Molecular evolution of chloroplast genomes in subfamily Zingiberoideae (Zingiberaceae)

Comparative chloroplast genomics, phylogenetic relationships and molecular markers development of Aglaonema commutatum and seven green cultivars of Aglaonema

Aglaonema commutatum is a famous species in the Aglaonema genus, which has important ornamental and economic value. However, its chloroplast genome information and phylogenetic relationships among popular green cultivars of Aglaonema in southern China have not been reported. Herein, chloroplast genomes of one variety of A. commutatum and seven green cultivars of Aglaonema, namely, A. commutatum 'San Remo', 'Kai Sa', 'Pattaya Beauty', 'Sapphire', 'Silver Queen', 'Snow White', 'White Gem', and 'White Horse Prince', were sequenced and assembled for comparative analysis and phylogeny. These eight genomes possessed a typical quadripartite structure that consisted of a LSC region (90,799-91,486 bp), an SSC region (20,508-21,137 bp) and a pair of IR regions (26,661-26,750 bp). Each genome contained 112 different genes, comprising 79 protein-coding genes, 29 tRNA genes and 4 rRNA genes. The gene orders, GC contents, codon usage frequency, and IR/SC boundaries were highly conserved among these eight genomes. Long repeats, SSRs, SNPs and indels were analyzed among these eight genomes. Comparative analysis of 15 Aglaonema chloroplast genomes identified 7 highly variable regions, including trnH-GUG-exon1-psbA, trnS-GCU-trnG-UCC-exon1, trnY-GUA-trnE-UUC, psbC-trnS-UGA, trnF-GAA-ndhJ, ccsA-ndhD, and rps15-ycf1-D2. Reconstruction of the phylogenetic trees based on chloroplast genomes, strongly supported that Aglaonema was a sister to Anchomanes, and that the Aglaonema genus was classified into two sister clades including clade I and clade II, which corresponded to two sections, Aglaonema and Chamaecaulon, respectively. One variety and five cultivars, including A. commutatum 'San Remo', 'Kai Sa', 'Pattaya Beauty', 'Silver Queen', 'Snow White', and 'White Horse Prince', were classified into clade I; and the rest of the two cultivars, including 'Sapphire' and 'White Gem', were classified into clade II. Positive selection was observed in 34 protein-coding genes at the level of the amino acid sites among 77 chloroplast genomes of the Araceae family. Based on the highly variable regions and SSRs, 4 DNA markers were developed to differentiate the clade I and clade II in Aglaonema. In conclusion, this study provided chloroplast genomic resources for Aglaonema, which were useful for its classification and phylogeny.

Plastid genomes provide insights into the phylogeny and chloroplast evolution of the paper daisy tribe Gnaphalieae (Asteraceae)

Chloroplast genomes, as an essential source of phylogenetic information, are increasingly utilized in the evolutionary study of angiosperms. Gnaphalieae is a medium-sized tribe of the sunflower family of Asteraceae, with about 2,100 species in 178 genera distributed in temperate habitats worldwide. There has been considerable progress in our understanding of their phylogenetic evolution using both nuclear and chloroplast sequences, but no focus on chloroplast genomic data. In this study, we performed sequencing, assembly, and annotation of 16 representative chloroplast genomes from all the major lineages of Gnaphalieae. Our results showed that the plastomes exhibited a typical circular tetrad structure with similar genomic structure gene content. But there were differences in genome size, SSRs, and codon usage within the tribe. Phylogenetic analysis revealed Relhania clade is the earliest diverged lineages with the Lasiopogon clade and the Gnaphalium s.s. clade diverged subsequently. The core group includes FLAG clade sister to the HAP and Australasian group. Compared with the outgroup species, chloroplast genome size of the FLAG clade is much reduced whereas those of Australasian, HAP, Gnaphalium s.s., Lasiopogon and Relhania clades are relatively expanded. Insertions and deletions in the intergenic regions associated with repetitive sequence variations are supposed to be the main factor leading to length variations in the chloroplast genomes of Gnaphalieae. The comparative analyses of chloroplast genomes would provide useful implications into understanding the taxonomic and evolutionary history of Gnaphalieae.

