The complete chloroplast genome sequences for four Amaranthus species (Amaranthaceae)

Comprehensive Chloroplast Genomic Insights into Amaranthus: Resolving the Phylogenetic and Taxonomic Status of A. powellii and A. bouchonii

Amaranthus, a genus in Amaranthaceae, is divided into three subgenera-Amaranthus, Acnida, and Albersia-and contains approximately 70 to 80 species. Understanding its phylogenetic relationships is essential for species classification, genetic diversity assessment, and evolutionary studies. This knowledge is vital for improving Amaranthus utilization in crop improvement and managing the ecological impacts of invasive weeds. In this study, we analyzed the chloroplast genomes of 27 Amaranthus species across all three subgenera to characterize their genomic features and construct a comprehensive phylogenetic tree. Our aim was to elucidate the phylogenetic relationships within the genus and evaluate interspecific affinities among the subgenera. We also addressed the taxonomic ambiguity surrounding A. bouchonii and A. powellii to determine their distinct species within the genus. Chloroplast genome sizes ranged from 149,949 to 150,818 bp, with GC content varying between 36.52% and 36.63%. Comparative structural analyses confirmed highly conserved quadripartite structures, gene content, and organization, comprising 87 protein-coding genes, 37 tRNAs, and 8 rRNAs. Repeat and codon usage analyses revealed conserved repeat patterns and a preference for codons ending in A or U. Selection pressure analysis indicated a predominantly purifying selection, with matK showing signs of positive selection, particularly in A. spinosus. Phylogenetic analysis of 80 protein-coding genes confirmed the monophyly of subgenus Amaranthus but found Alberisa and Acnida to be paraphyletic. Despite their morphological similarity, A. bouchonii and A. powellii were placed in separate clades within subgenus Amaranthus, with A. bouchonii clustering with A. retroflexus, and A. powellii aligning with the A. hybridus complex. Additionally, we identified 16 variable regions as potential molecular markers for species identification. Our study provides the most comprehensive Amaranthus chloroplast genome dataset to date, offering new insights into its evolutionary relationships and valuable genomic resources for taxonomy, germplasm management, and invasive risk assessment.

Analysis of codon usage patterns in complete plastomes of four medicinal Polygonatum species (Asparagaceae)

Polygonati Rhizoma and Polygonati odorati Rhizoma, known as "Huangjing" and "Yuzhu" in China, are medicinal Polygonatum species resources with top-grade medical and edible properties. The chloroplast (cp) genome has been used to study species diversity, evolution, and breeding of species for applications in genetic engineering. Codon usage bias (CUB), a common and complex natural phenomenon, is essential for studies of codon optimization of exogenous genes, genetic engineering, and molecular evolution. However, the CUB of medicinal Polygonatum species chloroplast genomes has not been systematically studied. In our study, a detailed analysis of CUB was performed in the medicinal Polygonatum species chloroplast genomes. We investigated the codon bias of 204 plastid protein-coding genes (PCGs) in 4 medicinal Polygonatum species using CodonW and CUSP online software. Through the analysis of the codon bias index, we found that the medicinal Polygonatum species chloroplast genomes had weak codon usage bias. In addition, our results also showed a high preference for AT bases in medicinal Polygonatum species chloroplast genomes, and the preference to use AT-ending codons was observed in these species chloroplast genomes. The neutrality plot, ENC plot, PR2-Bias plot, and correspondence analysis showed that compared with mutation pressure, natural selection was the most important factor of CUB. Based on the comparative analysis of high-frequency codons and high expression codons, we also determined the 10-11 optimal codons of investigative medicinal Polygonatum species. Furthermore, the result of RSCU-based cluster analysis showed that the genetic relationship between different medicinal Polygonatum species could be well reflected. This study provided an essential understanding of CUB and evolution in the medicinal Polygonatum species chloroplast genomes.

Isoform-resolved genome annotation enables mapping of tissue-specific betalain regulation in amaranth

Betalains are coloring pigments produced in some families of the order Caryophyllales, where they replace anthocyanins as coloring pigments. While the betalain pathway itself is well studied, the tissue-specific regulation of the pathway remains mostly unknown. We enhance the high-quality Amaranthus hypochondriacus reference genome and produce a substantially more complete genome annotation, incorporating isoform details. We annotate betalain and anthocyanin pathway genes along with their regulators in amaranth and map the genetic control and tissue-specific regulation of the betalain pathway. Our improved genome annotation allowed us to identify causal mutations that lead to a knock-out of red betacyanins in natural accessions of amaranth. We reveal the tissue-specific regulation of flower color via a previously uncharacterized MYB transcription factor, AhMYB2. Downregulation of AhMYB2 in the flower leads to reduced expression of key betalain enzyme genes and loss of red flower color. Our improved amaranth reference genome represents the most complete genome of amaranth to date and is a valuable resource for betalain and amaranth research. High similarity of the flower betalain regulator AhMYB2 to anthocyanin regulators and a partially conserved interaction motif support the co-option of anthocyanin regulators for the betalain pathway as a possible reason for the mutual exclusiveness of the two pigments.

