Comparative Analysis of Chloroplast Genome in Saccharum spp. and Related Members of ‘Saccharum Complex’

Insights into Aquilaria phylogenetics through comparative plastomic resources

The plastid is an essential organelle for its role in photosynthesis and energy production and its genomic information is always employed as important evolutionary markers to explore the relationship among species. Agarwood (Aquilaria), prized for its aromatic blend, finds extensive use in various cultures as incense and perfume. Despite its high economic importance, the phylogenetic status among Aquilaria based on plastomes remains ambiguous due to the lack of available plastomic resources. To bridge this knowledge gap, 22 Aquilaria plastomes were newly sequenced, similar variation patterns in this genus were determined, including a shared 16 bp extension of the rps19 gene and seven highly variable regions. The analysis highlighted the highest prevalence of the A/T motif among simple sequence repeats in these plastomes. Further phylogenetic analysis revealed Aquilaria's phylogenetic implications with an expanded dataset. This comprehensive plastomic resource not only enhances our understanding of Aquilaria evolution but also presents potential molecular markers for DNA barcoding.

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 genome analysis of four mitochondrial genomes from Saccharum complex species

Saccharum complex includes genera Saccharum, Miscanthus, Erianthus, Narenga, and Tripidium. Since the Saccharum complex/Saccharinae constitutes the gene pool used by sugarcane breeders to introduce useful traits into sugarcane, studying the genomic characterization of the Saccharum complex has become particularly important. Here, we assembled graph-based mitochondrial genomes (mitogenomes) of four Saccharinae species (T. arundinaceum, E. rockii, M. sinensis, and N. porphyrocoma) using Illumina and PacBio sequencing data. The total lengths of the mitogenomes of T. arundinaceum, M. sinensis, E. rockii and N. porphyrocoma were 549,593 bp, 514,248 bp, 481,576 bp and 513,095 bp, respectively. Then, we performed a comparative mitogenomes analysis of Saccharinae species, including characterization, organelles transfer sequence, collinear sequence, phylogenetics analysis, and gene duplicated/loss. Our results provided the mitogenomes of four species closely related to sugarcane breeding, enriching the mitochondrial genomic resources of the Saccharinae. Additionally, our study offered new insights into the evolution of mitogenomes at the family and genus levels and enhanced our understanding of organelle evolution in the highly polyploid Saccharum genus.

Graph-based mitochondrial genomes of three foundation species in the Saccharum genus

KEY MESSAGE

We reported the graph-based mitochondrial genomes of three foundation species (Saccharum spontaneum, S. robustum and S. officinarum) for the first time. The results revealed pan-structural variation and evolutionary processes in the mitochondrial genomes within Saccharum. Saccharum belongs to the Andropogoneae, and cultivars species in Saccharum contribute nearly 80% of sugar production in the world. To explore the genomic studies in Saccharum, we assembled 15 complete mitochondrial genomes (mitogenome) of three foundation species (Saccharum spontaneum, S. robustum and S. officinarum) using Illumina and Oxford Nanopore Technologies sequencing data. The mitogenomes of the three species were divided into a total of eight types based on contig numbers and linkages. All mitogenomes in the three species encoded 51 unique genes, including 32 protein-coding, 3 ribosomal RNA (rRNA) and 16 transfer RNA (tRNA) genes. The existence of long and short-repeat-mediated recombinations in the mitogenome of S. officinarum and S. robustum was revealed and confirmed through PCR validation. Furthermore, employing comparative genomics and phylogenetic analyses of the organelle genomes, we unveiled the evolutionary relationships and history of the major interspecific lineages in Saccharum genus. Phylogenetic analyses of homologous fragments between S. officinarum and S. robustum showed that S. officinarum and S. robustum are phylogenetically distinct and that they were likely parallel rather than domesticated. The variations between ancient (S. sinense and S. barberi) and modern cultivated species (S. hybrid) possibly resulted from hybridization involving different S. officinarum accessions. Lastly, this project reported the first graph-based mitogenomes of three Saccharum species, and a systematic comparison of the structural organization, evolutionary processes, and pan-structural variation of the Saccharum mitogenomes revealed the differential features of the Saccharum mitogenomes.

The complete chloroplast genome of Saccharum fulvum (Poaceae)

Saccharum species are of great importance as fruit crops due to their economic and food value. S. fulvum is a wild relative of sugarcane that has a wide geographic distribution and is well-adapted to various environmental conditions. It exhibits high resistance to pests, diseases, drought, cold, and degraded soils, making it a valuable resource for sugarcane research. Here, we report the chloroplast genome of S. fulvum. This chloroplast genome was 141,151 bp in length with a GC content of 38.41%. The large single-copy, small single-copy, and inverted repeat regions were 83,030 bp, 12,533 bp, and 22,794 bp in length, respectively. The chloroplast genome contained 111 different genes, including 77 protein-coding genes, 4 rRNA genes, and 30 tRNA genes. Phylogenetic analysis indicated that S. fulvum was closely related to S. narenga. This study not only enriches the genome information of Saccharum, but also will be useful for the evolutionary study of the family Poaceae.