Characterization of the chloroplast genome of Gleditsia species and comparative analysis

The genus Gleditsia has significant medicinal and economic value, but information about the chloroplast genomic characteristics of Gleditsia species has been limited. Using the Illumina sequencing, we assembled and annotated the whole chloroplast genomes of seven Gleditsia species (Gleditsia sinensis, Gleditsia japonica var. delavayi (G. delavayi), G. fera, G. japonica, G. microphylla, Fructus Gleditsiae Abnormalis (Zhū Yá Zào), G. microphylla mutant). The assembled genomes revealed that Gleditsia species have a typical circular tetrad structure, with genome sizes ranging from 162,746 to 170,907 bp. Comparative genomic analysis showed that most (65.8-75.8%) of the abundant simple sequence repeats in Gleditsia and Gymnocladus species were located in the large single copy region. The Gleditsia chloroplast genome prefer T/A-ending codons and avoid C/G-ending codons, positive selection was acting on the rpoA, rpl20, atpB, ndhA and ycf4 genes, most of the chloroplast genes of Gleditsia species underwent purifying selection. Expansion and contraction of the inverted repeat (IR)/single copy (SC) region showed similar patterns within the Gleditsia genus. Polymorphism analysis revealed that coding regions were more conserved than non-coding regions, and the IR region was more conserved than the SC region. Mutational hotspots were mostly found in intergenic regions such as "rps16-trnQ", "trnT-trnL", "ndhG-ndhI", and "rpl32-trnL" in Gleditsia. Phylogenetic analysis showed that G. fera is most closely related to G. sinensis,G. japonica and G. delavayi are relatively closely related. Zhū Yá Zào can be considered a bud mutation of the G. sinensis. The albino phenotype of G. microphylla mutant is not caused by variations in the chloroplast genome, and that the occurrence of the albino phenotype may be due to mutations in chloroplast-related genes involved in splicing or localization functions. This study will help us enhance our exploration of the genetic evolution and geographical origins of the Gleditsia genus.

Thirteen complete chloroplast genomes of the costaceae family: insights into genome structure, selective pressure and phylogenetic relationships

BACKGROUND

Costaceae, commonly known as the spiral ginger family, consists of approximately 120 species distributed in the tropical regions of South America, Africa, and Southeast Asia, of which some species have important ornamental, medicinal and ecological values. Previous studies on the phylogenetic and taxonomic of Costaceae by using nuclear internal transcribed spacer (ITS) and chloroplast genome fragments data had low resolutions. Additionally, the structures, variations and molecular evolution of complete chloroplast genomes in Costaceae still remain unclear. Herein, a total of 13 complete chloroplast genomes of Costaceae including 8 newly sequenced and 5 from the NCBI GenBank database, representing all three distribution regions of this family, were comprehensively analyzed for comparative genomics and phylogenetic relationships.

RESULT

The 13 complete chloroplast genomes of Costaceae possessed typical quadripartite structures with lengths from 166,360 to 168,966 bp, comprising a large single copy (LSC, 90,802 - 92,189 bp), a small single copy (SSC, 18,363 - 20,124 bp) and a pair of inverted repeats (IRs, 27,982 - 29,203 bp). These genomes coded 111 - 113 different genes, including 79 protein-coding genes, 4 rRNA genes and 28 - 30 tRNAs genes. The gene orders, gene contents, amino acid frequencies and codon usage within Costaceae were highly conservative, but several variations in intron loss, long repeats, simple sequence repeats (SSRs) and gene expansion on the IR/SC boundaries were also found among these 13 genomes. Comparative genomics within Costaceae identified five highly divergent regions including ndhF, ycf1-D2, ccsA-ndhD, rps15-ycf1-D2 and rpl16-exon2-rpl16-exon1. Five combined DNA regions (ycf1-D2 + ndhF, ccsA-ndhD + rps15-ycf1-D2, rps15-ycf1-D2 + rpl16-exon2-rpl16-exon1, ccsA-ndhD + rpl16-exon2-rpl16-exon1, and ccsA-ndhD + rps15-ycf1-D2 + rpl16-exon2-rpl16-exon1) could be used as potential markers for future phylogenetic analyses and species identification in Costaceae. Positive selection was found in eight protein-coding genes, including cemA, clpP, ndhA, ndhF, petB, psbD, rps12 and ycf1. Maximum likelihood and Bayesian phylogenetic trees using chloroplast genome sequences consistently revealed identical tree topologies with high supports between species of Costaceae. Three clades were divided within Costaceae, including the Asian clade, Costus clade and South American clade. Tapeinochilos was a sister of Hellenia, and Parahellenia was a sister to the cluster of Tapeinochilos + Hellenia with strong support in the Asian clade. The results of molecular dating showed that the crown age of Costaceae was about 30.5 Mya (95% HPD: 14.9 - 49.3 Mya), and then started to diverge into the Costus clade and Asian clade around 23.8 Mya (95% HPD: 10.1 - 41.5 Mya). The Asian clade diverged into Hellenia and Parahellenia at approximately 10.7 Mya (95% HPD: 3.5 - 25.1 Mya).