The complete plastome of Amaranthus roxburghianus (Amaranthaceae)

Amaranthus roxburghianus H.W. Kung 1935, belonging to the Amaranthaceae family, is recognized for its significant medicinal properties. However, molecular research on this species has been limited. This study represents the inaugural documentation of the sequencing and assembly of the complete plastome of A. roxburghianus. The genome spans a total length of 149,969 base pairs (bp), exhibiting a conventional quadripartite structure. This structure comprises a large single-copy (LSC) region of 83,917 bp, a small single-copy (SSC) region of 18,124 bp, and two inverted repeat (IR) regions, each extending to 23,964 bp. In its entirety, the A. roxburghianus plastome encompasses 128 genes, of which 107 are unique, encompassing 77 individual protein-coding genes, 26 unique tRNA genes, and four unique rRNA genes. Phylogenetic analysis has shown a close resemblance between A. roxburghianus and A. polygonoides, both part of the subgenus Albersia. Although the genus Amaranthus is roughly divided into three subgenera, additional plastid genomic data are required for a more accurate assignment of A. albus and A. blitoides. The sequencing of this plastome is a significant step forward, likely to expedite the development of molecular markers and significantly contribute to genetic assays involving this distinctive species.

Genetic resources and breeding approaches for improvement of amaranth (Amaranthus spp.) and quinoa (Chenopodium quinoa)

Nowadays, the human population is more concerned about their diet and very specific in choosing their food sources to ensure a healthy lifestyle and avoid diseases. So people are shifting to more smart nutritious food choices other than regular cereals and staple foods they have been eating for a long time. Pseudocereals, especially, amaranth and quinoa, are important alternatives to traditional cereals due to comparatively higher nutrition, essential minerals, amino acids, and zero gluten. Both Amaranchaceae crops are low-input demanding and hardy plants tolerant to stress, drought, and salinity conditions. Thus, these crops may benefit developing countries that follow subsistence agriculture and have limited farming resources. However, these are underutilized orphan crops, and the efforts to improve them by reducing their saponin content remain ignored for a long time. Furthermore, these crops have very rich variability, but the progress of their genetic gain for getting high-yielding genotypes is slow. Realizing problems in traditional cereals and opting for crop diversification to tackle climate change, research should be focused on the genetic improvement for low saponin, nutritionally rich, tolerant to biotic and abiotic stresses, location-specific photoperiod, and high yielding varietal development of amaranth and quinoa to expand their commercial cultivation. The latest technologies that can accelerate the breeding to improve yield and quality in these crops are much behind and slower than the already established major crops of the world. We could learn from past mistakes and utilize the latest trends such as CRISPR/Cas, TILLING, and RNA interference (RNAi) technology to improve these pseudocereals genetically. Hence, the study reviewed important nutrition quality traits, morphological descriptors, their breeding behavior, available genetic resources, and breeding approaches for these crops to shed light on future breeding strategies to develop superior genotypes.

Comparative analysis of dioecious Amaranthus plastomes and phylogenomic implications within Amaranthaceae s.s

BACKGROUND

The genus Amaranthus L. consists of 70-80 species distributed across temperate and tropical regions of the world. Nine species are dioecious and native to North America; two of which are agronomically important weeds of row crops. The genus has been described as taxonomically challenging and relationships among species including the dioecious ones are poorly understood. In this study, we investigated the phylogenetic relationships among the dioecious amaranths and sought to gain insights into plastid tree incongruence. A total of 19 Amaranthus species' complete plastomes were analyzed. Among these, seven dioecious Amaranthus plastomes were newly sequenced and assembled, an additional two were assembled from previously published short reads sequences and 10 other plastomes were obtained from a public repository (GenBank).

RESULTS

Comparative analysis of the dioecious Amaranthus species' plastomes revealed sizes ranged from 150,011 to 150,735 bp and consisted of 112 unique genes (78 protein-coding genes, 30 transfer RNAs and 4 ribosomal RNAs). Maximum likelihood trees, Bayesian inference trees and splits graphs support the monophyly of subgenera Acnida (7 dioecious species) and Amaranthus; however, the relationship of A. australis and A. cannabinus to the other dioecious species in Acnida could not be established, as it appears a chloroplast capture occurred from the lineage leading to the Acnida + Amaranthus clades. Our results also revealed intraplastome conflict at some tree branches that were in some cases alleviated with the use of whole chloroplast genome alignment, indicating non-coding regions contribute valuable phylogenetic signals toward shallow relationship resolution. Furthermore, we report a very low evolutionary distance between A. palmeri and A. watsonii, indicating that these two species are more genetically related than previously reported.

CONCLUSIONS

Our study provides valuable plastome resources as well as a framework for further evolutionary analyses of the entire Amaranthus genus as more species are sequenced.

Genomic profiling of dioecious Amaranthus species provides novel insights into species relatedness and sex genes

BACKGROUND

Amaranthus L. is a diverse genus consisting of domesticated, weedy, and non-invasive species distributed around the world. Nine species are dioecious, of which Amaranthus palmeri S. Watson and Amaranthus tuberculatus (Moq.) J.D. Sauer are troublesome weeds of agronomic crops in the USA and elsewhere. Shallow relationships among the dioecious Amaranthus species and the conservation of candidate genes within previously identified A. palmeri and A. tuberculatus male-specific regions of the Y (MSYs) in other dioecious species are poorly understood. In this study, seven genomes of dioecious amaranths were obtained by paired-end short-read sequencing and combined with short reads of seventeen species in the family Amaranthaceae from NCBI database. The species were phylogenomically analyzed to understand their relatedness. Genome characteristics for the dioecious species were evaluated and coverage analysis was used to investigate the conservation of sequences within the MSY regions.

