding genes (PCGs), composed of 24 core and 16 non-core genes, alongside multiple tRNA and rRNA genes. Repetitive element analysis identified 158 and 403 dispersed repeats in S. glabra and S. zeylanica, respectively, as well as 123 and 139 simple sequence repeats (SSRs). RNA editing site predictions revealed C-to-U conversions in both species. Additionally, chloroplast-to-mitochondrial DNA migration analysis detected 34 homologous fragments in S. glabra and 28 homologous fragments in S. zeylanica. Phylogenetically, S. glabra and S. zeylanica cluster within the Liliales order and Smilacaceae family, closely related to Lilium species. Collinearity analysis indicated numerous syntenic blocks between Smilax and three other Liliopsida species, though gene order was not conserved.

CONCLUSIONS

This study presents high-quality mitochondrial genome assemblies for S. glabra and S. zeylanica, providing valuable insights into molecular identification and conservation efforts of these traditional medicinal plants.

Unequally Abundant Chromosomes and Unusual Collections of Transferred Sequences Characterize Mitochondrial Genomes of Gastrodia (Orchidaceae), One of the Largest Mycoheterotrophic Plant Genera

The mystery of genomic alternations in heterotrophic plants is among the most intriguing in evolutionary biology. Compared to plastid genomes (plastomes) with parallel size reduction and gene loss, mitochondrial genome (mitogenome) variation in heterotrophic plants remains underexplored in many aspects. To further unravel the evolutionary outcomes of heterotrophy, we present a comparative mitogenomic study with 13 de novo assemblies of Gastrodia (Orchidaceae), one of the largest fully mycoheterotrophic plant genera, and its relatives. Analyzed Gastrodia mitogenomes range from 0.56 to 2.1 Mb, each consisting of numerous, unequally abundant chromosomes or contigs. Size variation might have evolved through chromosome rearrangements followed by stochastic loss of "dispensable" chromosomes, with deletion-biased mutations. The discovery of a hyper-abundant (∼15 times intragenomic average) chromosome in two assemblies represents the hitherto most extreme copy number variation in any mitogenomes, with similar architectures discovered in two metazoan lineages. Transferred sequence contents highlight asymmetric evolutionary consequences of heterotrophy: despite drastically reduced intracellular plastome transfers convergent across heterotrophic plants, their rarity of horizontally acquired sequences sharply contrasts parasitic plants, where massive transfers from their hosts prevail. Rates of sequence evolution are markedly elevated but not explained by copy number variation, extending prior findings of accelerated molecular evolution from parasitic to heterotrophic plants. Putative evolutionary scenarios for these mitogenomic convergence and divergence fit well with the common (e.g. plastome contraction) and specific (e.g. host identity) aspects of the two heterotrophic types. These idiosyncratic mycoheterotrophs expand known architectural variability of plant mitogenomes and provide mechanistic insights into their content and size variation.

Assembly and comparative analysis of the complete mitochondrial of Spodiopogon sagittifolius, an endemic and protective species from Yunnan, China

BACKGROUND

Spodiopogon sagittifolius, a C4 plant closely related to cultivated crops, is an edible resource and a Class II nationally protected species in China. Endemic to Yunnan, its populations are declining due to habitat destruction, highlighting its resource and conservation importance. Despite its significance, the molecular phylogenetic relationships and genetic mechanisms of adaptive evolution in the genus Spodiopogon remain poorly understood.

RESULTS

We successfully assembled and annotated the first mitochondrial genome of S. sagittifolius using HiFi sequencing and the PMAT tool. The genome is 500,699 bp in length with a GC content of 43.15%. Synteny and dN/dS analyses revealed structural and functional conservation of mitochondrial genomes in closely related species, with most protein-coding genes under purifying selection (dN/dS < 1). Notably, nad2 exhibited signs of positive selection (dN/dS = 1.49), indicating potential adaptive evolution. Extensive RNA editing events were detected across 27 protein-coding genes, predominantly involving C-to-U conversions, with synonymous mutations accounting for 49.65% of the edits. Strong codon usage bias favoring A/U-ending codons and the identification of repeat sequences suggest enhanced mitochondrial efficiency and stress adaptation. Phylogenetic analyses confirmed the taxonomic placement of S. sagittifolius within the Andropogoneae tribe.

CONCLUSIONS

This study provides the first insights into the mitochondrial genome evolution of S. sagittifolius, highlighting key features linked to stress tolerance and adaptive evolution. These findings establish a foundation for its conservation and potential domestication, with implications for crop improvement and ecological resilience.

Mitogenome of Uncaria rhynchophylla: genome structure, characterization, and phylogenetic relationships

BACKGROUND

Uncaria rhynchophylla is listed in the Chinese pharmacopoeia as one of the five botanical sources of the traditional medicine Gou-Teng, which has been utilized for the treatment of mental and cardiovascular disorders. This particular species is well-known in China for its hook-like structures originating from the leaf axils. Despite available reports on its chloroplast genome, there persists a notable lack of understanding concerning the structural variations and evolution of its mitochondrial genome. This knowledge gap hinders our ability to fully comprehend its genetic attributes.

RESULTS

We successfully assembled the mitochondrial genome of U. rhynchophylla by seamlessly integrating Illumina short reads with Nanopore long reads, resulting in a non-circular genome comprising 1 circular contig and 2 linear contigs. The total length of this genome is 421,660 bp, encompassing 36 PCGs. The identification of 4 distinct pairs of repeats has unveiled their pivotal role in repeat-mediated recombination. Of the 28 homologous fragments derived from chloroplasts, the majority were observed to have been transferred from the inverted repeat (IR) regions of the chloroplast genome to the mitochondrial genome. The mitochondrial DNA provides a distinctive resolution for the positioning of several species within the Gentianales phylogenetic framework, which remains unresolved by chloroplast DNA.

CONCLUSION

By utilizing a newly assembled, high-quality mitochondrial genome of U. rhynchophylla, we have elucidated its intricate genomic structure, distinctive sequence characteristics, and potential for phylogenetic analysis. These findings mark significant strides in advancing our comprehension of the genetics of Uncaria.

Recombination and retroprocessing in broomrapes reveal a universal roadmap for mitochondrial evolution in heterotrophic plants

The altered life history strategies of heterotrophic organisms often leave a profound genetic footprint on energy metabolism related functions. In parasitic plants, the reliance on host-derived nutrients and loss of photosynthesis in holoparasites have led to highly degraded to absent plastid genomes, but its impact on mitochondrial genome (mitogenome) evolution has remained controversial. By examining mitogenomes from 45 Orobanchaceae species including three independent transitions to holoparasitism and key evolutionary intermediates, we identified measurable and predictable genetic alterations in genomic shuffling, RNA editing, and intracellular (IGT) and horizontal gene transfer (HGT) en route to a nonphotosynthetic lifestyle. In-depth comparative analyses revealed DNA recombination and repair processes, especially RNA-mediated retroprocessing, as significant drivers for genome structure evolution. In particular, we identified a novel RNA-mediated IGT and HGT mechanism, which has not been demonstrated in cross-species and inter-organelle transfers. Based on this, we propose a generalized dosage effect mechanism to explain the biased transferability of plastid DNA to mitochondria across green plants, especially in heterotrophic lineages like parasites and mycoheterotrophs. Evolutionary rates scaled with these genomic changes, but the direction and strength of selection varied substantially among genes and clades, resulting in high contingency in mitochondrial genome evolution. Finally, we describe a universal roadmap for mitochondrial evolution in heterotrophic plants where increased recombination and repair activities, rather than relaxed selection alone, lead to differentiated genome structure compared to free-living species.

The multi-chromosomal structure of mitogenomes provided new insights into the accurate authentication of medicinal Dendrobium species

BACKGROUND

The global prevalence of herbal-based health care rapidly promoted requirements for medicinal plant resources. Accurate classification and identification are crucial to assuring the safety of these herbal sources.

RESULTS

Here, we took Dendrobium (Orchidaceae), a famous horticultural and medicinal plant taxon, as the study focus to establish an effective authentication approach for medicinal plants based on new mtDNA barcodes. We first de novo assembled three complete mitogenomes using Illumina and Nanopore data. These three mitogenomes were 635,454 bp-831,745 bp long with multichromosomal structures. Moreover, the three mitogenomes were compared to the other four published Dendrobium mitogenomes. The results revealed great variations of the structure and repeat contents among these mitogenomes, while gene contents and genomic sequences were relatively conserved. The analysis of mutational hotspots showed eight mitochondrial DNA regions with high sequence variability (> 5%) at the interspecific level, which could provide abundant informatic loci for phylogeny, genetic diversity, and identification analyses. We also newly obtained mitochondrial sequences of 45 individuals from 15 Dendrobium species for authentication analysis. These 15 Dendrobium species were successfully identified by the whole mitogenome sequences and the isoform combination (Mt17 + Mt19) respectively.