CONCLUSION

The complete chloroplast genomes can resolve the phylogenetic relationships of Costaceae and provide new insights into genome structures, variations and evolution. The identified DNA divergent regions would be useful for species identification and phylogenetic inference in Costaceae.

Comparative chloroplast genomics of 24 species shed light on the genome evolution and phylogeny of subtribe Coelogyninae (Orchidaceae)

BACKGROUND

The orchids of the subtribe Coelogyninae are among the most morphologically diverse and economically important groups within the subfamily Epidendroideae. Previous molecular studies have revealed that Coelogyninae is an unambiguously monophyletic group. However, intergeneric and infrageneric relationships within Coelogyninae are largely unresolved. There has been long controversy over the classification among the genera within the subtribe.

RESULTS

The complete chloroplast (cp.) genomes of 15 species in the subtribe Coelogyninae were newly sequenced and assembled. Together with nine available cp. genomes in GenBank from representative clades of the subtribe, we compared and elucidated the characteristics of 24 Coelogyninae cp. genomes. The results showed that all cp. genomes shared highly conserved structure and contained 135 genes arranged in the same order, including 89 protein-coding genes, 38 tRNAs, and eight rRNAs. Nevertheless, structural variations in relation to particular genes at the IR/SC boundary regions were identified. The diversification pattern of the cp. genomes showed high consistency with the phylogenetic placement of Coelogyninae. The number of different types of SSRs and long repeats exhibited significant differences in the 24 Coelogyninae cp. genomes, wherein mononucleotide repeats (A/T), and palindromic repeats were the most abundant. Four mutation hotspot regions (ycf1a, ndhF-rp132, psaC-ndhE, and rp132-trnL) were determined, which could serve as effective molecular markers. Selection pressure analysis revealed that three genes (ycf1a, rpoC2 and ycf2 genes) might have experienced apparent positive selection during the evolution. Using the alignments of whole cp. genomes and protein-coding sequences, this study presents a well-resolved phylogenetic framework of Coelogyninae.

CONCLUSION

The inclusion of 55 plastid genome data from a nearly complete generic-level sampling provide a comprehensive view of the phylogenetic relationships among genera and species in subtribe Coelogyninae and illustrate the diverse genetic variation patterns of plastid genomes in this species-rich plant group. The inferred relationships and informally recognized major clades within the subtribe are presented. The genetic markers identified here will facilitate future studies on the genetics and phylogeny of subtribe Coelogyninae.

The complete chloroplast genome sequences of four Liparis species (Orchidaceae) and phylogenetic implications

Liparis Richard (Malaxideae, Epidendroideae) is a large and diverse genus of the family Orchidaceae, the taxonomy of which is complicated and controversial. In this study, we sequenced, assembled and analyzed four complete chloroplast genomes of Liparis species including L. kumokiri, L. makinoana, L. pauliana, and L. viridiflora, and evaluated their phylogenetic relationships with related species for the first time. These four chloroplast genomes (size range 153,095 to 158,239 bp) possess typical quadripartite structures that consist of a large single copy (LSC, 83,533-86,752 bp), a small single copy (SSC, 17,938-18,156 bp) and a pair of inverted repeats (IRs, 26,421-26,933 bp). The genomes contain 133 genes, including 87 protein coding genes, 38 tRNAs and 8 rRNA genes. The genome arrangements, gene contents, gene order, long repeats and simple sequence repeats were similar with small differences observed among these four chloroplast genomes. Five highly variable regions including ycf1, ndhA, ndhF, trnQ and trnK were identified from the comparative analysis with other nine related Liparis species, which had the potential to be used as DNA markers for species identification and phylogenetic studies of Liparis species. Phylogenetic analysis based on the complete chloroplast genome sequences strongly supported the polyphyly of Liparis and its further division into three branches. These results provided valuable information to illustrate the complicated taxonomy, phylogeny and evolution process of the Liparis genus.

Chloroplast Genome Comparison and Phylogenetic Analysis of the Commercial Variety Actinidia chinensis 'Hongyang'