RESULTS

We provide genome size, heterozygosity, and ploidy level inference for seven newly sequenced dioecious Amaranthus species and two additional dioecious species from the NCBI database. We report a pattern of transposable element proliferation in the species, in which seven species had more Ty3 elements than copia elements while A. palmeri and A. watsonii had more copia elements than Ty3 elements, similar to the TE pattern in some monoecious amaranths. Using a Mash-based phylogenomic analysis, we accurately recovered taxonomic relationships among the dioecious Amaranthus species that were previously identified based on comparative morphology. Coverage analysis revealed eleven candidate gene models within the A. palmeri MSY region with male-enriched coverages, as well as regions on scaffold 19 with female-enriched coverage, based on A. watsonii read alignments. A previously reported FLOWERING LOCUS T (FT) within A. tuberculatus MSY contig was also found to exhibit male-enriched coverages for three species closely related to A. tuberculatus but not for A. watsonii reads. Additional characterization of the A. palmeri MSY region revealed that 78% of the region is made of repetitive elements, typical of a sex determination region with reduced recombination.

CONCLUSIONS

The results of this study further increase our understanding of the relationships among the dioecious species of the Amaranthus genus as well as revealed genes with potential roles in sex function in the species.

Genetic variation and structure of complete chloroplast genome in alien monoecious and dioecious Amaranthus weeds

Amaranthus is a complex taxon with economic importance as well as harmful weeds. We studied the genetic variation and structure of the chloroplast genomes of 22 samples from 17 species of three subgenera. It was found that the length of the chloroplast genome of Amaranthus varied from 149,949 bp of A. polygonoides to 150,757 bp of A. albus. The frequencies of SNPs and InDels in chloroplast genomes were 1.79% and 2.86%, and the variation mainly occurred in the non-coding regions. The longest InDel was 387 bp, which occurred on ycf2, followed by 384 bp InDel on psbM-trnD. Two InDels in ndhE-I on the SSC make the three subgenera clearly distinguished. In LSC, SSC and IRs regions, there were four 30 bp forward and reverse repeats, and the repeats in SSC and LSC were in nearly opposite positions in circular genome structure, and almost divided the circular genome into symmetrical structures. In the topological tree constructed by chloroplast genome, species in subgen. Amaranthus and subgen. Acnida form monophyletic branches separately and cluster together. A. albus, A. blitoides and A. polygonoides were separated from subgen. Albersia, and the rest of subgen. Albersia were clustered into a monophyletic branch. The rpoC2, ycf1, ndhF-rpl32 were good at distinguishing most amaranths. The trnk-UUU-atpF, trnT-UGU-atpB, psbE-clpP, rpl14-rps19, and ndhF-D can distinguish several similar species. In general, the chloroplast genome is of certain value for the identification of the similar species of Amaranthus, which provides more evidence for clarifying the phylogenetic relationships within the genus.

Comparative Analysis of Codon Bias in the Chloroplast Genomes of Theaceae Species

Theaceae species are dicotyledonous angiosperms with extremely high ornamental and economic value. The chloroplast genome is traditionally used to study species evolution, expression of chloroplast genes and chloroplast transformation. Codon usage bias (CUB) analysis is beneficial for investigations of evolutionary relationships and can be used to improve gene expression efficiency in genetic transformation research. However, there are relatively few systematic studies of the CUB in the chloroplast genomes of Theaceae species. In this study, CUB and nucleotide compositions parameters were determined by the scripts written in the Perl language, CodonW 1.4.2, CU.Win2000, RStudio and SPSS 23.0. The chloroplast genome data of 40 Theaceae species were obtained to analyse the codon usage (CU) characteristics of the coding regions and the influence of the source of variation on CUB. To explore the relationship between the CUB and gene expression levels in these 40 Theaceae plastomes, the synonymous codon usage order (SCUO) and measure independent of length and composition (MILC) values were determined. Finally, phylogenetic analysis revealed the genetic evolutionary relationships among these Theaceae species. Our results showed that based on the chloroplast genomes of these 40 Theaceae species, the CUB was for codons containing A/T bases and those that ended with A/T bases. Moreover, there was great commonality in the CUB of the Theaceae species according to comparative analysis of relative synonymous codon usage (RSCU) and relative frequency of synonymous codon (RFSC): these species had 29 identical codons with bias (RSCU > 1), and there were 19 identical high-frequency codons. The CUB of Theaceae species is mainly affected by natural selection. The SCUO value of the 40 Theaceae species was 0.23 or 0.24, and the chloroplast gene expression level was moderate, according to MILC values. Additionally, we observed a positive correlation between the SCUO and MILC values, which indicated that CUB might affect gene expression. Furthermore, the phylogenetic analysis showed that the evolutionary relationships in these 40 Theaceae species were relatively conserved. A systematic study on the CUB and expression of Theaceae species provides further evidence for their evolution and phylogeny.

Chloroplast genome analysis of Chrysotila dentata

Chrysotila dentata is an ecologically important marine alga contributing to the coccolith formation. In this study, a complete chloroplast (cp DNA) genome of Chrysotila dentata was sequenced by using Illumina Hiseq and was analyzed with the help of a bioinformatics tool CPGAVAS2. The circular chloroplast genome of Chrysotila dentata has a size of 109,017 bp with two inverted repeats (IRs) regions (4513 bp each) which is a common feature in most land plants and algal species. The Chrysotila dentata cp genome consists of 61 identified protein-coding genes, 30 tRNA genes and 6 rRNAs with 21 microsatellites. The phylogenetic relationship with other select algal species revealed a close phylogeny of Chrysotila dentata with Phaeocystis antarctica. This is the first report of the cp genome analysis of genus Chrysotila and the results from this study will be helpful for understanding the genetic structure and function of chloroplast in other species of Chrysotila.