Assessing the genetic integrity of sugarcane germplasm in the USDA-ARS National Plant Germplasm System collection using single-dose SNP markers

The World Collection of Sugarcane and Related Grasses, maintained at the USDA-ARS in Miami, FL, is one of the largest sugarcane germplasm repositories in the world. However, the genetic integrity of the Saccharum spp. germplasm in this collection has not been fully analyzed. In this study, we employed a single-dose SNP panel to genotype 901 sugarcane accessions, representing six Saccharum species and various hybrids. Our analysis uncovered a high rate of clone mislabeling in the collection. Specifically, we identified 86 groups of duplicates, characterized by identical SNP genotypes, which encompassed 211 accessions (23% of the total clones), while 135 groups, constituting 471 clones (52% of the total), exhibited near-identical genotypes. In addition, twenty-seven homonymous groups were detected, which shared the same clone name but differed in SNP genotypes. Hierarchical analysis of population structure partitioned the Saccharum germplasm into five clusters, corresponding to S. barberi, S. sinense, S. officinarum, S. spontaneum and S. robustum/S. edule. An assignment test, based on the five Saccharum species, enabled correcting 141 instances of mislabeled species memberships and inaccuracies. Moreover, we clarified the species membership and parentage of 298 clones that had ambiguous passport records (e.g., 'Saccharum spp', 'unknown', and 'hybrid'). Population structure and genetic diversity in these five species were further supported by Principal Coordinate Analysis and neighbor-joining clustering analysis. Analysis of Molecular Variance revealed that within-species genetic variations accounted for 85% of the total molecular variance, with the remaining 15% attributed to among-species genetic variations. The single-dose SNP markers developed in this study offer a robust tool for characterizing sugarcane germplasm worldwide. These findings have important implications for sugarcane genebank management, germplasm exchange, and crop genetic improvement.

Comparative Plastid Genome and Phylogenomic Analyses of Potamogeton Species

Potamogetonaceae are aquatic plants divided into six genera. The largest genus in the family is Potamogeton, which is morphologically diverse with many hybrids and polyploids. Potamogetonaceae plastomes were conserved in genome size (155,863 bp-156,669 bp), gene contents (113 genes in total, comprising 79 protein-coding genes and 30 tRNA and 4 rRNA genes), and GC content (36.5%). However, we detected a duplication of the trnH gene in the IR region of the Potamogeton crispus and P. maakianus plastomes. A comparative analysis of Alismatales indicated that the plastomes of Potamogetonaceae, Cymodaceae, and Ruppiaceae have experienced a 6-kb inversion of the rbcL-trnV region and the ndh complex has been lost in the Najas flexilis plastome. Five divergent hotspots (rps16-trnQ, atpF intron, rpoB-trnC, trnC-psbM, and ndhF-rpl32) were identified among the Potamogeton plastomes, which will be useful for species identification. Phylogenetic analyses showed that the family Potamogetonaceae is a well-defined with 100% bootstrap support and divided into two different clades, Potamogeton and Stuckenia. Compared to the nucleotide substitution rates among Alismatales, we found neutral selection in all plastid genes of Potamogeton species. Our results reveal the complete plastome sequences of Potamogeton species, and will be helpful for taxonomic identification, the elucidation of phylogenetic relationships, and the plastome structural analysis of aquatic plants.

The population genomic analyses of chloroplast genomes shed new insights on the complicated ploidy and evolutionary history in Fragaria