CONCLUSIONS

Our findings revealed that mitochondrial isoforms (chromosomes) could be used as super-barcodes for Dendrobium species authentication. The multi-chromosomal structure of mitogenomes provided new insights into the accurate authentication of medical plants.

Features and evolutionary adaptations of the mitochondrial genome of Garuga forrestii W. W. Sm

Introduction

Garuga forrestii W. W. Sm. is a tree species of the Burseraceae family, endemic to China, found in hot/warm-dry valleys. This species plays a crucial role in maintaining biodiversity in these ecosystems.

Methods

We performed de novo assembly of the Garuga forrestii mitochondrial genome using PMAT (v.1.5.4), resulting in a typical circular molecule of 606,853 bp. The genome consists of 31 tRNA genes, 3 rRNA genes, 35 protein-coding genes, and 1 pseudogene. The study also investigates RNA editing sites and evolutionary patterns.

Results

The mitochondrial genome exhibits a low proportion of repetitive sequences (3.30%), suggesting a highly conserved structure. A high copy number of the trnM-CAT gene (4 copies) is noted, which may contribute to genomic rearrangement and adaptive evolution. Among the 476 RNA editing sites, hydrophilic-hydrophobic and hydrophobic-hydrophobic editing events are most common, accounting for 77.10%. Negative selection predominates among most genes (Ka/Ks < 1), while a few genes (e.g., matR, nad3, rps1, rps12, and rps4) show signs of positive selection (Ka/Ks > 1), potentially conferring evolutionary advantages. Additionally, a significant A/T bias is observed at the third codon position. Phylogenomic analysis supports the APG IV classification, with no evidence of horizontal gene transfer.

Discussion

This mitochondrial genome offers valuable insights into the adaptive mechanisms and evolutionary processes of Garuga forrestii. It enhances our understanding of the species' biogeography in tropical Southeast Asia and Southwest China, providing key information on the evolutionary history of this genus.

Mitochondrial genome structural variants and candidate cytoplasmic male sterility-related gene in sugarcane

BACKGROUND

Sugarcane is a crucial crop for both sugar and bioethanol production. The nobilization breeding and utilization of wild germplasm have significantly enhanced its productivity. However, the pollen sterility in Saccharum officinarum restricts its role to being a female parent in crosses with Saccharum spontaneum during nobilization breeding, resulting in a narrow genetic basis for modern sugarcane cultivars. Mitochondria, often referred to as the intracellular "energy factories", provide energy for plant life activities, and are also implicated in cytoplasmic male sterility (CMS).

RESULTS

We performed mitochondrial genome assembly and structural analysis of two Saccharum founding species. We discovered that the proportions of repeat sequences are the primary factor contributing to the variations in mitochondrial genome structure and size between the two Saccharum species. Heterologous expression of the mitochondrial chimeric gene ORF113, which is highly expressed in male-sterile S. officinarum flowers, significantly inhibits growth and ATP synthesis in yeast cells, making it a key candidate CMS-related gene in sugarcane. Furthermore, we developed two co-dominant simple sequence repeat (SSR) markers based on the mitochondrial genome, which can effectively distinguish the cytoplasmic types of the two Saccharum species.

CONCLUSION

In this study, we identified structural variants and developed SSR molecular markers in the mitochondrial genomes of both S. officinarum and S. spontaneum. We also identified a novel mitochondrial chimeric ORF as a key candidate CMS-related gene. These findings offer valuable insights into variety identification, genetic resource development, and cross-breeding strategies in sugarcane.

Comprehensive analysis of the multi-rings mitochondrial genome of Populus tomentosa

BACKGROUND

Populus tomentosa, known as Chinese white poplar, is indigenous and distributed across large areas of China, where it plays multiple important roles in forestry, agriculture, conservation, and urban horticulture. However, limited accessibility to the mitochondrial (mt) genome of P. tomentosa impedes phylogenetic and population genetic analyses and restricts functional gene research in Salicaceae family.

RESULTS

Single-molecule real-time (SMRT) sequencing technology was used to sequence, assemble, and annotate the mt genome of P. tomentosa. This genome has a complex structure composed of four circular molecules ranging from 153,004 to 330,873 base pairs (bp). Each of these four circular molecules contains unique gene sequences that constitute the mt genome of P. tomentosa. The mt genome comprises 69 functional genes, including 38 protein-coding genes (PCGs), 26 tRNA genes, and 5 rRNA genes. After removing duplications, 19 different tRNA coding genes remain, though only 10 amino acids can be recognized. The noncoding region constitutes 93.38% of the mt genome, comprising a large number of repetitive sequences, gene spacer regions, and insertion from chloroplast sequences. Specifically, 40 chloroplast-derived sequences, with a total length of 24,381 bp, were identified in P. tomentosa.

CONCLUSIONS

In the current study, the results provide mitochondrial genomic evidence for the maternal origin of P. tomentosa and enhance understanding of the gene dialog between organelle genomes, contributing to the conservation and utilization of the genetic resources of P. tomentosa.

Comparative analysis of mitochondrial genomes of Stemona tuberosa lour. reveals heterogeneity in structure, synteny, intercellular gene transfer, and RNA editing

BACKGROUND

Stemona tuberosa, a vital species in traditional Chinese medicine, has been extensively cultivated and utilized within its natural distribution over the past decades. While the chloroplast genome of S. tuberosa has been characterized, its mitochondrial genome (mitogenome) remains unexplored.

RESULTS

This paper details the assembly of the complete S. tuberosa mitogenome, achieved through the integration of Illumina and Nanopore sequencing technologies. The assembled mitogenome is 605,873 bp in size with a GC content of 45.63%. It comprises 66 genes, including 38 protein-coding genes, 25 tRNA genes, and 3 rRNA genes. Our analysis delved into codon usage, sequence repeats, and RNA editing within the mitogenome. Additionally, we conducted a phylogenetic analysis involving S. tuberosa and 17 other taxa to clarify its evolutionary and taxonomic status. This study provides a crucial genetic resource for evolutionary research within the genus Stemona and other related genera in the Stemonaceae family.

CONCLUSION

Our study provides the inaugural comprehensive analysis of the mitochondrial genome of S. tuberosa, revealing its unique multi-branched structure. Through our investigation of codon usage, sequence repeats, and RNA editing within the mitogenome, coupled with a phylogenetic analysis involving S. tuberosa and 17 other taxa, we have elucidated its evolutionary and taxonomic status. These investigations provide a crucial genetic resource for evolutionary research within the genus Stemona and other related genera in the Stemonaceae family.

TIPPo: A User-Friendly Tool for De Novo Assembly of Organellar Genomes with High-Fidelity Data

Plant cells have two major organelles with their own genomes: chloroplasts and mitochondria. While chloroplast genomes tend to be structurally conserved, the mitochondrial genomes of plants, which are much larger than those of animals, are characterized by complex structural variation. We introduce TIPPo, a user-friendly, reference-free assembly tool that uses PacBio high-fidelity long-read data and that does not rely on genomes from related species or nuclear genome information for the assembly of organellar genomes. TIPPo employs a deep learning model for initial read classification and leverages k-mer counting for further refinement, significantly reducing the impact of nuclear insertions of organellar DNA on the assembly process. We used TIPPo to completely assemble a set of 54 complete chloroplast genomes. No other tool was able to completely assemble this set. TIPPo is comparable with PMAT in assembling mitochondrial genomes from most species but does achieve even higher completeness for several species. We also used the assembled organelle genomes to identify instances of nuclear plastid DNA (NUPTs) and nuclear mitochondrial DNA (NUMTs) insertions. The cumulative length of NUPTs/NUMTs positively correlates with the size of the nuclear genome, suggesting that insertions occur stochastically. NUPTs/NUMTs show predominantly C:G to T:A changes, with the mutated cytosines typically found in CG and CHG contexts, suggesting that degradation of NUPT and NUMT sequences is driven by the known elevated mutation rate of methylated cytosines. Small interfering RNA loci are enriched in NUPTs and NUMTs, consistent with the RdDM pathway mediating DNA methylation in these sequences.

De novo assembly of the mitochondrial genome of Glycyrrhiza glabra and identification of two types of homologous recombination configurations caused by repeat sequences

BACKGROUND

Glycyrrhiza glabra, which is widely used in medicine and therapy, is known as the 'king of traditional Chinese medicine'. In this study, we successfully assembled and annotated the mitochondrial and chloroplast genomes of G. glabra via high-throughput sequencing technology, combining the advantages of short-read (Illumina) and long-read (Oxford Nanopore) sequencing.

RESULTS

We revealed the ring structure of the mitochondrial genome, which spans 421,293 bp with 45.1% GC content and 56 annotated genes. Notably, we identified 514 repetitive sequences, including 123 Simple sequence repeats (SRs), 3 Tndem sequence repeats (TSRs), and 388 Dispersed sequence repeats (DSRs). We identified 79 out of the 388 DSRs as potentially involved in homologous recombination. We identified five forward repeats and four palindromic repeats that facilitate homologous recombination and induce alterations in the mitochondrial genome structure. We corroborated this finding via polymerase chain reaction (PCR). Furthermore, we identified chloroplast-derived sequence fragments within the mitochondrial genome, offering novel insights into the evolutionary history of plant mitochondrial genomes. We predicted 460 potential RNA editing sites, primarily involving cytosine-to-uracil transitions. This study reveals the complexity of repetitive sequence-mediated homologous recombination in the mitochondrial genome of G. glabra and provides new insights into its structure, function, and evolution.