Comparative and phylogenetic analyses of the chloroplast genomes of species of Paeoniaceae

Plants belonging to family Paeoniaceae are not only economically important ornamental plants but also medicinal plants used as an important source of traditional Chinese medicine. Owing to the complex network evolution and polyploidy evolution of this family, its systematics and taxonomy are controversial and require a detailed investigation. In this study, three complete chloroplast genomes of sect. Paeonia, one of the sections of Paeonia, were sequenced and then analysed together with 16 other published chloroplast genomes of Paeoniaceae species. The total lengths of the chloroplast genomes of these species were 152,153–154,405 bp. A total of 82–87 protein-coding genes, 31–40 tRNA genes and 8 rRNA genes were annotated. Bioinformatics analysis revealed 61–74 simple sequence repeats (SSRs) in the chloroplast genomes, most of which have A/T base preference. Codon usage analysis showed that A/U-ending codons were more positive than C/G-ending codons, and a slight bias in codon usage was observed in these species. A comparative analysis of these 19 species of Paeoniaceae was then conducted. Fourteen highly variable regions were selected for species relationship study. Phylogenetic analysis revealed that the species of sect. Paeonia gathered in one branch and then divided into different small branches. P. lactiflora, P. anomala, P. anomala subsp. veitchii and P. mairei clustered together. P. intermedia was related to P. obovata and P. obovata subsp. willmottiae. P. emodi was the sister to all other species in the sect. Paeonia.

Amaranths

Chance W. Riggins and Rita H. Mumm introduce the ancient amaranth genus, highlighting the ancient crop's controversial history and its contemporary use in improving food security.

Identification and phylogenetic analysis of five Crataegus species (Rosaceae) based on complete chloroplast genomes

The chloroplast genomes of the five Crataegus species were shown to have a conserved genome structure. Complete chloroplast genome sequences were more suitable than highly variable regions for the identification and phylogenetic analysis of Crataegus species. Hawthorn, which is commonly used as a traditional Chinese medicine, is one of the most popular sour fruits and has high economic value. Crataegus pinnatifida var. pinnatifida and C. pinnatifida var. major are frequently adulterated with other Crataegus species on the herbal medicine market. However, most Crataegus plants are difficult to identify using traditional morphological methods. Here, we compared five Crataegus chloroplast (CP) genomes comprising two newly sequenced (i.e., C. pinnatifida var. pinnatifida and C. pinnatifida var. major) and three previously published CP genomes. The CP genomes of the five Crataegus species had a conserved genome structure, gene content and codon usage. The total length of the CP genomes was 159,654-159,865 bp. A total of 129-130 genes, including 84-85 protein-coding genes, 37 tRNA genes and 8 rRNA genes, were annotated. Bioinformatics analysis revealed 96-103 simple sequence repeats (SSRs) and 48-70 long repeats in the five CP genomes. Combining the results of mVISTA and nucleotide diversity, five highly variable regions were screened for species identification and relationship studies. Maximum likelihood trees were constructed on the basis of complete CP genome sequences and highly variable regions. The results showed that the former had higher discriminatory power for Crataegus species, indicating that the complete CP genome could be used as a super-barcode to accurately authenticate the five Crataegus species.

Comparative chloroplast genomes and phylogenetic analysis of Aquilegia

PREMISE

Aquilegia is an ideal taxon for studying the evolution of adaptive radiation. Current phylogenies of Aquilegia based on different molecular markers are inconsistent, and therefore a clear and accurate phylogeny remains uncertain. Analyzing the chloroplast genome, with its simple structure and low recombination rate, may help solve this problem.

METHODS

Next-generation sequencing data were generated or downloaded for Aquilegia species, enabling their chloroplast genomes to be assembled. The assemblies were used to estimate the genome characteristics and infer the phylogeny of Aquilegia.

RESULTS

In this study, chloroplast genome sequences were assembled for Aquilegia species distributed across Asia, North America, and Europe. Three of the genes analyzed (petG, rpl36, and atpB) were shown to be under positive selection and may be related to adaptation. The phylogenetic tree of Aquilegia showed that its member species formed two clades with high support, North American and European species, with the Asian species being paraphyletic; A. parviflora and A. amurensis clustered with the North American species, while the remaining Asian species were found in the European clade. In addition, A. oxysepala var. kansuensis should be considered as a separate species rather than a variety.

DISCUSSION

The complete chloroplast genomes of these Aquilegia species provide new insights into the reconstruction of the phylogeny of related species and contribute to the further study of this genus.

Methods of analysis of chloroplast genomes of C3, Kranz type C4 and Single Cell C4 photosynthetic members of Chenopodiaceae

BACKGROUND

Chloroplast genome information is critical to understanding forms of photosynthesis in the plant kingdom. During the evolutionary process, plants have developed different photosynthetic strategies that are accompanied by complementary biochemical and anatomical features. Members of family Chenopodiaceae have species with C3 photosynthesis, and variations of C4 photosynthesis in which photorespiration is reduced by concentrating CO2 around Rubisco through dual coordinated functioning of dimorphic chloroplasts. Among dicots, the family has the largest number of C4 species, and greatest structural and biochemical diversity in forms of C4 including the canonical dual-cell Kranz anatomy, and the recently identified single cell C4 with the presence of dimorphic chloroplasts separated by a vacuole. This is the first comparative analysis of chloroplast genomes in species representative of photosynthetic types in the family.