Actinidia chinensis 'Hongyang', also known as red yangtao (red heart kiwifruit), is a vine fruit tree native to China possessing significant nutritional and economic value. However, information on its genetic diversity and phylogeny is still very limited. The first chloroplast (cp) genome of A. chinensis 'Hongyang' cultivated in China was sequenced using de novo technology in this study. A. chinensis 'Hongyang' possesses a cp genome that spans 156,267 base pairs (bp), exhibiting an overall GC content of 37.20%. There were 132 genes that were annotated, with 85 of them being protein-coding genes, 39 transfer RNA (tRNA) genes, and 8 ribosomal RNA (rRNA) genes. A total of 49 microsatellite sequences (SSRs) were detected, mainly single nucleotide repeats, mostly consisting of A or T base repeats. Compared with 14 other species, the cp genomes of A. chinensis 'Hongyang' were biased towards the use of codons containing A/U, and the non-protein coding regions in the A. chinensis 'Hongyang' cpDNA showed greater variation than the coding regions. The nucleotide polymorphism analysis (Pi) yielded nine highly variable region hotspots, most in the large single copy (LSC) region. The cp genome boundary analysis revealed a conservative order of gene arrangement in the inverted repeats (IRs) region of the cp genomes of 15 Actinidia plants, with small expansions and contractions of the boundaries. Furthermore, phylogenetic tree indicated that A. chinensis 'Hongyang' was the closest relative to A. indochinensis. This research provides a useful basis for future genetic and evolutionary studies of A. chinensis 'Hongyang', and enriches the biological information of Actinidia species.

Phylogenetics Study to Compare Chloroplast Genomes in Four Magnoliaceae Species

Magnoliaceae, a family of perennial woody plants, contains several endangered species whose taxonomic status remains ambiguous. The study of chloroplast genome information can help in the protection of Magnoliaceae plants and confirmation of their phylogenetic relationships. In this study, the chloroplast genomes were sequenced, assembled, and annotated in Woonyoungia septentrionalis and three Michelia species (Michelia champaca, Michelia figo, and Michelia macclurei). Comparative analyses of genomic characteristics, repetitive sequences, and sequence differences were performed among the four Magnoliaceae plants, and phylogenetic relationships were constructed with twenty different magnolia species. The length of the chloroplast genomes varied among the four studied species ranging from 159,838 bp (Woonyoungia septentrionalis) to 160,127 bp (Michelia macclurei). Four distinct hotspot regions were identified based on nucleotide polymorphism analysis. They were petA-psbJ, psbJ-psbE, ndhD-ndhE, and rps15-ycf1. These gene fragments may be developed and utilized as new molecular marker primers. By using Liriodendron tulipifera and Liriodendron chinense as outgroups reference, a phylogenetic tree of the four Magnoliaceae species and eighteen other Magnoliaceae species was constructed with the method of Shared Coding Sequences (CDS). Results showed that the endangered species, W. septentrionalis, is relatively genetically distinct from the other three species, indicating the different phylogenetic processes among Magnoliaceae plants. Therefore, further genetic information is required to determine the relationships within Magnoliaceae. Overall, complete chloroplast genome sequences for four Magnoliaceae species reported in this paper have shed more light on phylogenetic relationships within the botanical group.

Complete plastid genome structure of 13 Asian Justicia (Acanthaceae) species: comparative genomics and phylogenetic analyses

BACKGROUND

Justicia L. is the largest genus in Acanthaceae Juss. and widely distributed in tropical and subtropical regions of the world. Previous phylogenetic studies have proposed a general phylogenetic framework for Justicia based on several molecular markers. However, their studies were mainly focused on resolution of phylogenetic issues of Justicia in Africa, Australia and South America due to limited sampling from Asia. Additionally, although Justicia plants are of high medical and ornamental values, little research on its genetics was reported. Therefore, to improve the understanding of its genomic structure and relationships among Asian Justicia plants, we sequenced complete chloroplast (cp.) genomes of 12 Asian plants and combined with the previously published cp. genome of Justicia leptostachya Hemsl. for further comparative genomics and phylogenetic analyses.

RESULTS

All the cp. genomes exhibit a typical quadripartite structure without genomic rearrangement and gene loss. Their sizes range from 148,374 to 151,739 bp, including a large single copy (LSC, 81,434-83,676 bp), a small single copy (SSC, 16,833-17,507 bp) and two inverted repeats (IR, 24,947-25,549 bp). GC contents range from 38.1 to 38.4%. All the plastomes contain 114 genes, including 80 protein-coding genes, 30 tRNAs and 4 rRNAs. IR variation and repetitive sequences analyses both indicated that Justicia grossa C. B. Clarke is different from other Justicia species because its lengths of ndhF and ycf1 in IRs are shorter than others and it is richest in SSRs and dispersed repeats. The ycf1 gene was identified as the candidate DNA barcode for the genus Justicia. Our phylogenetic results showed that Justicia is a polyphyletic group, which is consistent with previous studies. Among them, J. grossa belongs to subtribe Tetramerinae of tribe Justicieae while the other Justicia members belong to subtribe Justiciinae. Therefore, based on morphological and molecular evidence, J. grossa should be undoubtedly recognized as a new genus. Interestingly, the evolutionary history of Justicia was discovered to be congruent with the morphology evolution.