CONCLUSIONS

These findings have important implications for conservation biology, population genetics, and evolutionary studies, underscoring the role of repetitive sequences in genome dynamics and highlighting the need for further research on mitochondrial genome evolution and function in plants.

Unveiling the intricate structural variability induced by repeat-mediated recombination in the complete mitochondrial genome of Cuscuta gronovii Willd

Cuscuta gronovii Willd., a member of the Convolvulaceae family, is noted for its potential medicinal and nutritional benefits. In this study, we utilized a combination of Illumina and Oxford Nanopore sequencing technologies to successfully assemble the complete circular mitochondrial genome (mitogenome) of C. gronovii. The mitogenome, spanning 304,467 base pairs, includes 54 genes: 33 protein-coding genes, three ribosomal RNA (rRNA) genes, and 18 transfer RNA (tRNA) genes. Beyond its primary circular structure, we discovered and validated several alternative genomic conformations, driven by five specific repeat sequences. Three inverted repeats were found to initiate rearrangements, resulting in the creation of seven distinct chromosomal structures, while two direct repeats split a larger molecule into two subgenomic entities. We also mapped 421 RNA editing sites across the protein-coding sequences, influencing 33 protein-coding genes with varying distribution, particularly noting high frequencies in the nad4 and ccmB genes. Sixteen of these RNA editing sites were experimentally validated through PCR amplification and Sanger sequencing, confirming their presence with 100 % accuracy. This research not only introduces the first mitochondrial genome of C. gronovii but also highlights its complex conformational variability induced by repeat-mediated recombination, providing a valuable genomic resource for further molecular breeding efforts and phylogenetic evolution within the genus Cuscuta.

The evolutionarily diverged single-stranded DNA-binding proteins SSB1/SSB2 differentially affect the replication, recombination and mutation of organellar genomes in Arabidopsis thaliana

Single-stranded DNA-binding proteins (SSBs) play essential roles in the replication, recombination and repair processes of organellar DNA molecules. In Arabidopsis thaliana, SSBs are encoded by a small family of two genes (SSB1 and SSB2). However, the functional divergence of these two SSB copies in plants remains largely unknown, and detailed studies regarding their roles in the replication and recombination of organellar genomes are still incomplete. In this study, phylogenetic, gene structure and protein motif analyses all suggested that SSB1 and SSB2 probably diverged during the early evolution of seed plants. Based on accurate long-read sequencing results, ssb1 and ssb2 mutants had decreased copy numbers for both mitochondrial DNA (mtDNA) and plastid DNA (ptDNA), accompanied by a slight increase in structural rearrangements mediated by intermediate-sized repeats in mt genome and small-scale variants in both genomes. Our findings provide an important foundation for further investigating the effects of DNA dosage in the regulation of mutation frequencies in plant organellar genomes.

Characterization of the complete mitochondrial genome of the rice bean (Vigna umbellata)

BACKGROUND

Rice bean (Vigna umbellata), an underrated legume crop, demonstrates strong adaptability to poor soil fertility and has significant potential to enhance global food security. It is valuable both as a vegetable and fodder crop due to its high protein content, essential fatty acids, and micronutrients. Despite the sequencing of a high-quality genome of rice bean, its mitochondrial genome (mitogenome) sequence has not yet been reported.

RESULTS

For the first time, the rice bean mitogenome was assembled and annotated using PacBio HiFi sequencing and Geseq software. The mitogenome is a circular molecule with a length of 404,493 bp, containing 32 protein-coding genes, 17 tRNAs, and 3 rRNAs. Codon usage and sequence repeats were also determined. Six gene migration events from the chloroplast to the mitogenome were detected in rice bean. A phylogenetic analysis, including the rice bean mitogenome and 25 other taxa (23 of which are Fabales species), clarified the evolutionary and taxonomic status of rice bean. Additionally, a collinearity analysis of seven Fabales mitogenomes revealed high structural variability. In total, 473 RNA editing sites in protein-coding genes were identified.

CONCLUSIONS

This study presents the first sequencing, assembly, annotation, and analysis of the rice bean mitogenome, providing valuable background information for understanding the evolution of this species. These findings lay the groundwork for future genetic studies and molecular breeding efforts aimed at improving rice bean.

Comprehensive analysis of the first complete mitogenome and plastome of a traditional Chinese medicine Viola diffusa

BACKGROUND

Viola diffusa is used in the formulation of various Traditional Chinese Medicines (TCMs), including antiviral, antimicrobial, antitussive, and anti-inflammatory drugs, due to its richness in flavonoids and triterpenoids. The biosynthesis of these compounds is largely mediated by cytochrome P450 enzymes, which are primarily located in the membranes of mitochondria and the endoplasmic reticulum.

RESULTS

This study presents the complete assembly of the mitogenome and plastome of Viola diffusa. The circular mitogenome spans 474,721 bp with a GC content of 44.17% and encodes 36 unique protein-coding genes, 21 tRNA, and 3 rRNA. Except for the RSCU values of 1 observed for the start codon (AUG) and tryptophan (UGG), the mitochondrial protein-coding genes exhibited a codon usage bias, with most estimates deviating from 1, similar to patterns observed in closely related species. Analysis of repetitive sequences in the mitogenome demonstrated potential homologous recombination mediated by these repeats. Sequence transfer analysis revealed 24 homologous sequences shared between the mitogenome and plastome, including nine full-length genes. Collinearity was observed among Viola diffusa species within the other members of Malpighiales order, indicated by the presence of homologous fragments. The length and arrangement of collinear blocks varied, and the mitogenome exhibited a high frequency of gene rearrangement.

CONCLUSIONS

We present the first complete assembly of the mitogenome and plastome of Viola diffusa, highlighting its implications for pharmacological, evolutionary, and taxonomic studies. Our research underscores the multifaceted importance of comprehensive mitogenome analysis.

Assembly and analysis of the complete mitochondrial genome of Carya illinoinensis to provide insights into the conserved sequences of tRNA genes

Carya illinoinensis is an economically important nut tree, and its chloroplast (cp.) genome has been reported; however, its mitochondrial (mt) genome remains unknown. In the present study, we assembled the first mt genome of C. illinoinensis. The circular mt genome of C. illinoinensis is 495,205 bp long, with 37 protein-coding genes(PCGs), 24 tRNA genes, and 3 rRNA genes. All the tRNAs could be folded into typical cloverleaf secondary structures, with lengths of 58-88 bp. A conserved U-U-C-x-A-x2 consensus nucleotide sequence was discovered in the Ψ-loops of tRNA sequences. In addition, 447 dispersed repeats were detected, as well as found 482 RNA editing sites and 9,960 codons in the mt genome. Furthermore, a total of 27 DNA sequences with a length of 43,277 bp were transferred from the cp. to the mt genome, and eight integrated cp-derived genes (trnL-CAA, trnV-GAC, trnD-GUC, trnW-CCA, trnN-GUU, trnH-GUG, trnM-CAU, and rps7) were identified. We also obtained 1,086 hits, including 364.023 kp of nuclear genome sequences, that were transferred to the mt genome. To determine the evolutionary position of C. illinoinensis, we conducted a phylogenetic analysis of the mitogenomes of C. illinoinensis and 14 other taxa. The results strongly suggested that C. illinoinensis and Fagus sylvatica formed a single clade with 100% bootstrap support. This study sequenced comprehensive data on the C. illinoinensis mitochondrial genome and provided insights into the conserved sequences of tRNA genes, which could facilitate evolutionary research in other Carya trees in the future.

The complete mitochondrial genome assembly of Capsicum pubescens reveals key evolutionary characteristics of mitochondrial genes of two Capsicum subspecies

BACKGROUND

Pepper (Capsicum pubescens), one of five domesticated pepper species, has unique characteristics, such as numerous hairs on the epidermis of its leaves and stems, black seeds, and vibrant purple flowers. To date, no studies have reported on the complete assembly of the mitochondrial genome (mitogenome) of C. pubescens. Understanding the mitogenome is crucial for further research on C. pubescens.