RESULTS

Methodology with high throughput sequencing complemented with Sanger sequencing of selected loci provided high quality and complete chloroplast genomes of seven species in the family and one species in the closely related Amaranthaceae family, representing C3, Kranz type C4 and single cell C4 (SSC4) photosynthesis six of the eight chloroplast genomes are new, while two are improved versions of previously published genomes. The depth of coverage obtained using high-throughput sequencing complemented with targeted resequencing of certain loci enabled superior resolution of the border junctions, directionality and repeat region sequences. Comparison of the chloroplast genomes with previously sequenced plastid genomes revealed similar genome organization, gene order and content with a few revisions. High-quality complete chloroplast genome sequences resulted in correcting the orientation the LSC region of the published Bienertia sinuspersici chloroplast genome, identification of stop codons in the rpl23 gene in B. sinuspersici and B. cycloptera, and identifying an instance of IR expansion in the Haloxylon ammodendron inverted repeat sequence. The rare observation of a mitochondria-to-chloroplast inter-organellar gene transfer event was identified in family Chenopodiaceae.

CONCLUSIONS

This study reports complete chloroplast genomes from seven Chenopodiaceae and one Amaranthaceae species. The depth of coverage obtained using high-throughput sequencing complemented with targeted resequencing of certain loci enabled superior resolution of the border junctions, directionality, and repeat region sequences. Therefore, the use of high throughput and Sanger sequencing, in a hybrid method, reaffirms to be rapid, efficient, and reliable for chloroplast genome sequencing.

Adaptation Evolution and Phylogenetic Analyses of Species in Chinese Allium Section Pallasia and Related Species Based on Complete Chloroplast Genome Sequences

The section Pallasia is one of the components of the genus Allium subgenus Allium (Amaryllidaceae), and species relationship in this section is still not resolved very well, which hinders further evolutionary and adaptive studies. Here, the complete chloroplast genomes of five sect. Pallasia species were reported, and a comparative analysis was performed with other three related Allium species. The genome size of the eight species ranged from 151,672 bp to 153,339 bp in length, GC content changed from 36.7% to 36.8%, and 130 genes (except Allium pallasii), 37 tRNA, and 8 rRNA were identified in each genome. By analyzing the IR/LSC and IR/SSC boundary, A. pallasii exhibited differences compared with other seven species. Phylogenetic analysis achieved high supports in each branch, seven of the eight Allium species cluster into a group, and A. pallasii exhibit a close relationship with A. obliquum. Higher pairwise Ka/Ks ratios were found in A. schoenoprasoides compared to A. caeruleum and A. macrostemon while a lower value of Ka/Ks ratios was detected between A. caeruleum and A. macrostemon. This study will be a great contribution to the future phylogenetic and adaptive research in Allium.

Can we use it? On the utility of de novo and reference-based assembly of Nanopore data for plant plastome sequencing

The chloroplast genome harbors plenty of valuable information for phylogenetic research. Illumina short-read data is generally used for de novo assembly of whole plastomes. PacBio or Oxford Nanopore long reads are additionally employed in hybrid approaches to enable assembly across the highly similar inverted repeats of a chloroplast genome. Unlike for PacBio, plastome assemblies based solely on Nanopore reads are rarely found, due to their high error rate and non-random error profile. However, the actual quality decline connected to their use has rarely been quantified. Furthermore, no study has employed reference-based assembly using Nanopore reads, which is common with Illumina data. Using Leucanthemum Mill. as an example, we compared the sequence quality of seven chloroplast genome assemblies of the same species, using combinations of two sequencing platforms and three analysis pipelines. In addition, we assessed the factors which might influence Nanopore assembly quality during sequence generation and bioinformatic processing. The consensus sequence derived from de novo assembly of Nanopore data had a sequence identity of 99.59% compared to Illumina short-read de novo assembly. Most of the errors detected were indels (81.5%), and a large majority of them is part of homopolymer regions. The quality of reference-based assembly is heavily dependent upon the choice of a close-enough reference. When using a reference with 0.83% sequence divergence from the studied species, mapping of Nanopore reads results in a consensus comparable to that from Nanopore de novo assembly, and of only slightly inferior quality compared to a reference-based assembly with Illumina data. For optimal de novo assembly of Nanopore data, appropriate filtering of contaminants and chimeric sequences, as well as employing moderate read coverage, is essential. Based on these results, we conclude that Nanopore long reads are a suitable alternative to Illumina short reads in plastome phylogenomics. Few errors remain in the finalized assembly, which can be easily masked in phylogenetic analyses without loss in analytical accuracy. The easily applicable and cost-effective technology might warrant more attention by researchers dealing with plant chloroplast genomes.

Comparative Analysis of the Complete Chloroplast Genomes in Allium Subgenus Cyathophora (Amaryllidaceae): Phylogenetic Relationship and Adaptive Evolution

Recent advances in molecular phylogenetics provide us with information of Allium L. taxonomy and evolution, such as the subgenus Cyathophora, which is monophyletic and contains five species. However, previous studies detected distinct incongruence between the nrDNA and cpDNA phylogenies, and the interspecies relationships of this subgenus need to be furtherly resolved. In our study, we newly assembled the whole chloroplast genome of four species in subgenus Cyathophora and two allied Allium species. The complete cp genomes were found to possess a quadripartite structure, and the genome size ranged from 152,913 to 154,174 bp. Among these cp genomes, there were subtle differences in the gene order, gene content, and GC content. Seven hotspot regions (infA, rps16, rps15, ndhF, trnG-UCC, trnC-GCA, and trnK-UUU) with nucleotide diversity greater than 0.02 were discovered. The selection analysis showed that some genes have elevated Ka/Ks ratios. Phylogenetic analysis depended on the complete chloroplast genome (CCG), and the intergenic spacer regions (IGS) and coding DNA sequences (CDS) showed same topologies with high support, which revealed that subgenus Cyathophora was a monophyletic group, containing four species, and A. cyathophorum var. farreri was sister to A. spicatum with 100% bootstrap value. Our study revealed selective pressure may exert effect on several genes of the six Allium species, which may be useful for them to adapt to their specific living environment. We have well resolved the phylogenetic relationship of species in the subgenus Cyathophora, which will contribute to future evolutionary studies or phylogeographic analysis of Allium.

