Plastome Structural Evolution and Homoplastic Inversions in Neo-Astragalus (Fabaceae)

Polyphyly of Boehmeria (Urticaceae) congruent with plastome structural variation

Boehmeria is a taxonomically challenging group within the nettle family (Urticaceae). The polyphyly of the genus has been proposed by previous studies with respect to five genera (Debregeasia, Cypholophus, Sarcochlamys, Archiboehmeria, and Astrothalamus). Extensive homoplasy of morphological characters has made generic delimitation problematic. Previous studies in other plant groups suggest that plastome structural variations have the potential to provide characters useful in reconstructing evolutionary relationships. We aimed to test this across Boehmeria and its allied genera by mapping plastome structural variations onto a resolved strongly supported phylogeny. In doing so, we expanded the sampling of the plastome to include Cypholophus, Sarcochlamys, Archiboehmeria, and Astrothalamus for the first time. The results of our phylogenomic analyses provide strong support for Sarcochlamys as being more closely related to Leucosyke puya than to Boehmeria and for the clustering of Boehmeria s.l. into four subclades. The sizes of the plastomes in Boehmeria s.l. ranged from 142,627 bp to 170,958 bp. The plastomes recovered a typical quadripartite structure comprising 127~146 genes. We observe several obvious structural variations across the taxa such as gene loss and multiple gene duplication, inverted repeat (IR) contraction and wide expansions, and inversions. Moreover, we recover a trend for these variations that the early clades were relatively conserved in evolution, whereas the later diverging clades were variable. We propose that the structural variations documented may be linked to the adaptation of Boehmeria s.l. to a wide range of habitats, from moist broadleaf forests in Asia to xeric shrublands and deserts in Africa. This study confirms that variation in plastome gene loss/duplication, IR contraction/expansion, and inversions can provide evidence useful for the reconstruction of evolutionary relationships.

Insight into chloroplast genome structural variation of the Mongolian endemic species Adonis mongolica (Ranunculaceae) in the Adonideae tribe

Adonis mongolica is a threatened species that is endemic to Mongolia. It is a medicinal plant from the Adonis genus and has been used to treat heart diseases. However, the genomics and evolution of this species have not been thoroughly studied. We sequenced the first complete plastome of A. mongolica and compared it with ten Adonideae species to describe the plastome structure and infer phylogenetic relationships. The complete plastome of A. mongolica was 157,521 bp long and had a typical quadripartite structure with numerous divergent regions. The plastomes of Adonideae had relatively constant genome structures and sizes, except for those of Adonis. The plastome structure was consistent across Adonis. We identified a 44.8 kb large-scale inversion within the large single-copy region and rpl32 gene loss in the Adonis plastomes compared to other members of the Adonideae tribe. Additionally, Adonis had a smaller plastome size (156,917-157,603 bp) than the other genera within the tribe (159,666-160,940 bp), which was attributed to deletions of intergenic regions and partial and complete gene losses. These results suggested that an intramolecular mutation occurred in the ancestor of the Adonis genus. Based on the phylogenetic results, Adonis separated earlier than the other genera within the Adonideae tribe. The genome structures and divergences of specific regions in the Adonis genus were unique to the Adonideae tribe. This study provides fundamental knowledge for further genomic research in Mongolia and a better understanding of the evolutionary history of endemic plants.

More than a spiny morphology: plastome variation in the prickly pear cacti (Opuntieae)

BACKGROUND

Plastid genomes (plastomes) have long been recognized as highly conserved in their overall structure, size, gene arrangement and content among land plants. However, recent studies have shown that some lineages present unusual variations in some of these features. Members of the cactus family are one of these lineages, with distinct plastome structures reported across disparate lineages, including gene losses, inversions, boundary movements or loss of the canonical inverted repeat (IR) region. However, only a small fraction of cactus diversity has been analysed so far.

METHODS

Here, we investigated plastome features of the tribe Opuntieae, the remarkable prickly pear cacti, which represent one of the most diverse and important lineages of Cactaceae. We assembled de novo the plastome of 43 species, representing a comprehensive sampling of the tribe, including all seven genera, and analysed their evolution in a phylogenetic comparative framework. Phylogenomic analyses with different datasets (full plastome sequences and genes only) were performed, followed by congruence analyses to assess signals underlying contentious nodes.

KEY RESULTS

Plastomes varied considerably in length, from 121 to 162 kbp, with striking differences in the content and size of the IR region (contraction and expansion events), including a lack of the canonical IR in some lineages and the pseudogenization or loss of some genes. Overall, nine different types of plastomes were reported, deviating in the presence of the IR region or the genes contained in the IR. Overall, plastome sequences resolved phylogenetic relationships within major clades of Opuntieae with high bootstrap values but presented some contentious nodes depending on the dataset analysed (e.g. whole plastome vs. genes only). Congruence analyses revealed that most plastidial regions lack phylogenetic resolution, while few markers are supporting the most likely topology. Likewise, alternative topologies are driven by a handful of plastome markers, suggesting recalcitrant nodes in the phylogeny.