RESULTS

In our study, we successfully assembled the first mitogenome of C. pubescens, which was assigned the GenBank accession number OP957066. This mitogenome has a length of 454,165 bp and exhibits the typical circular structure observed in most mitogenomes. We annotated a total of 70 genes, including 35 protein-coding genes (PCGs), 30 tRNA genes, 3 rRNA genes, and 2 pseudogenes. Compared to the other three pepper mitogenomes (KJ865409, KJ865410, and MN196478), C. pubescens OP957066 exhibited four unique PCGs (atp4, atp8, mttB, and rps1), while two PCGs (rpl10 and rps3) were absent. Notably, each of the three pepper mitogenomes from C. annuum (KJ865409, KJ865410, and MN196478) experienced the loss of four PCGs (atp4, atp8, mttB, and rps1). To further explore the evolutionary relationships, we reconstructed a phylogenetic tree using the mitogenomes of C. pubescens and fourteen other species. Structural comparison and synteny analysis of the above four pepper mitogenomes revealed that C. pubescens shares high sequence similarity with KJ865409 and that C. pubescens has rearranged with the other three pepper mitogenomes. Interestingly, we observed 72 similar sequences between the mitochondrial and chloroplast genomes, which accounted for 12.60% of the mitogenome, with a total length of 57,207 bp. These sequences encompassed 12 tRNA genes and the rRNA gene (rrn18). Remarkably, selective pressure analysis suggested that the nad5 gene underwent obvious positive selection. Furthermore, a single-base mutation in three genes (nad1, nad2, and nad4) resulted in an amino acid change.

CONCLUSION

This study provides a high-quality mitogenome of pepper, providing valuable molecular data for future investigations into the exchange of genetic information between pepper organelle genomes.

Assembly and comparative analysis of the first complete mitochondrial genome of Astragalus membranaceus (Fisch.) Bunge: an invaluable traditional Chinese medicine

BACKGROUND

Astragalus membranaceus (Fisch.) Bunge is one of the most well-known tonic herbs in traditional Chinese medicine, renowned for its remarkable medicinal value in various clinical contexts. The corresponding chloroplast (cp) and nuclear genomes have since been accordingly sequenced, providing valuable information for breeding and phylogeny studies. However, the mitochondrial genome (mitogenome) of A. membranaceus remains unexplored, which hinders comprehensively understanding the evolution of its genome.

RESULTS

For this study, we de novo assembled the mitogenome of A. membranaceus (Fisch.) Bunge var. mongholicus (Bunge) P. K. Hsiao using a strategy integrating Illumina and Nanopore sequencing technology and subsequently performed comparative analysis with its close relatives. The mitogenome has a multi-chromosome structure, consisting of two circular chromosomes with a total length of 398,048 bp and an overall GC content of 45.3%. It encodes 54 annotated functional genes, comprising 33 protein-coding genes (PCGs), 18 tRNA genes, and 3 rRNA genes. An investigation of codon usage in the PCGs revealed an obvious preference for codons ending in A or U (T) bases, given their high frequency. RNA editing identified 500 sites in the coding regions of mt PCGs that exhibit a perfect conversion of the base C to U, a process that tends to lead to the conversion of hydrophilic amino acids into hydrophobic amino acids. From the mitogenome analysis, a total of 399 SSRs, 4 tandem repeats, and 77 dispersed repeats were found, indicating that A. membranaceus possesses fewer repeats compared to its close relatives with similarly sized mitogenomes. Selection pressure analysis indicated that most mt PCGs were purifying selection genes, while only five PCGs (ccmB, ccmFc, ccmFn, nad3, and nad9) were positive selection genes. Notably, positive selection emerged as a critical factor in the evolution of ccmB and nad9 in all the pairwise species comparisons, suggesting the extremely critical role of these genes in the evolution of A. membranaceus. Moreover, we inferred that 22 homologous fragments have been transferred from cp to mitochondria (mt), in which 5 cp-derived tRNA genes remain intact in the mitogenome. Further comparative analysis revealed that the syntenic region and mt gene organization are relatively conserved within the provided legumes. The comparison of gene content indicated that the gene composition of Fabaceae mitogenomes differed. Finally, the phylogenetic tree established from analysis is largely congruent with the taxonomic relationships of Fabaceae species and highlights the close relationship between Astragalus and Oxytropis.

CONCLUSIONS

We provide the first report of the assembled and annotated A. membranaceus mitogenome, which enriches the genetic resources available for the Astragalus genus and lays the foundation for comprehensive exploration of this invaluable medicinal plant.

Assembly and comparative analysis of the multichromosomal mitochondrial genome of globally endangered seagrass Halophila beccarii

BACKGROUND

Halophila beccarii is one of the oldest two generations of seagrass plants and one of the 10 species of seagrass currently at risk of extinction worldwide. Therefore, how to effectively protect the H. beccarii resources from extinction is a huge challenge. Molecular biology research can provide a scientific basis for species conservation. So far, there has been no detailed analysis of the mitochondrial genome of the genus Halophila.

RESULTS

The mitochondrial genome of H. beccarii was assembled into 28 circular chromosomes, ranging in length from 41,738 bp to 104,744 bp, with a total length of 1,964,072 bp and a GC content of 46.71%. It contains 39 genes, including 26 protein coding genes, 10 tRNA genes, and 3 rRNA genes. Repeat sequence analysis and prediction of RNA editing sites revealed a total of 850 dispersed repeats, 1,205 simple repeats, 61 tandem repeats, and 120 RNA editing sites. Analysis of codon usage indicates that codons ending in A/U are preferred. Gene migration between the mitochondrial genome and the chloroplast genome was observed through homologous fragment detection. In addition, Ka/Ks analysis showed that most protein coding genes in the mitochondrial genome experienced negative selection, while only the nad3 gene experienced potential positive selection in most Alismatales. Nucleotide polymorphism analysis revealed variations in each gene, with rpl10 being the most significant. In addition, comparative analysis shows that the GC content is conserved, but there are significant differences in the size and structure of mitochondrial genomes among different species of Alismatales. The phylogenetic analysis based on the mitochondrial genome reflects the exact evolutionary and taxonomic status of H. beccarii.

CONCLUSION

In this study, we sequenced and annotated the mitochondrial genome of H. beccarii, and compared it with the mitochondrial genomes of other plants in Alismatales. Our findings enrich the mitogenome database of seagrass plants and highlight the potential for mitochondrial genes to help decipher plant evolutionary history.

Peering through the hedge: Multiple datasets yield insights into the phylogenetic relationships and incongruences in the tribe Lilieae (Liliaceae)

The increasing use of genome-scale data has significantly facilitated phylogenetic analyses, contributing to the dissection of the underlying evolutionary mechanisms that shape phylogenetic incongruences, such as incomplete lineage sorting (ILS) and hybridization. Lilieae, a prominent member of the Liliaceae family, comprises four genera and approximately 260 species, representing 43% of all species within Liliaceae. They possess high ornamental, medicinal and edible values. Yet, no study has explored the validity of various genome-scale data in phylogenetic analyses within this tribe, nor have potential evolutionary mechanisms underlying its phylogenetic incongruences been investigated. Here, transcriptome, Angiosperms353, plastid and mitochondrial data, were collected from 50 to 93 samples of Lilieae, covering all four recognized genera. Multiple datasets were created and used for phylogenetic analyses based on concatenated and coalescent-based methods. Evolutionary rates of different datasets were calculated, and divergence times were estimated. Various approaches, including coalescence simulation, Quartet Sampling (QS), calculation of concordance factors (gCF and sCF), as well as MSCquartets and reticulate network inference, were carried out to infer the phylogenetic discordances and analyze their underlying mechanisms using a reduced 33-taxon dataset. Despite extensive phylogenetic discordances among gene trees, robust phylogenies were inferred from nuclear and plastid data compared to mitochondrial data, with lower synonymous substitution detected in mitochondrial genes than in nuclear and plastid genes. Significant ILS was detected across the phylogeny of Lilieae, with clear evidence of reticulate evolution identified. Divergence time estimation indicated that most of lineages in Lilieae diverged during a narrow time frame (ranging from 5.0 Ma to 10.0 Ma), consistent with the notion of rapid radiation evolution. Our results suggest that integrating transcriptomic and plastid data can serve as cost-effective and efficient tools for phylogenetic inference and evolutionary analysis within Lilieae, and Angiosperms353 data is also a favorable choice. Mitochondrial data are more suitable for phylogenetic analyses at higher taxonomic levels due to their stronger conservation and lower synonymous substitution rates. Significant phylogenetic incongruences detected in Lilieae were caused by both incomplete lineage sorting (ILS) and reticulate evolution, with hybridization and "ghost introgression" likely prevalent in the evolution of Lilieae species. Our findings provide new insights into the phylogeny of Lilieae, enhancing our understanding of the evolution of species in this tribe.

Comparative analysis of mitochondrial genomes of invasive weed Mikania micrantha and its indigenous congener Mikania cordata

Mikania micrantha and Mikania cordata are two distinct species in China. The former is notorious as one of the top 100 worst invasive species, whereas the latter is an indigenous species harmless to native plants or the environment. They form an ideal congener pair for comparative studies aimed at deeply understanding the invasion mechanisms of the exotic weed. In this study, we have assembled and annotated the mitogenomes of both species using Illumina and PacBio sequencing data and compared their characteristic differences. The complete mitogenome of M. micrantha is a double-stranded DNA with a length of 336,564 bp, while the mitogenome of M. cordata exhibits a branching structure, consisting of two small circular molecules and six linear molecules, with a combined length totaling 335,444 bp. Compared to M. cordata, M. micrantha has less SSRs, tandem repeats, dispersed repeats, mitochondrial protein coding genes (PCGs). The two plants show similar codon usage patterns. This comparative study has revealed the structure and function of the mitogenomes of the two species and laid a solid foundation for investigating the effects of gene loss and duplication on the development of invasive traits in M. micrantha.