CONCLUSION

Our study not only elucidates basic features of Justicia whole plastomes, but also sheds light on interspecific relationships of Asian Justicia plants for the first time.

The complete chloroplast genome of Hedychium flavum Roxb., an ornamental, edible and medicinal plant

Hedychium flavum Roxb. 1820 is a perennial herb mainly distributed in China, India, Myanmar and Thailand with ornamental, edible and medicinal value. It is extensively cultivated as a source of aromatic essential oils, ornamental plant, food flavorings and vegetables, and folk medicine. In this study, we sequence the complete chloroplast genome of H. flavum by de novo assembly. The assembled genome has a typical quadripartite circular structure with 163,909 bp in length, containing a large single-copy region (LSC, 88,589 bp), a small single-copy region (SSC, 15,762 bp), and two inverted repeat regions (IRs, 29,779 bp). The cp genome contains 133 genes, including 87 protein-coding genes, 38 tRNA genes and 8 rRNA genes. Phylogenetic analysis based on the complete cp genome shows a close affinity of H. flavum and H. neocarneum with 100% bootstrap support. This study will provide useful genetic resource for further phylogenetic analysis of the genus Hedychium and Zingiberaceae.

Comparative Chloroplast Genomics of 21 Species in Zingiberales with Implications for Their Phylogenetic Relationships and Molecular Dating

Zingiberales includes eight families and more than 2600 species, with many species having important economic and ecological value. However, the backbone phylogenetic relationships of Zingiberales still remain controversial, as demonstrated in previous studies, and molecular dating based on chloroplast genomes has not been comprehensively studied for the whole order. Herein, 22 complete chloroplast genomes from 21 species in Zingiberales were sequenced, assembled, and analyzed. These 22 genomes displayed typical quadripartite structures, which ranged from 161,303 bp to 163,979 bp in length and contained 111-112 different genes. The genome structures, gene contents, simple sequence repeats, long repeats, and codon usage were highly conserved, with slight differences among these genomes. Further comparative analysis of the 111 complete chloroplast genomes of Zingiberales, including 22 newly sequenced ones and the remaining ones from the national center for biotechnology information (NCBI) database, identified three highly divergent regions comprising ccsA, psaC, and psaC-ndhE. Maximum likelihood and Bayesian inference phylogenetic analyses based on chloroplast genome sequences found identical topological structures and identified a strongly supported backbone of phylogenetic relationships. Cannaceae was sister to Marantaceae, forming a clade that was collectively sister to the clade of (Costaceae, Zingiberaceae) with strong support (bootstrap (BS) = 100%, and posterior probability (PP) = 0.99-1.0); Heliconiaceae was sister to the clade of (Lowiaceae, Strelitziaceae), then collectively sister to Musaceae with strong support (BS = 94-100%, and PP = 0.93-1.0); the clade of ((Cannaceae, Marantaceae), (Costaceae, Zingiberaceae)) was sister to the clade of (Musaceae, (Heliconiaceae, (Lowiaceae, Strelitziaceae))) with robust support (BS = 100%, and PP = 1.0). The results of divergence time estimation of Zingiberales indicated that the crown node of Zingiberales occurred approximately 85.0 Mya (95% highest posterior density (HPD) = 81.6-89.3 million years ago (Mya)), with major family-level lineages becoming from 46.8 to 80.5 Mya. These findings proved that chloroplast genomes could contribute to the study of phylogenetic relationships and molecular dating in Zingiberales, as well as provide potential molecular markers for further taxonomic and phylogenetic studies of Zingiberales.

Plastome evolution and phylogenomic insights into the evolution of Lysimachia (Primulaceae: Myrsinoideae)

BACKGROUND

Lysimachia L., the second largest genus within the subfamily Myrsinoideae of Primulaceae, comprises approximately 250 species worldwide. China is the species diversity center of Lysimachia, containing approximately 150 species. Despite advances in the backbone phylogeny of Lysimachia, species-level relationships remain poorly understood due to limited genomic information. This study analyzed 50 complete plastomes for 46 Lysimachia species. We aimed to identify the plastome structure features and hypervariable loci of Lysimachia. Additionally, the phylogenetic relationships and phylogenetic conflict signals in Lysimachia were examined.

RESULTS

These fifty plastomes within Lysimachia had the typical quadripartite structure, with lengths varying from 152,691 to 155,784 bp. Plastome size was positively correlated with IR and intron length. Thirteen highly variable regions in Lysimachia plastomes were identified. Additionally, ndhB, petB and ycf2 were found to be under positive selection. Plastid ML trees and species tree strongly supported that L. maritima as sister to subg. Palladia + subg. Lysimachia (Christinae clade), while the nrDNA ML tree clearly placed L. maritima and subg. Palladia as a sister group.