Complete chloroplast genome sequences of four Allium species: comparative and phylogenetic analyses

The genus Allium is one of the largest monocotyledonous genera, containing over 850 species, and most of these species are found in temperate climates of the Northern Hemisphere. Furthermore, as a large number of new Allium species continue to be identified, phylogenetic classification based on morphological characteristics and a few genetic markers will gradually exhibit extremely low discriminatory power. In this study, we present the use of complete chloroplast genome sequences in genome-scale phylogenetic studies of Allium. We sequenced and assembled four Allium chloroplast genomes and retrieved five published chloroplast genomes from GenBank. All nine chloroplast genomes were used for genomic comparison and phylogenetic inference. The chloroplast genomes, ranging from 152,387 bp to 154,482 bp in length, exhibited conservation of genomic structure, and gene organization and order. Subsequently, we observed the expansion of IRs from the basal monocot Acorus americanus to Allium, identified 814 simple sequence repeats, 131 tandem repeats, 154 dispersed repeats and 109 palindromic repeats, and found six highly variable regions. The phylogenetic relationships of the Allium species inferred from the chloroplast genomes obtained high support, indicating that chloroplast genome data will be useful for further resolution of the phylogeny of the genus Allium.

Mitochondrial and chloroplast genomes provide insights into the evolutionary origins of quinoa (Chenopodium quinoa Willd.)

Quinoa has recently gained international attention because of its nutritious seeds, prompting the expansion of its cultivation into new areas in which it was not originally selected as a crop. Improving quinoa production in these areas will benefit from the introduction of advantageous traits from free-living relatives that are native to these, or similar, environments. As part of an ongoing effort to characterize the primary and secondary germplasm pools for quinoa, we report the complete mitochondrial and chloroplast genome sequences of quinoa accession PI 614886 and the identification of sequence variants in additional accessions from quinoa and related species. This is the first reported mitochondrial genome assembly in the genus Chenopodium. Inference of phylogenetic relationships among Chenopodium species based on mitochondrial and chloroplast variants supports the hypotheses that 1) the A-genome ancestor was the cytoplasmic donor in the original tetraploidization event, and 2) highland and coastal quinoas were independently domesticated.

Assembly of chloroplast genomes with long- and short-read data: a comparison of approaches using Eucalyptus pauciflora as a test case

BACKGROUND

Chloroplasts are organelles that conduct photosynthesis in plant and algal cells. The information chloroplast genome contained is widely used in agriculture and studies of evolution and ecology. Correctly assembling chloroplast genomes can be challenging because the chloroplast genome contains a pair of long inverted repeats (10-30 kb). Typically, it is simply assumed that the gross structure of the chloroplast genome matches the most commonly observed structure of two single-copy regions separated by a pair of inverted repeats. The advent of long-read sequencing technologies should remove the need to make this assumption by providing sufficient information to completely span the inverted repeat regions. Yet, long-reads tend to have higher error rates than short-reads, and relatively little is known about the best way to combine long- and short-reads to obtain the most accurate chloroplast genome assemblies. Using Eucalyptus pauciflora, the snow gum, as a test case, we evaluated the effect of multiple parameters, such as different coverage of long-(Oxford nanopore) and short-(Illumina) reads, different long-read lengths, different assembly pipelines, with a view to determining the most accurate and efficient approach to chloroplast genome assembly.

RESULTS

Hybrid assemblies combining at least 20x coverage of both long-reads and short-reads generated a single contig spanning the entire chloroplast genome with few or no detectable errors. Short-read-only assemblies generated three contigs (the long single copy, short single copy and inverted repeat regions) of the chloroplast genome. These contigs contained few single-base errors but tended to exclude several bases at the beginning or end of each contig. Long-read-only assemblies tended to create multiple contigs with a much higher single-base error rate. The chloroplast genome of Eucalyptus pauciflora is 159,942 bp, contains 131 genes of known function.

CONCLUSIONS

Our results suggest that very accurate assemblies of chloroplast genomes can be achieved using a combination of at least 20x coverage of long- and short-reads respectively, provided that the long-reads contain at least ~5x coverage of reads longer than the inverted repeat region. We show that further increases in coverage give little or no improvement in accuracy, and that hybrid assemblies are more accurate than long-read-only or short-read-only assemblies.

Complete Chloroplast Genome Sequence of Malus hupehensis: Genome Structure, Comparative Analysis, and Phylogenetic Relationships

Malus hupehensis belongs to the Malus genus (Rosaceae) and is an indigenous wild crabapple of China. This species has received more and more attention, due to its important medicinal, and excellent ornamental and economical, values. In this study, the whole chloroplast (cp) genome of Malus hupehensis, using a Hiseq X Ten sequencing platform, is reported. The M. hupehensis cp genome is 160,065 bp in size, containing a large single copy region (LSC) of 88,166 bp and a small single copy region (SSC) of 19,193 bp, separated by a pair of inverted repeats (IRs) of 26,353 bp. It contains 112 genes, including 78 protein-coding genes (PCGs), 30 transfer RNA genes (tRNAs), and four ribosomal RNA genes (rRNAs). The overall nucleotide composition is 36.6% CG. A total of 96 simple sequence repeats (SSRs) were identified, most of them were found to be mononucleotide repeats composed of A/T. In addition, a total of 49 long repeats were identified, including 24 forward repeats, 21 palindromic repeats, and four reverse repeats. Comparisons of the IR boundaries of nine Malus complete chloroplast genomes presented slight variations at IR/SC boundaries regions. A phylogenetic analysis, based on 26 chloroplast genomes using the maximum likelihood (ML) method, indicates that M. hupehensis clustered closer ties with M. baccata, M. micromalus, and M. prunifolia than with M. tschonoskii. The availability of the complete chloroplast genome using genomics methods is reported here and provides reliable genetic information for future exploration on the taxonomy and phylogenetic evolution of the Malus and related species.