Mitochondrial genome insights into the spatio-temporal distribution and genetic diversity of Dendrobium hancockii Rolfe (Orchidaceae)

Introduction

With its distinctive evolutionary rate and inheritance patterns separate from the nuclear genome, mitochondrial genome analysis has become a prominent focus of current research. Dendrobium hancockii Rolfe, a species of orchid with both medicinal and horticultural value, will benefit from the application of the fully assembled and annotated mitochondrial genome. This will aid in elucidating its phylogenetic relationships, comparative genomics, and population genetic diversity.

Methods

Based on sequencing results from Illumina combined with PacBio and Nanopore, the mitochondrial genome map of D. hancockii was constructed. Comparative analysis was conducted from the perspectives of phylogeny across multiple species, selection pressure on protein-coding genes, and homologous segments. The population diversity of D. hancockii was analyzed using single nucleotide polymorphism (SNP) data from the mitochondrial genome and single-copy nuclear genes.

Results and discussion

This research constructed a circular mitochondrial map for D. hancockii, spanning 523,952 bp, containing 40 unique protein-coding genes, 37 transfer RNA genes, and 4 ribosomal RNA genes. Comparative analysis of mitochondrial genes from 26 land plants revealed a conserved gene cluster, "rpl16-ccmFn-rps3-rps19," particularly within the Dendrobium genus. The mitochondrial genome of D. hancockii exhibits a lower point mutation rate but significant structural variation. Analysis of 103 resequencing samples identified 19,101 SNP sites, dividing D. hancockii into two major groups with limited gene flow between them, as supported by population diversity, genetic structure analysis, principal component analysis, and phylogenetic trees. The geographical distribution and genetic differentiation of D. hancockii into two major groups suggest a clear phytogeographical division, likely driven by ancient geological or climatic events. The close alignment of mitochondrial data with nuclear gene data highlights the potential of the mitochondrial genome for future studies on genetic evolution in this species.

The complete mitochondrial genome of Castanopsis carlesii and Castanea henryi reveals the rearrangement and size differences of mitochondrial DNA molecules

BACKGROUND

Castanopsis carlesii is a dominant tree species in subtropical evergreen broad-leaved forests and holds significant ecological value. It serves as an excellent timber tree species and raw material for cultivating edible fungi. Henry Chinquapin (Castanea henryi) wood is known for its hardness and resistance to water and moisture, making it an exceptional timber species. Additionally, its fruit has a sweet and fruity taste, making it a valuable food source. However, the mitogenomes of these species have not been previously reported. To gain a better understanding of them, this study successfully assembled high-quality mitogenomes of C. carlesii and Ca. henryi for the first time.

RESULTS

Our research reveals that the mitochondrial DNA (mtDNA) of C. carlesii exhibits a unique multi-branched conformation, while Ca. henryi primarily exists in the form of two independent molecules that can be further divided into three independent molecules through one pair of long repetitive sequences. The size of the mitogenomes of C. carlesii and Ca. henryi are 592,702 bp and 379,929 bp respectively, which are currently the largest and smallest Fagaceae mitogenomes recorded thus far. The primary factor influencing mitogenome size is dispersed repeats. Comparison with published mitogenomes from closely related species highlights differences in size, gene loss patterns, codon usage preferences, repetitive sequences, as well as mitochondrial plastid DNA segments (MTPTs).

CONCLUSIONS

Our study enhances the understanding of mitogenome structure and evolution in Fagaceae, laying a crucial foundation for future research on cell respiration, disease resistance, and other traits in this family.

Rapid evolution of mitochondrion-related genes in haplodiploid arthropods

BACKGROUND

Mitochondrial genes and nuclear genes cooperate closely to maintain the functions of mitochondria, especially in the oxidative phosphorylation (OXPHOS) pathway. However, mitochondrial genes among arthropod lineages have dramatic evolutionary rate differences. Haplodiploid arthropods often show fast-evolving mitochondrial genes. One hypothesis predicts that the small effective population size of haplodiploid species could enhance the effect of genetic drift leading to higher substitution rates in mitochondrial and nuclear genes. Alternatively, positive selection or compensatory changes in nuclear OXPHOS genes could lead to the fast-evolving mitochondrial genes. However, due to the limited number of arthropod genomes, the rates of evolution for nuclear genes in haplodiploid species, besides hymenopterans, are largely unknown. To test these hypotheses, we used data from 76 arthropod genomes, including 5 independently evolved haplodiploid lineages, to estimate the evolutionary rates and patterns of gene family turnover of mitochondrial and nuclear genes.

RESULTS

We show that five haplodiploid lineages tested here have fast-evolving mitochondrial genes and fast-evolving nuclear genes related to mitochondrial functions, while nuclear genes not related to mitochondrion showed no significant evolutionary rate differences. Among hymenopterans, bees and ants show faster rates of molecular evolution in mitochondrial genes and mitochondrion-related nuclear genes than sawflies and wasps. With genome data, we also find gene family expansions and contractions in mitochondrion-related genes of bees and ants.

CONCLUSIONS

Our results reject the small population size hypothesis in haplodiploid species. A combination of positive selection and compensatory changes could lead to the observed patterns in haplodiploid species. The elevated evolutionary rates in OXPHOS complex 2 genes of bees and ants suggest a unique evolutionary history of social hymenopterans.

Deciphering the multi- partite mitochondrial genome of Crataegus pinnatifida: insights into the evolution and genetics of cultivated Hawthorn

Flowering plant (angiosperm) mitochondrial genomes are remarkably dynamic in their structures. We present the complete mitochondrial genome of hawthorn (Crataegus pinnatifida Bunge), a shrub that bears fruit and is celebrated for its extensive medicinal history. We successfully assembled the hawthorn mitogenome utilizing the PacBio long-read sequencing technique, which yielded 799,862 reads, and the Illumina novaseq6000 sequencing platform, which producing 6.6 million raw paired reads. The C. pinnatifida mitochondria sequences encompassed a total length of 440,295 bp with a GC content of 45.42%. The genome annotates 54 genes, including 34 that encode proteins, 17 that encode tRNA, and three genes for rRNA. A fascinating interplay was observed between the chloroplast and mitochondrial genomes, which share 17 homologous sequences sequences that rotal 1,933 bp. A total of 134 SSRs, 22 tandem repeats and 42 dispersed repeats were identified in the mitogenome. Four conformations of C. pinnatifida mitochondria sequences recombination were verified through PCR experiments and Sanger sequencing, and C. pinnatifida mitogenome is more likely to be assembled into three circular-mapping chromosomes. All the RNA editing sites that were identified C-U edits, which predominantly occurred at the first and second positions of the codons. Phylogenetic and collinearity analyses identified the evolutionary trajectory of C. pinnatifida, which reinforced the genetic identity of the hawthorn section. This unveiling of the unique multi-partite structure of the hawthorn mitogenome offers a foundational reference for future study into the evolution and genetics of C. pinnatifida.

The mitochondrial genome of Lavandula angustifolia Mill. (Lamiaceae) sheds light on its genome structure and gene transfer between organelles

BACKGROUND

Lavandula angustifolia holds importance as an aromatic plant with extensive applications spanning the fragrance, perfume, cosmetics, aromatherapy, and spa sectors. Beyond its aesthetic and sensory applications, this plant offers medicinal benefits as a natural herbal remedy and finds use in household cleaning products. While extensive genomic data, inclusive of plastid and nuclear genomes, are available for this species, researchers have yet to characterize its mitochondrial genome. This gap in knowledge hampers deeper understanding of the genome organization and its evolutionary significance.

RESULTS

Through the course of this study, we successfully assembled and annotated the mitochondrial genome of L. angustifolia, marking a first in this domain. This assembled genome encompasses 61 genes, which comprise 34 protein-coding genes, 24 transfer RNA genes, and three ribosomal RNA genes. We identified a chloroplast sequence insertion into the mitogenome, which spans a length of 10,645 bp, accounting for 2.94% of the mitogenome size. Within these inserted sequences, there are seven intact tRNA genes (trnH-GUG, trnW-CCA, trnD-GUC, trnS-GGA, trnN-GUU, trnT-GGU, trnP-UGG) and four complete protein-coding genes (psbA, rps15, petL, petG) of chloroplast derivation. Additional discoveries include 88 microsatellites, 15 tandem repeats, 74 palindromic repeats, and 87 forward long repeats. An RNA editing analysis highlighted an elevated count of editing sites in the cytochrome c oxidase genes, notably ccmB with 34 editing sites, ccmFN with 32, and ccmC with 29. All protein-coding genes showed evidence of cytidine-to-uracil conversion. A phylogenetic analysis, utilizing common protein-coding genes from 23 Lamiales species, yielded a tree with consistent topology, supported by high confidence values.