CONCLUSIONS

The structures of these plastomes of Lysimachia were generally conserved, but potential plastid markers and signatures of positive selection were detected. These genomic data provided new insights into the interspecific relationships of Lysimachia, including the cytonuclear discordance of the position of L. maritima, which may be the result of ghost introgression in the past. Our findings have established a basis for further exploration of the taxonomy, phylogeny and evolutionary history within Lysimachia.

The complete chloroplast genome sequence of Zingiber teres S. Q. Tong & Y. M. Xia (Zingiberaceae)

Here, the complete chloroplast genome sequence of Zingiber teres is described using MGI paired-end sequencing. The genome is 163,428 bp in length and contains a small single-copy region (SSC) of 15,782 bp, a large single-copy region (LSC) of 88,142 bp, and two inverted repeat (IR) regions of 29,752 bp. The overall GC content is 36.1%, and the GC content of the IR regions is 41.1%, which is higher than that of both the LSC region (33.8%) and SSC region (29.5%). The genome of Z. teres contains 133 complete genes, including 88 protein-coding genes (79 protein-coding gene species), 38 tRNA genes (28 tRNA species), and 8 rRNA genes (four rRNA species). Maximum likelihood phylogenetic analysis yielded a well-resolved tree of the genus Zingiber, and Z. teres and Zingiber mioga were sister species in this tree. The development of DNA barcodes could aid the identification of Zingiber species.

Chloroplast genome analysis of three Acanthus species reveal the adaptation of mangrove to intertidal habitats

Acanthus is a distinctive genus that covers three species with different ecological niches including Acanthus mollis (arid terrestrial), Acanthus leucostachyus (damp forest) and Acanthus ilicifolius (coastal intertidal). It is an intriguing question how these species evolved from terrestrial to coastal intertidal. In the present study, we assembled chloroplast genomes of A. ilicifolius, A. leucostachyus and A. mollis, which exhibited typical quadripartite structures. The sizes were 150,758, 154,686 and 150,339 bp that comprised a large single copy (LSC, 82,963, 86,461 and 82,612 bp), a small single copy (SSC, 17,191, 17,511 and 17,019 bp), and a pair of inverted repeats (IRs, 25,302, 25,357 and 25,354 bp), respectively. Gene annotation revealed that A. ilicifolius, A. leucostachyus and A. mollis contained 113, 112 and 108 unique genes, each of which contained 79, 79 and 74 protein-coding genes, 30, 29 and 30 tRNAs, and 4 rRNA genes, respectively. Differential gene analysis revealed plenty of ndhs gene deletions in the terrestrial plant A. mollis. Nucleotide diversity analysis showed that the psbK, ycf1, ndhG, and rpl22 have the highest nucleotide variability. Compared to A. leucostachyus and A. mollis, seven genes in A. ilicifolius underwent positive selection. Among them, the atpF gene showed a strong positive selection throughout terrestrial to marine evolution and was important for adaptation to coastal intertidal habitats. Phylogenetic analysis indicated that A. ilicifolius has a closer genetic relationship with A. leucostachyus than A. mollis which further confirmed the evolutionary direction of Acanthus going from terrestrial to coastal intertidal zones.

Small cardamom genome: development and utilization of microsatellite markers from a draft genome sequence of Elettaria cardamomum Maton

Small cardamom (Elettaria cardamomum Maton), the queen of spices, is the third most expensive spice in the world after saffron and vanilla, valued highly for its aroma and taste. This perennial herbaceous plant is a native of coastal parts of Southern India and displays a significant amount of morphological diversity. Its genetic potential has not been exploited due to lack of genomic resources limiting our understanding of the genome and important metabolic pathways which give it the economic advantage in the spice industry. Here, we report upon the de novo assembled, draft whole genome sequence of cardamom variety, Njallani Green Gold. We used a hybrid assembly strategy using the reads from the Oxford Nanopore, Illumina and 10x Genomics GemCode sequencing chemistries. The assembled genome length was 1.06 Gb (gigabases) which is close to the estimated genome size of cardamom. More than 75% of the genome was captured in 8000 scaffolds with a N50 of 0.15 Mb. The genome appears to have a high repeat content and 68055 gene models were predicted. The genome is close to Musa species and displays an expansion and contraction in different gene families. The draft assembly was used for in silico mining of simple sequence repeats (SSRs). A total of 2,50,571 SSRs were identified of which 2,18,270 were perfect SSRs and 32,301 were compound SSRs. Among the perfect SSRs, trinucleotides were most abundant (1,25,329) and hexanucleotide repeats appear least (2,380). From the 2,50,571 SSRs mined, 2,27,808 primer pairs were designed based on flanking sequence information. Wet lab validation was performed for 246 SSR loci and based on their amplification profiles, 60 SSR markers were used for diversity analysis of a set of 60 diverse cardamom accessions. The average number of alleles detected per locus were 14.57 with a minimum of 4 and maximum of 30 alleles. Population structure analysis revealed the presence of high degree of admixtures which could primarily be due to cross-pollination prevalent in this species. The SSR markers identified would help in the development of gene or trait-linked markers which can be subsequently used for marker-assisted breeding for crop improvement in cardamom. The information on utilization of the SSR loci for generation of markers has been developed into a public database, 'cardamomSSRdb' that is freely available for use by the cardamom community.