The complete plastid genomes of Ophrys iricolor and O. sphegodes (Orchidaceae) and comparative analyses with other orchids

Sexually deceptive orchids of the genus Ophrys may rapidly evolve by adaptation to pollinators. However, understanding of the genetic basis of potential changes and patterns of relationships is hampered by a lack of genomic information. We report the complete plastid genome sequences of Ophrys iricolor and O. sphegodes, representing the two most species-rich lineages of the genus Ophrys. Both plastomes are circular DNA molecules (146754 bp for O. sphegodes and 150177 bp for O. iricolor) with the typical quadripartite structure of plastid genomes and within the average size of photosynthetic orchids. 213 Simple Sequence Repeats (SSRs) (31.5% polymorphic between O. iricolor and O. sphegodes) were identified, with homopolymers and dipolymers as the most common repeat types. SSRs were mainly located in intergenic regions but SSRs located in coding regions were also found, mainly in ycf1 and rpoC2 genes. The Ophrys plastome is predicted to encode 107 distinct genes, 17 of which are completely duplicated in the Inverted Repeat regions. 83 and 87 putative RNA editing sites were detected in 25 plastid genes of the two Ophrys species, all occurring in the first or second codon position. Comparing the rate of nonsynonymous (dN) and synonymous (dS) substitutions, 24 genes (including rbcL and ycf1) display signature consistent with positive selection. When compared with other members of the orchid family, the Ophrys plastome has a complete set of 11 functional ndh plastid genes, with the exception of O. sphegodes that has a truncated ndhF gene. Comparative analysis showed a large co-linearity with other related Orchidinae. However, in contrast to O. iricolor and other Orchidinae, O. sphegodes has a shift of the junction between the Inverted Repeat and Small Single Copy regions associated with the loss of the partial duplicated gene ycf1 and the truncation of the ndhF gene. Data on relative genomic coverage and validation by PCR indicate the presence, with a different ratio, of the two plastome types (i.e. with and without ndhF deletion) in both Ophrys species, with a predominance of the deleted type in O. sphegodes. A search for this deleted plastid region in O. sphegodes nuclear genome shows that the deleted region is inserted in a retrotransposon nuclear sequence. The present study provides useful genomic tools for studying conservation and patterns of relationships of this rapidly radiating orchid genus.

From zero to hero: the past, present and future of grain amaranth breeding

Grain amaranth is an underutilized crop with high nutritional quality from the Americas. Emerging genomic and biotechnological tools are becoming available that allow the integration of novel breeding techniques for rapid improvement of amaranth and other underutilized crops. Out of thousands of edible plants, only three cereals-maize, wheat and rice-are the major food sources for a majority of people worldwide. While these crops provide high amounts of calories, they are low in protein and other essential nutrients. The dependence on only few crops, with often narrow genetic basis, leads to a high vulnerability of modern cropping systems to the predicted climate change and accompanying weather extremes. Broadening our food sources through the integration of so-called orphan crops can help to mitigate the effects of environmental change and improve qualitative food security. Thousands of traditional crops are known, but have received little attention in the last century and breeding efforts were limited. Amaranth is such an underutilized pseudocereal that is of particular interest because of its balanced amino acid and micronutrient profiles. Additionally, the C₄ photosynthetic pathway and ability to withstand environmental stress make the crop a suitable choice for future agricultural systems. Despite the potential of amaranth, efforts of genetic improvement lag considerably behind those of major crops. The progress in novel breeding methods and molecular techniques developed in model plants and major crops allow a rapid improvement of underutilized crops. Here, we review the history of amaranth and recent advances in genomic tools and give a concrete perspective how novel breeding techniques can be implemented into breeding programs. Our perspectives are transferable to many underutilized crops. The implementation of these could improve the nutritional quality and climate resilience of future cropping systems.

The Chloroplast Genome of Lilium henrici: Genome Structure and Comparative Analysis

Lilium henrici Franchet, which belongs to the family Liliaceae, is an endangered plant native to China. The wild populations of L. henrici have been largely reduced by habitat degradation or loss. In our study, we determined the whole chloroplast genome sequence for L. henrici and compared its structure with other Lilium (including Nomocharis) species. The chloroplast genome of L. henrici is a circular structure and 152,784 bp in length. The large single copy and small single copy is 82,429 bp and 17,533 bp in size, respectively, and the inverted repeats are 26,411 bp in size. The L. henrici chloroplast genome contains 116 different genes, including 78 protein coding genes, 30 tRNA genes, 4 rRNA genes, and 4 pseudogenes. There were 51 SSRs detected in the L. henrici chloroplast genome sequence. Genic comparison among L. henrici with other Lilium (including Nomocharis) chloroplast genomes shows that the sequence lengths and gene contents show little variation, the only differences being in three pseudogenes. Phylogenetic analysis revealed that N. pardanthina was a sister species to L. henrici. Overall, this study, providing L. henrici genomic resources and the comparative analysis of Lilium chloroplast genomes, will be beneficial for the evolutionary study and phylogenetic reconstruction of the genus Lilium, molecular barcoding in population genetics.