CONCLUSIONS

Analysis of the current mitogenome resource revealed its typical circular genome structure. Notably, sequences originally from the chloroplast genome were found within the mitogenome, pointing to the occurrence of horizontal gene transfer between organelles. This assembled mitogenome stands as a valuable resource for subsequent studies on mitogenome structures, their evolution, and molecular biology.

Assembly and comparative analysis of the first complete mitochondrial genome of Salix psammophila, a good windbreak and sand fixation shrub

Salix psammophila, commonly known as the sandlive willow, is a vital shrub species within the Salicaceae family, particularly significant for its ecological role in regions susceptible to desertification and sandy soils. In this study, we assembled the complete S. psammophila mitochondrial genome using Pacbio HiFi third-generation sequencing data. The genome was found to be a typical single circular structure, with a total length of 715,555 bp and a GC content of 44.89%. We annotated 33 unique protein-coding genes (PCGs), which included 24 core mitochondrial genes and 9 variable genes, as well as 18 tRNA genes (5 of which were multicopy genes) and 3 rRNA genes. Comparative analysis of the PCGs from the mitochondrial genomes of S. psammophila, Populus deltoides, Populus simonii, Salix wilsonii, and Salix suchowensis revealed that these genes are relatively conserved within the Salicaceae family, with variability primarily occurring in the ribosomal protein genes. The absence of the rps14, which encodes a ribosomal protein, may have played a role in the evolution of stress tolerance in Salicaceae plants. Additionally, we identified 232 SSRs, 19 tandem repeat sequences, and 236 dispersed repeat sequences in the S. psammophila mitochondrial genome, with palindromic and forward repeats being the most abundant. The longest palindromic repeat measured 260 bp, while the longest forward repeat was 86,068 bp. Furthermore, 324 potential RNA editing sites were discovered, all involving C-to-U edits, with the nad4 having the highest number of edits. These findings provide valuable insights into the phylogenetic and genetic research of Salicaceae plants.

Characterization of the complete mitogenome of Tiarella polyphylla, commonly known as Asian foamflower: insights into the multi-chromosomes structure and DNA transfers

BACKGROUND

Tiarella polyphylla D. Don has been traditionally used to cure asthma and skin eruptions. However, the sequence and the structure of the mitogenome of T. polyphylla remained elusive, limiting the genomic and evolution analysis based on the mitogenome.

RESULTS

Using a combination of Illumina and Nanopore sequencing reads, we de novo assembled the complete mitogenome of T. polyphylla. In addition to unveiling the major configuration of the T. polyphylla mitogenome was three circular chromosomes with lengths of 430,435 bp, 126,943 bp, and 55,269 bp, we revealed five (R01-R05) and one (R06) repetitive sequence could mediate the intra- and inter-chromosomal recombination, respectively. Furthermore, we identified 208 short and 25 long tandem segments, seven cp-derived mtDNAs, 106 segments of mtDNAs transferred to the nuclear genome, and 653 predicted RNA editing sites. Based on the sequence of the mitogenomes, we obtained the resolved phylogeny of the seven Saxifragales species.

CONCLUSIONS

These results presented the mitogenome features and expanded its potential applications in phylogenetics, species identification, and cytoplasmic male sterility (CMS) in the future.

Cytoplasmic genomes of Jasminum sambac reveal divergent sub-mitogenomic conformations and a large nuclear chloroplast-derived insertion

BACKGROUND

Jasminum sambac, a widely recognized ornamental plant prized for its aromatic blossoms, exhibits three flora phenotypes: single-petal ("SP"), double-petal ("DP"), and multi-petal ("MP"). The lack of detailed characterization and comparison of J. sambac mitochondrial genomes (mitogenomes) hinders the exploration of the genetic and structural diversity underlying the varying floral phenotypes in jasmine accessions.

RESULTS

Here, we de novo assembled three mitogenomes of typical phenotypes of J. sambac, "SP", "DP", and "MP-hutou" ("HT"), with PacBio reads and the "HT" chloroplast (cp) genome with Illumina reads, and verified them with read mapping and fluorescence in situ hybridization (FISH). The three mitogenomes present divergent sub-genomic conformations, with two, two, and four autonomous circular chromosomes ranging in size from 35.7 kb to 405.3 kb. Each mitogenome contained 58 unique genes. Ribosome binding sites with conserved AAGAAx/AxAAAG motifs were detected upstream of uncanonical start codons TTG, CTG and GTG. The three mitogenomes were similar in genomic content but divergent in structure. The structural variations were mainly attributed to recombination mediated by a large (~ 5 kb) forward repeat pair and several short repeats. The three jasmine cp. genomes showed a well-conserved structure, apart from a 19.9 kb inversion in "HT". We identified a 14.3 kb "HT"-specific insertion on Chr7 of the "HT" nuclear genome, consisting of two 7 kb chloroplast-derived fragments with two intact ndhH and rps15 genes, further validated by polymerase chain reaction (PCR). The well-resolved phylogeny suggests faster mitogenome evolution in J. sambac compared to other Oleaceae species and outlines the mitogenome evolutionary trajectories within Lamiales. All evidence supports that "DP" and "HT" evolved from "SP", with "HT" being the most recent derivative of "DP".

CONCLUSION

The comprehensive characterization of jasmine organelle genomes has added to our knowledge of the structural diversity and evolutionary trajectories behind varying jasmine traits, paving the way for in-depth exploration of mechanisms and targeted genetic research.

Comparative mitochondrial genomics of Terniopsis yongtaiensis in Malpighiales: structural, sequential, and phylogenetic perspectives

BACKGROUND

Terniopsis yongtaiensis, a member of the Podostemaceae family, is an aquatic flowering plant displaying remarkable adaptive traits that enable survival in submerged, turbulent habitats. Despite the progressive expansion of chloroplast genomic information within this family, mitochondrial genome sequences have yet to be reported.

RESULTS

In current study, the mitochondrial genome of the T. yongtaiensis was characterized by a circular genome of 426,928 bp encoding 31 protein-coding genes (PCGs), 18 tRNAs, and 3 rRNA genes. Our comprehensive analysis focused on gene content, repeat sequences, RNA editing processes, intracellular gene transfer, phylogeny, and codon usage bias. Numerous repeat sequences were identified, including 130 simple sequence repeats, 22 tandem repeats, and 220 dispersed repeats. Phylogenetic analysis positioned T. yongtaiensis (Podostemaceae) within the Malpighiales order, showing a close relationship with the Calophyllaceae family, which was consistent with the APG IV classification. A comparative analysis with nine other Malpighiales species revealed both variable and conserved regions, providing insights into the genomic evolution within this order. Notably, the GC content of T. yongtaiensis was distinctively lower compared to other Malpighilales, primarily due to variations in non-coding regions and specific protein-coding genes, particularly the nad genes. Remarkably, the number of RNA editing sites was low (276), distributed unevenly across 27 PCGs. The dN/dS analysis showed only the ccmB gene of T. yongtaiensis was positively selected, which plays a crucial role in cytochrome c biosynthesis. Additionally, there were 13 gene-containing homologous regions between the mitochondrial and chloroplast genomes of T. yongtaiensis, suggesting the gene transfer events between these organellar genomes.

CONCLUSIONS

This study assembled and annotated the first mitochondrial genome of the Podostemaceae family. The comparison results of mitochondrial gene composition, GC content, and RNA editing sites provided novel insights into the adaptive traits and genetic reprogramming of this aquatic eudicot group and offered a foundation for future research on the genomic evolution and adaptive mechanisms of Podostemaceae and related plant families in the Malpighiales order.

Methods for assembling complex mitochondrial genomes in land plants

The large size and complex structural rearrangements inherent in the mitochondrial genomes of land plants pose challenges for their sequencing. Originally, the assembly of these genomes required the cloning of mitochondrial DNA fragments followed by Sanger sequencing. Subsequently, the advent of next-generation sequencing significantly expedited the process. This review highlights examples of plant mitochondrial genome assembly employing various technologies, including 454 sequencing, Illumina short sequencing reads, and Pacific Biosciences or Oxford Nanopore Technology long sequencing reads. The combination of short and long reads in hybrid assembly has proven to be the most efficient approach for achieving reliable assemblies of land plant mitochondrial genomes.