Complete chloroplast genomes provide insights into evolution and phylogeny of Zingiber (Zingiberaceae)

BACKGROUND

The genus Zingiber of the Zingiberaceae is distributed in tropical, subtropical, and in Far East Asia. This genus contains about 100-150 species, with many species valued as important agricultural, medicinal and horticultural resources. However, genomic resources and suitable molecular markers for species identification are currently sparse.

RESULTS

We conducted comparative genomics and phylogenetic analyses on Zingiber species. The Zingiber chloroplast genome (size range 162,507-163,711 bp) possess typical quadripartite structures that consist of a large single copy (LSC, 86,986-88,200 bp), a small single copy (SSC, 15,498-15,891 bp) and a pair of inverted repeats (IRs, 29,765-29,934 bp). The genomes contain 113 unique genes, including 79 protein coding genes, 30 tRNA and 4 rRNA genes. The genome structures, gene contents, amino acid frequencies, codon usage patterns, RNA editing sites, simple sequence repeats and long repeats are conservative in the genomes of Zingiber. The analysis of sequence divergence indicates that the following genes undergo positive selection (ccsA, ndhA, ndhB, petD, psbA, psbB, psbC, rbcL, rpl12, rpl20, rpl23, rpl33, rpoC2, rps7, rps12 and ycf3). Eight highly variable regions are identified including seven intergenic regions (petA-pabJ, rbcL-accD, rpl32-trnL-UAG, rps16-trnQ-UUG, trnC-GCA-psbM, psbC-trnS-UGA and ndhF-rpl32) and one genic regions (ycf1). The phylogenetic analysis revealed that the sect. Zingiber was sister to sect. Cryptanthium rather than sect. Pleuranthesis.

CONCLUSIONS

This study reports 14 complete chloroplast genomes of Zingiber species. Overall, this study provided a solid backbone phylogeny of Zingiber. The polymorphisms we have uncovered in the sequencing of the genome offer a rare possibility (for Zingiber) of the generation of DNA markers. These results provide a foundation for future studies that seek to understand the molecular evolutionary dynamics or individual population variation in the genus Zingiber.

Insights into the phylogeny and chloroplast genome evolution of Eriocaulon (Eriocaulaceae)

BACKGROUND

Eriocaulon is a wetland plant genus with important ecological value, and one of the famous taxonomically challenging groups among angiosperms, mainly due to the high intraspecific diversity and low interspecific variation in the morphological characters of species within this genus. In this study, 22 samples representing 15 Eriocaulon species from China, were sequenced and combined with published samples of Eriocaulon to test the phylogenetic resolution using the complete chloroplast genome. Furthermore, comparative analyses of the chloroplast genomes were performed to investigate the chloroplast genome evolution of Eriocaulon.

RESULTS

The 22 Eriocaulon chloroplast genomes and the nine published samples were proved highly similar in genome size, gene content, and order. The Eriocaulon chloroplast genomes exhibited typical quadripartite structures with lengths from 150,222 bp to 151,584 bp. Comparative analyses revealed that four mutation hotspot regions (psbK-trnS, trnE-trnT, ndhF-rpl32, and ycf1) could serve as effective molecular markers for further phylogenetic analyses and species identification of Eriocaulon species. Phylogenetic results supported Eriocaulon as a monophyletic group. The identified relationships supported the taxonomic treatment of section Heterochiton and Leucantherae, and the section Heterochiton was the first divergent group. Phylogenetic tree supported Eriocaulon was divided into five clades. The divergence times indicated that all the sections diverged in the later Miocene and most of the extant Eriocaulon species diverged in the Quaternary. The phylogeny and divergence times supported rapid radiation occurred in the evolution history of Eriocaulon.