The genus Fragaria consists of a rich diversity of ploidy levels with diploid (2x), tetraploid (4x), pentaploid (5x), hexaploidy (6x), octoploid (8x) and decaploid (10x) species. Only a few studies have explored the origin of diploid and octoploid strawberry, and little is known about the roles of tetraploidy and hexaploidy during the evolution of octoploid strawberry. The chloroplast genome is usually a stable circular genome and is widely used in investigating the evolution and matrilineal identification. Here, we assembled the chloroplast genomes of F. x ananassa cv. 'Benihoppe' (8x) using Illumina and HiFi data seperately. The genome alignment results showed that more InDels were located in the chloroplast genomes based on the PacBio HiFi data than Illumina data. We obtain highly accurate chloroplast genomes assembled through GetOrganelle using Illumina reads. We assembled 200 chloroplast genomes including 198 Fragaria (21 species) and 2 Potentilla samples. Analyses of sequence variation, phylogenetic and PCA analyses showed that Fragaria was divided into five groups. F. iinumae, F. nilgerrensis and all octoploid accessions formed Group A, C and E separately. Species native to western China were clustered into Group B. Group D consisted of F. virdis, F. orientalis, F. moschata, and F. vesca. STRUCTURE and haplotype network confirmed that the diploid F. vesca subsp. bracteata was the last maternal donator of octoploid strawberry. The dN/dS ratio estimated for the protein-coding genes revealed that genes involved in ATP synthase and photosystem function were under positive selection. These findings demonstrate the phylogeny of totally 21 Fragaria species and the origin of octoploid species. F. vesca was the last female donator of octoploid, which confirms the hypothesis that the hexaploid species F. moschata may be an evolutionary intermediate between the diploids and wild octoploid species.

Comprehensive Comparative Analysis and Development of Molecular Markers for Dianthus Species Based on Complete Chloroplast Genome Sequences

Dianthus spp. is a genus with high economic and ornamental value in the Caryophyllaceae, which include the famous fresh-cut carnation and the traditional Chinese herbal medicine, D. superbus. Despite the Dianthus species being seen everywhere in our daily lives, its genome information and phylogenetic relationships remain elusive. Thus, we performed the assembly and annotation of chloroplast genomes for 12 individuals from seven Dianthus species. On this basis, we carried out the first comprehensive and systematic analysis of the chloroplast genome sequence characteristics and the phylogenetic evolution of Dianthus. The chloroplast genome of 12 Dianthus individuals ranged from 149,192 bp to 149,800 bp, containing 124 to 126 functional genes. Sequence repetition analysis showed the number of simple sequence repeats (SSRs) ranged from 75 to 80, tandem repeats ranged from 23 to 41, and pair-dispersed repeats ranged from 28 to 43. Next, we calculated the synonymous nucleotide substitution rates (Ks) of all 76 protein coding genes to obtain the evolution rate of these coding genes in Dianthus species; rpl22 showed the highest Ks (0.0471), which suggested that it evolved the swiftest. By reconstructing the phylogenetic relationships within Dianthus and other species of Caryophyllales, 16 Dianthus individuals (12 individuals reported in this study and four individuals downloaded from NCBI) were divided into two strongly supported sister clades (Clade A and Clade B). The Clade A contained five species, namely D. caryophyllus, D. barbatus, D. gratianopolitanus, and two cultivars (‘HY’ and ‘WC’). The Clade B included four species, in which D. superbus was a sister branch with D. chinensis, D. longicalyx, and F1 ‘87M’ (the hybrid offspring F1 from D. chinensis and ‘HY’). Further, based on sequence divergence analysis and hypervariable region analysis, we selected several regions that had more divergent sequences, to develop DNA markers. Additionally, we found that one DNA marker can be used to differentiate Clade A and Clade B in Dianthus. Taken together, our results provide useful information for our understanding of Dianthus classification and chloroplast genome evolution.

Comparative Genomics and Phylogenetic Analysis of the Chloroplast Genomes in Three Medicinal Salvia Species for Bioexploration

To systematically determine their phylogenetic relationships and develop molecular markers for species discrimination of Salvia bowleyana, S. splendens, and S. officinalis, we sequenced their chloroplast genomes using the Illumina Hiseq 2500 platform. The chloroplast genomes length of S. bowleyana, S. splendens, and S. officinalis were 151,387 bp, 150,604 bp, and 151,163 bp, respectively. The six genes ndhB, rpl2, rpl23, rps7, rps12, and ycf2 were present in the IR regions. The chloroplast genomes of S. bowleyana, S. splendens, and S. officinalis contain 29 tandem repeats; 35, 29, 24 simple-sequence repeats, and 47, 49, 40 interspersed repeats, respectively. The three specific intergenic sequences (IGS) of rps16-trnQ-UUG, trnL-UAA-trnF-GAA, and trnM-CAU-atpE were found to discriminate the 23 Salvia species. A total of 91 intergenic spacer sequences were identified through genetic distance analysis. The two specific IGS regions (trnG-GCC-trnM-CAU and ycf3-trnS-GGA) have the highest K2p value identified in the three studied Salvia species. Furthermore, the phylogenetic tree showed that the 23 Salvia species formed a monophyletic group. Two pairs of genus-specific DNA barcode primers were found. The results will provide a solid foundation to understand the phylogenetic classification of the three Salvia species. Moreover, the specific intergenic regions can provide the probability to discriminate the Salvia species between the phenotype and the distinction of gene fragments.