Comparative Analysis of the Mitochondrial Genome of Eggplant (Solanum melongena L.) to Identify Cytoplasmic Male Sterility Candidate Genes

Cytoplasmic male sterility (CMS) is important for commercial hybrid seed production. However, it is still not used in eggplant (Solanum melongena L.), and corresponding regulatory genes and mechanisms of action have not been reported. We report CMS line 327A, which was derived from the hybridization between cultivated and wild eggplants. By looking at different stages of anther development under a microscope, we saw that the 327A anther's tapetum layer vacuolized during meiosis, which caused abortion. To investigate the 327A CMS regulatory genes, the mitochondrial genomes of 327A and its maintainer line 327B were assembled de novo. It was found that 15 unique ORFs (Open Reading Frame) were identified in 327A. RT-PCR and RT-QPCAR tests confirmed that orf312a and orf172a, 327A-specific ORFs with a transmembrane domain, were strongly expressed in sterile anthers of 327A. In addition, orf312a has a chimeric structure with the ribosomal protein subunit rpl16. Therefore, orf312a and orf172a can be considered strong candidate genes for CMS. Concurrently, we analyzed the characteristics of CMS to develop a functional molecular marker, CMS312, targeting a future theoretical basis for eggplant CMS three-line molecular breeding.

The assembly and comparative analysis of the first complete mitogenome of Lindera aggregata

Lindera aggregata, a member belongs to the genus Lindera of Lauraceae family. Its roots and leaves have been used as traditional Chinese medicine or functional food for thousands of years. However, its mitochondrial genome has not been explored. Our aim is to sequence and assemble the mitogenome of L. aggregata to elucidate the genetic mechanism and evolutionary pathway. The results had shown that the mitogenome was extremely complex and had a unique multi-branched conformation with total size of 912,473 bp. Comprehensive analysis of protein coding genes of 7 related species showed that there were 40 common genes in their mitogenome. Interestingly, positive selection had become an important factor in the evolution of ccmB, ccmFC, rps10, rps11 and rps7 genes. Furthermore, our data highlighted the repeated trend of homologous fragment migrations between chloroplast and mitochondrial organelles, and 38 homologous fragments were identified. Phylogenetic analysis identified a tree that was basically consistent with the phylogeny of Laurales species described in the APG IV system. To sum up, this study will be helpful to the study of population genetics and evolution of Lindera species.

De novo assembly and comprehensive analysis of the mitochondrial genome of Taxus wallichiana reveals different repeats mediate recombination to generate multiple conformations

Taxus plants are the exclusive source of paclitaxel, an anticancer drug with significant medicinal and economic value. Interspecies hybridization and gene introgression during evolution have obscured distinctions among Taxus species, complicating their phylogenetic classification. While the chloroplast genome of Taxus wallichiana, a widely distributed species in China, has been sequenced, its mitochondrial genome (mitogenome) remains uncharacterized.We sequenced and assembled the T. wallichiana mitogenome using BGI short reads and Nanopore long reads, facilitating comparisons with other gymnosperm mitogenomes. The T. wallichiana mitogenome spanning 469,949 bp, predominantly forms a circular configuration with a GC content of 50.51%, supplemented by 3 minor configurations mediated by one pair of LRs and two pairs of IntRs. It includes 32 protein-coding genes, 7 tRNA genes, and 3 rRNA genes, several of which exist in multiple copies.We detailed the mitogenome's structure, codon usage, RNA editing, and sequence migration between organelles, constructing a phylogenetic tree to elucidate evolutionary relationships. Unlike typical gymnosperm mitochondria, T. wallichiana shows no evidence of mitochondrial-plastid DNA transfer (MTPT), highlighting its unique genomic architecture. Synteny analysis indicated extensive genomic rearrangements in T. wallichiana, likely driven by recombination among abundant repetitive sequences. This study offers a high-quality T. wallichiana mitogenome, enhancing our understanding of gymnosperm mitochondrial evolution and supporting further cultivation and utilization of Taxus species.

Horizontally transferred mitochondrial DNA tracts become circular by microhomology-mediated repair pathways

The holoparasitic plant Lophophytum mirabile exhibits remarkable levels of mitochondrial horizontal gene transfer (HGT). Gathering comparative data from other individuals and host plants can provide insights into the HGT process. We sequenced the mitochondrial genome (mtDNA) from individuals of two species of Lophophytum and from mimosoid hosts. We applied a stringent phylogenomic approach to elucidate the origin of the whole mtDNAs, estimate the timing of the transfers, and understand the molecular mechanisms involved. Ancestral and recent HGT events replaced and enlarged the multichromosomal mtDNA of Lophophytum spp., with the foreign DNA ascending to 74%. A total of 14 foreign mitochondrial chromosomes originated from continuous regions in the host mtDNA flanked by short direct repeats. These foreign tracts are circularized by microhomology-mediated repair pathways and replicate independently until they are lost or they eventually recombine with other chromosomes. The foreign noncoding chromosomes are variably present in the population and likely evolve by genetic drift. We present the 'circle-mediated HGT' model in which foreign mitochondrial DNA tracts become circular and are maintained as plasmid-like molecules. This model challenges the conventional belief that foreign DNA must be integrated into the recipient genome for successful HGT.

Structural analysis of the mitochondrial genome of Santalum album reveals a complex branched configuration

BACKGROUND

Santalum album L. is an evergreen tree which is mainly distributes throughout tropical and temperate regions. And it has a great medicinal and economic value.

RESULTS

In this study, the complete mitochondrial genome of S. album were assembled and annotated, which could be descried by a complex branched structure consisting of three contigs. The lengths of these three contigs are 165,122 bp, 93,430 bp and 92,491 bp. We annotated 34 genes coding for proteins (PCGs), 26 tRNA genes, and 4 rRNA genes. The analysis of repeated elements shows that there are 89 SSRs and 242 pairs of dispersed repeats in S. album mitochondrial genome. Also we found 20 MTPTs among the chloroplast and mitochondria. The 20 MTPTs sequences span a combined length of 22,353 bp, making up 15.52 % of the plastome, 6.37 % of the mitochondrial genome. Additionally, by using the Deepred-mt tool, we found 628 RNA editing sites in 34 PCGs. Moreover, significant genomic rearrangement is observed between S. album and its associated mitochondrial genomes. Finally, based on mitochondrial genome PCGs, we deduced the phylogenetic ties between S. album and other angiosperms.

CONCLUSIONS

We reported the mitochondrial genome from Santalales for the first time, which provides a crucial genetic resource for our study of the evolution of mitochondrial genome.

Complete mitochondrial genome assembly of Zizania latifolia and comparative genome analysis

Zizania latifolia (Griseb.) Turcz. ex Stapf has been cultivated as a popular aquatic vegetable in China due to its important nutritional, medicinal, ecological, and economic values. The complete mitochondrial genome (mitogenome) of Z. latifolia has not been previously studied and reported, which has hindered its molecular systematics and understanding of evolutionary processes. Here, we assembled the complete mitogenome of Z. latifolia and performed a comprehensive analysis including genome organization, repetitive sequences, RNA editing event, intercellular gene transfer, phylogenetic analysis, and comparative mitogenome analysis. The mitogenome of Z. latifolia was estimated to have a circular molecule of 392,219 bp and 58 genes consisting of three rRNA genes, 20 tRNA genes, and 35 protein-coding genes (PCGs). There were 46 and 20 simple sequence repeats (SSRs) with different motifs identified from the mitogenome and chloroplast genome of Z. latifolia, respectively. Furthermore, 49 homologous fragments were observed to transfer from the chloroplast genome to the mitogenome of Z. latifolia, accounting for 47,500 bp, presenting 12.1% of the whole mitogenome. In addition, there were 11 gene-containing homologous regions between the mitogenome and chloroplast genome of Z. latifolia. Also, approximately 85% of fragments from the mitogenome were duplicated in the Z. latifolia nuclear genome. Selection pressure analysis revealed that most of the mitochondrial genes were highly conserved except for ccmFc, ccmFn, matR, rps1, and rps3. A total of 93 RNA editing sites were found in the PCGs of the mitogenome. Z. latifolia and Oryza minuta are the most closely related, as shown by collinear analysis and the phylogenetic analysis. We found that repeat sequences and foreign sequences in the mitogenomes of Oryzoideae plants were associated with genome rearrangements. In general, the availability of the Z. latifolia mitogenome will contribute valuable information to our understanding of the molecular and genomic aspects of Zizania.

Assembly and comparative analysis of the complete mitochondrial genome of Fritillaria ussuriensis Maxim. (Liliales: Liliaceae), an endangered medicinal plant

BACKGROUND

Fritillaria ussuriensis is an endangered medicinal plant known for its notable therapeutic properties. Unfortunately, its population has drastically declined due to the destruction of forest habitats. Thus, effectively protecting F. ussuriensis from extinction poses a significant challenge. A profound understanding of its genetic foundation is crucial. To date, research on the complete mitochondrial genome of F. ussuriensis has not yet been reported.