CONCLUSION

Our study mostly supported the taxonomic treatment at the section level for Eriocaulon species in China and demonstrated the power of phylogenetic resolution using whole chloroplast genome sequences. Comparative analyses of the Eriocaulon chloroplast genome developed molecular markers that can help us better identify and understand the evolutionary history of Eriocaulon species in the future.

The complete chloroplast genome of Zingiber striolatum Diels (Zingiberaceae)

The chloroplast genome of Zingiber striolatum Diels was sequenced using the MGI paired-end sequencing method and assembled. The chloroplast genome was 163,711 bp in length, containing a large single-copy (LSC) region of 88,205 bp, a small single-copy (SSC) region of 15,750 bp, and two inverted repeat (IR) regions of 29,752 bp. The overall GC content was 36.1%, whereas the corresponding value in the IR regions was 41.1%, which was higher than that in the LSC region (33.8%) and SSC region (29.6%). A total of 136 complete genes were annotated in the chloroplast genome of Z. striolatum, including 87 protein-coding genes (79 protein-coding gene species), 40 tRNA genes (29 tRNA species), and 8 rRNA genes (4 rRNA species). A phylogenetic tree was constructed using the maximum likelihood (ML) method, and the results showed that the phylogeny of Zingiber was well resolved with high support values, and Z. striolatum was sister to Z. mioga. The assembly and sequence analysis of the chloroplast genome can provide a basis for developing high-resolution genetic makers.

Comparative chloroplast genome analyses of Amomum: insights into evolutionary history and species identification

BACKGROUND

Species in genus Amomum always have important medicinal and economic values. Classification of Amomum using morphological characters has long been a challenge because they exhibit high similarity. The main goals of this study were to mine genetic markers from cp genomes for Amomum species identification and discover their evolutionary history through comparative analysis.

RESULTS

Three species Amomum villosum, Amomum maximum and Amomum longipetiolatum were sequenced and annotated for the complete chloroplast (cp) genomes, and the cp genomes of A. longipetiolatum and A. maximum were the first reported. Three cp genomes exhibited typical quadripartite structures with 163,269-163,591 bp in length. Each genome encodes 130 functional genes including 79 protein-coding, 26 tRNAs and 3 rRNAs genes. 113-152 SSRs and 99 long repeats were identified in the three cp genomes. By designing specific primers, we amplified the highly variable loci and the mined genetic marker ccsA exhibited a relatively high species identification resolution in Amomum. The nonsynonymous and synonymous substitution ratios (Ka/Ks) in Amomum and Alpinia showed that most genes were subjected to a purifying selection. Phylogenetic analysis revealed the evolutionary relationships of Amomum and Alpinia species and proved that Amomum is paraphyletic. In addition, the sequenced sample of A. villosum was found to be a hybrid, becoming the first report of natural hybridization of this genus. Meanwhile, the high-throughput sequencing-based ITS2 analysis was proved to be an efficient tool for interspecific hybrid identification and with the help of the chloroplast genome, the hybrid parents can be also be determined.

CONCLUSION

The comparative analysis and mined genetic markers of cp genomes were conducive to species identification and evolutionary relationships of Amomum.

Comparative Analyses of Plastomes of Four Anubias (Araceae) Taxa, Tropical Aquatic Plants Endemic to Africa

Anubias Schott (Araceae) have high ornamental properties as aquarium plants. However, the genus has difficulties in species identification, and the mechanism of its adaptation to the aquatic environment is unknown. To better identify species and understand the evolutionary history of Anubias, the plastomes of Anubias barteri Schott, A. barteri var. nana (Engl.) Crusio, and A. hastifolia Engl., were sequenced. The sizes of the plastomes of Anubias ranged from 169,841 bp to 170,037 bp. These plastomes were composed of conserved quadripartite circular structures and comprised 112 unique genes, including 78 protein-coding genes, 30 transfer RNA genes, and 4 ribosomal RNA genes. The comparative analysis of genome structure, repeat sequences, codon usage and RNA editing sites revealed high similarities among the Anubias plastomes, indicating the conservation of plastomes of Anubias. Three spacer regions with relatively high nucleotide diversity, trnL-CAA-ndhB, ycf1-ndhF, and rps15-ycf1, were found within the plastomes of Anubias. Phylogenetic analysis, based on 75 protein-coding genes, showed that Anubias was sister to Montrichardia arborescens (L.) Schott (BS = 99). In addition, four genes (ccsA, matK, ndhF, and ycf4) that contain sites undergoing positive selection were identified within the Anubias plastomes. These genes may play an important role in the adaptation of Anubias to the aquatic environment. The present study provides a valuable resource for further studies on species identification and the evolutionary history of Anubias.

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.