The Complete Chloroplast Genome of Heimia myrtifolia and Comparative Analysis within Myrtales

Heimia myrtifolia is an important medicinal plant with several pharmacologically active alkaloids and is also used as an ornamental landscape plant. The purpose of this study is to complete and characterize the chloroplast (cp) genome of H. myrtifolia and compare genomic features to other Myrtales species' cp genomes. The analysis showed that H. myrtifolia has a total length of 159,219 bp with a typical quadripartite structure containing two identical inverted repeats (IRs) of 25,643 bp isolated by one large single copy (LSC) of 88,571 bp and one small single copy (SSC) of 18,822 bp. The H. myrtifolia cp genome contains 129 genes with eight ribosomal RNAs, 30 transfer RNAs, and 78 protein coding genes, in which 17 genes are duplicated in two IR regions. The genome organization including gene type and number and guanine-cytosine (GC) content is analyzed among the 12 cp genomes in this study. Approximately 255 simple sequence repeats (SSRs) and 16 forward, two reverses, and two palindromic repeats were identified in the H. myrtifolia cp genome. By comparing the whole H. myrtifolia cp genome with 11 other Myrtales species, the results showed that the sequence similarity was high between coding regions while sequence divergence was high between intergenic regions. By employing the full cp genomes for phylogenetic analysis, structural and sequence differences were characterized between H. myrtifolia and 11 Myrtales species illustrating what patterns are common in the evolution of cp genomes within the Myrtales. The first entire cp genome in the genus Heimia provides a valuable resource for further studies in these medicinally and ornamentally important taxa.

Application of Chloroplast Phylogenomics to Resolve Species Relationships Within the Plant Genus Amaranthus

Amaranthus species are an emerging and promising nutritious traditional vegetable food source. Morphological plasticity and poorly resolved dendrograms have led to the need for well resolved species phylogenies. We hypothesized that whole chloroplast phylogenomics would result in more reliable differentiation between closely related amaranth species. The aims of the study were therefore: to construct a fully assembled, annotated chloroplast genome sequence of Amaranthus tricolor; to characterize Amaranthus accessions phylogenetically by comparing barcoding genes (matK, rbcL, ITS) with whole chloroplast sequencing; and to use whole chloroplast phylogenomics to resolve deeper phylogenetic relationships. We generated a complete A. tricolor chloroplast sequence of 150,027 bp. The three barcoding genes revealed poor inter- and intra-species resolution with low bootstrap support. Whole chloroplast phylogenomics of 59 Amaranthus accessions increased the number of parsimoniously informative sites from 92 to 481 compared to the barcoding genes, allowing improved separation of amaranth species. Our results support previous findings that two geographically independent domestication events of Amaranthus hybridus likely gave rise to several species within the Hybridus complex, namely Amaranthus dubius, Amaranthus quitensis, Amaranthus caudatus, Amaranthus cruentus and Amaranthus hypochondriacus. Poor resolution of species within the Hybridus complex supports the recent and ongoing domestication within the complex, and highlights the limitation of chloroplast data for resolving recent evolution. The weedy Amaranthus retroflexus and Amaranthus powellii was found to share a common ancestor with the Hybridus complex. Leafy amaranth, Amaranthus tricolor, Amaranthus blitum, Amaranthus viridis and Amaranthus graecizans formed a stable sister lineage to the aforementioned species across the phylogenetic trees. This study demonstrates the power of next-generation sequencing data and reference-based assemblies to resolve phylogenies, and also facilitated the identification of unknown Amaranthus accessions from a local genebank. The informative phylogeny of the Amaranthus genus will aid in selecting accessions for breeding advanced genotypes to satisfy global food demand.

Complete Chloroplast Genome Sequences and Comparative Analysis of Chenopodium quinoa and C. album

The Chenopodium genus comprises ~150 species, including Chenopodium quinoa and Chenopodium album, two important crops with high nutritional value. To elucidate the phylogenetic relationship between the two species, the complete chloroplast (cp) genomes of these species were obtained by next generation sequencing. We performed comparative analysis of the sequences and, using InDel markers, inferred phylogeny and genetic diversity of the Chenopodium genus. The cp genome is 152,099 bp (C. quinoa) and 152,167 bp (C. album) long. In total, 119 genes (78 protein-coding, 37 tRNA, and 4 rRNA) were identified. We found 14 (C. quinoa) and 15 (C. album) tandem repeats (TRs); 14 TRs were present in both species and C. album and C. quinoa each had one species-specific TR. The trnI-GAU intron sequences contained one (C. quinoa) or two (C. album) copies of TRs (66 bp); the InDel marker was designed based on the copy number variation in TRs. Using the InDel markers, we detected this variation in the TR copy number in four species, Chenopodium hybridum, Chenopodium pumilio, Chenopodium ficifolium, and Chenopodium koraiense, but not in Chenopodium glaucum. A comparison of coding and non-coding regions between C. quinoa and C. album revealed divergent sites. Nucleotide diversity >0.025 was found in 17 regions-14 were located in the large single copy region (LSC), one in the inverted repeats, and two in the small single copy region (SSC). A phylogenetic analysis based on 59 protein-coding genes from 25 taxa resolved Chenopodioideae monophyletic and sister to Betoideae. The complete plastid genome sequences and molecular markers based on divergence hotspot regions in the two Chenopodium taxa will help to resolve the phylogenetic relationships of Chenopodium.