RESULTS

The complete mitochondrial genome of F. ussuriensis was sequenced and assembled by integrating PacBio and Illumina sequencing technologies, revealing 13 circular chromosomes totaling 737,569 bp with an average GC content of 45.41%. A total of 55 genes were annotated in this mitogenome, including 2 rRNA genes, 12 tRNA genes, and 41 PCGs. The mitochondrial genome of F. ussuriensis contained 192 SSRs and 4,027 dispersed repeats. In the PCGs of F. ussuriensis mitogenome, 90.00% of the RSCU values exceeding 1 exhibited a preference for A-ended or U-ended codons. In addition, 505 RNA editing sites were predicted across these PCGs. Selective pressure analysis suggested negative selection on most PCGs to preserve mitochondrial functionality, as the notable exception of the gene nad3 showed positive selection. Comparison between the mitochondrial and chloroplast genomes of F. ussuriensis revealed 20 homologous fragments totaling 8,954 bp. Nucleotide diversity analysis revealed the variation among genes, and gene atp9 was the most notable. Despite the conservation of GC content, mitogenome sizes varied significantly among six closely related species, and colinear analysis confirmed the lack of conservation in their genomic structures. Phylogenetic analysis indicated a close relationship between F. ussuriensis and Lilium tsingtauense.

CONCLUSIONS

In this study, we sequenced and annotated the mitogenome of F. ussuriensis and compared it with the mitogenomes of other closely related species. In addition to genomic features and evolutionary position, this study also provides valuable genomic resources to further understand and utilize this medicinal plant.

Complete mitochondrial genome of Hippophae tibetana: insights into adaptation to high-altitude environments

Hippophae tibetana, belonging to the Elaeagnaceae family, is an endemic plant species of the Qinghai-Tibet Plateau, valued for its remarkable ecological restoration capabilities, as well as medicinal and edible properties. Despite being acknowledged as a useful species, its mitochondrial genome data and those of other species of the Elaeagnaceae family are lacking to date. In this study, we, for the first time, successfully assembled the mitochondrial genome of H. tibetana, which is 464,208 bp long and comprises 31 tRNA genes, 3 rRNA genes, 37 protein-coding genes, and 3 pseudogenes. Analysis of the genome revealed a high copy number of the trnM-CAT gene and a high prevalence of repetitive sequences, both of which likely contribute to genome rearrangement and adaptive evolution. Through nucleotide diversity and codon usage bias analyses, we identified specific genes that are crucial for adaptation to high-altitude conditions. Notably, genes such as atp6, ccmB, nad4L, and nad7 exhibited signs of positive selection, indicating the presence of unique adaptive traits for survival in extreme environments. Phylogenetic analysis confirmed the close relationship between the Elaeagnaceae family and other related families, whereas intergenomic sequence transfer analysis revealed a substantial presence of homologous fragments among the mitochondrial, chloroplast, and whole genomes, which may be linked to the high-altitude adaptation mechanisms of H. tibetana. The findings of this study not only enrich our knowledge of H. tibetana molecular biology but also advance our understanding of the adaptive evolution of plants on the Qinghai-Tibet Plateau. This study provides a solid scientific foundation for the molecular breeding, conservation, and utilization of H. tibetana genetic resources.

Evolution and maintenance of mtDNA gene content across eukaryotes

Across eukaryotes, most genes required for mitochondrial function have been transferred to, or otherwise acquired by, the nucleus. Encoding genes in the nucleus has many advantages. So why do mitochondria retain any genes at all? Why does the set of mtDNA genes vary so much across different species? And how do species maintain functionality in the mtDNA genes they do retain? In this review, we will discuss some possible answers to these questions, attempting a broad perspective across eukaryotes. We hope to cover some interesting features which may be less familiar from the perspective of particular species, including the ubiquity of recombination outside bilaterian animals, encrypted chainmail-like mtDNA, single genes split over multiple mtDNA chromosomes, triparental inheritance, gene transfer by grafting, gain of mtDNA recombination factors, social networks of mitochondria, and the role of mtDNA dysfunction in feeding the world. We will discuss a unifying picture where organismal ecology and gene-specific features together influence whether organism X retains mtDNA gene Y, and where ecology and development together determine which strategies, importantly including recombination, are used to maintain the mtDNA genes that are retained.

Accumulation of Large Lineage-Specific Repeats Coincides with Sequence Acceleration and Structural Rearrangement in Plantago Plastomes

Repeats can mediate rearrangements and recombination in plant mitochondrial genomes and plastid genomes. While repeat accumulations are linked to heightened evolutionary rates and complex structures in specific lineages, debates persist regarding the extent of their influence on sequence and structural evolution. In this study, 75 Plantago plastomes were analyzed to investigate the relationships between repeats, nucleotide substitution rates, and structural variations. Extensive repeat accumulations were associated with significant rearrangements and inversions in the large inverted repeats (IRs), suggesting that repeats contribute to rearrangement hotspots. Repeats caused infrequent recombination that potentially led to substoichiometric shifting, supported by long-read sequencing. Repeats were implicated in elevating evolutionary rates by facilitating localized hypermutation, likely through DNA damage and repair processes. This study also observed a decrease in nucleotide substitution rates for loci translocating into IRs, supporting the role of biased gene conversion in maintaining lower substitution rates. Combined with known parallel changes in mitogenomes, it is proposed that potential dysfunction in nuclear-encoded genes associated with DNA replication, recombination, and repair may drive the evolution of Plantago organellar genomes. These findings contribute to understanding how repeats impact organellar evolution and stability, particularly in rapidly evolving plant lineages.

Dinoflagellate chloroplasts as a model for extreme genome reduction and fragmentation in organelles - The COCOA principle for gene retention

The genomes of peridinin-containing dinoflagellate chloroplasts have a very unusual organisation. These genomes are highly fragmented and greatly reduced, with most of the usual complement of chloroplast genes relocated to the nucleus. Dinoflagellate chloroplasts highlight evolutionary changes that are found to varying extents in a number of other organelle genomes. These include the chloroplast genome of the green alga Boodlea and other Cladophorales, and the mitochondrial genomes of blood-sucking and chewing lice, the parasitic plant Rhopalocnemis phalloides, the red alga Rhodosorus marinus and other members of the Stylonematophyceae, diplonemid flagellates, and some Cnidaria. Consideration of the coding content of the remnant chloroplast genomes indicates that organelles may preferentially retain genes for proteins important in initiating assembly of complexes, and the same is largely true for mitochondria. We propose a new principle, of CO-location for COntrol of Assembly (COCOA), indicating the importance of retaining these genes in the organelle. This adds to, but does not invalidate, the existing hypotheses of the multisubunit completion principle, CO-location for Redox Regulation (CORR) and Control by Epistasy of Synthesis (CES).

A Systematic Review and Developmental Perspective on Origin of CMS Genes in Crops

Cytoplasmic male sterility (CMS) arises from the incompatibility between the nucleus and cytoplasm as typical representatives of the chimeric structures in the mitochondrial genome (mitogenome), which has been extensively applied for hybrid seed production in various crops. The frequent occurrence of chimeric mitochondrial genes leading to CMS is consistent with the mitochondrial DNA (mtDNA) evolution. The sequence conservation resulting from faithfully maternal inheritance and the chimeric structure caused by frequent sequence recombination have been defined as two major features of the mitogenome. However, when and how these chimeric mitochondrial genes appear in the context of the highly conserved reproduction of mitochondria is an enigma. This review, therefore, presents the critical view of the research on CMS in plants to elucidate the mechanisms of this phenomenon. Generally, distant hybridization is the main mechanism to generate an original CMS source in natural populations and in breeding. Mitochondria and mitogenomes show pleomorphic and dynamic changes at key stages of the life cycle. The promitochondria in dry seeds develop into fully functioning mitochondria during seed imbibition, followed by massive mitochondria or mitogenome fusion and fission in the germination stage along with changes in the mtDNA structure and quantity. The mitogenome stability is controlled by nuclear loci, such as the nuclear gene Msh1. Its suppression leads to the rearrangement of mtDNA and the production of heritable CMS genes. An abundant recombination of mtDNA is also often found in distant hybrids and somatic/cybrid hybrids. Since mtDNA recombination is ubiquitous in distant hybridization, we put forward a hypothesis that the original CMS genes originated from mtDNA recombination during the germination of the hybrid seeds produced from distant hybridizations to solve the nucleo-cytoplasmic incompatibility resulting from the allogenic nuclear genome during seed germination.

Overexpression of RPOTmp Being Targeted to Either Mitochondria or Chloroplasts in Arabidopsis Leads to Overall Transcriptome Changes and Faster Growth

The transcription of Arabidopsis organellar genes is performed by three nuclear-encoded RNA polymerases: RPOTm, RPOTmp, and RPOTp. The RPOTmp protein possesses ambiguous transit peptides, allowing participation in gene expression control in both mitochondria and chloroplasts, although its function in plastids is still under discussion. Here, we show that the overexpression of RPOTmp in Arabidopsis, targeted either to mitochondria or chloroplasts, disturbs the dormant seed state, and it causes the following effects: earlier germination, decreased ABA sensitivity, faster seedling growth, and earlier flowering. The germination of RPOTmp overexpressors is less sensitive to NaCl, while rpotmp knockout is highly vulnerable to salt stress. We found that mitochondrial dysfunction in the rpotmp mutant induces an unknown retrograde response pathway that bypasses AOX and ANAC017. Here, we show that RPOTmp transcribes the accD, clpP, and rpoB genes in plastids and up to 22 genes in mitochondria.