CONCLUSIONS

Our study reveals a dynamic nature of plastome evolution across closely related lineages, shedding light on peculiar features of plastomes. Variation of plastome types across Opuntieae is remarkable in size, structure and content and can be important for the recognition of species in some major clades. Unravelling connections between the causes of plastome variation and the consequences for species biology, physiology, ecology, diversification and adaptation is a promising and ambitious endeavour in cactus research. Although plastome data resolved major phylogenetic relationships, the generation of nuclear genomic data is necessary to confront these hypotheses and assess the recalcitrant nodes further.

A phylogenomic study of Iridaceae Juss. based on complete plastid genome sequences

The plastid genome has proven to be an effective tool for examining deep correlations in plant phylogenetics, owing to its highly conserved structure, uniparental inheritance, and limited variation in evolutionary rates. Iridaceae, comprising more than 2,000 species, includes numerous economically significant taxa that are frequently utilized in food industries and medicines and for ornamental and horticulture purposes. Molecular studies on chloroplast DNA have confirmed the position of this family in the order Asparagales with non-asparagoids. The current subfamilial classification of Iridaceae recognizes seven subfamilies-Isophysioideae, Nivenioideae, Iridoideae, Crocoideae, Geosiridaceae, Aristeoideae, and Patersonioideae-which are supported by limited plastid DNA regions. To date, no comparative phylogenomic studies have been conducted on the family Iridaceae. We assembled and annotated (de novo) the plastid genomes of 24 taxa together with seven published species representing all the seven subfamilies of Iridaceae and performed comparative genomics using the Illumina MiSeq platform. The plastomes of the autotrophic Iridaceae represent 79 protein-coding, 30 tRNA, and four rRNA genes, with lengths ranging from 150,062 to 164,622 bp. The phylogenetic analysis of the plastome sequences based on maximum parsimony, maximum likelihood, and Bayesian inference analyses suggested that Watsonia and Gladiolus were closely related, supported by strong support values, which differed considerably from recent phylogenetic studies. In addition, we identified genomic events, such as sequence inversions, deletions, mutations, and pseudogenization, in some species. Furthermore, the largest nucleotide variability was found in the seven plastome regions, which can be used in future phylogenetic studies. Notably, three subfamilies-Crocoideae, Nivenioideae, and Aristeoideae-shared a common ycf2 gene locus deletion. Our study is a preliminary report of a comparative study of the complete plastid genomes of 7/7 subfamilies and 9/10 tribes, elucidating the structural characteristics and shedding light on plastome evolution and phylogenetic relationships within Iridaceae. Additionally, further research is required to update the relative position of Watsonia within the tribal classification of the subfamily Crocoideae.

Comparative analysis of 84 chloroplast genomes of Tylosema esculentum reveals two distinct cytotypes

Tylosema esculentum (marama bean) is an important orphan legume from southern Africa that has long been considered to have the potential to be domesticated as a crop. The chloroplast genomes of 84 marama samples collected from various geographical locations in Namibia and Pretoria were compared in this study. The cp genomes were analyzed for diversity, including SNPs, indels, structural alterations, and heteroplasmy. The marama cp genomes ranged in length from 161,537 bp to 161,580 bp and contained the same sets of genes, including 84 protein-coding genes, 37 tRNA genes, and 8 rRNA genes. The genes rpoC2 and rpoB, and the intergenic spacers trnT-trnL and ndhG-ndhI were found to be more diverse than other regions of the marama plastome. 15 haplotypes were found to be divided into two groups, differing at 122 loci and at a 230 bp inversion. One type appears to have greater variability within the major genome present, and variations amongst individuals with this type of chloroplast genome seems to be distributed within specific geographic regions but with very limited sampling for some regions. However, deep sequencing has identified that within most of the individuals, both types of chloroplast genomes are present, albeit one is generally at a very low frequency. The inheritance of this complex of chloroplast genomes appears to be fairly constant, providing a conundrum of how the two genomes co-exist and are propagated through generations. The possible consequences for adaptation to the harsh environment in which T. esculentum survives are considered. The results pave the way for marama variety identification, as well as for understanding the origin and evolution of the bean.

Analysis of Complete Chloroplast Genome: Structure, Phylogenetic Relationships of Galega orientalis and Evolutionary Inference of Galegeae

Galega orientalis, a leguminous herb in the Fabaceae family, is an ecologically and economically important species widely cultivated for its strong stress resistance and high protein content. However, genomic information of Galega orientalis has not been reported, which limiting its evolutionary analysis. The small genome size makes chloroplast relatively easy to obtain genomic sequence for phylogenetic studies and molecular marker development. Here, the chloroplast genome of Galega orientalis was sequenced and annotated. The results showed that the chloroplast genome of G. orientalis is 125,280 bp in length with GC content of 34.11%. A total of 107 genes were identified, including 74 protein-coding genes, 29 tRNAs and four rRNAs. One inverted repeat (IR) region was lost in the chloroplast genome of G. orientalis. In addition, five genes (rpl22, ycf2, rps16, trnE-UUC and pbf1) were lost compared with the chloroplast genome of its related species G. officinalis. A total of 84 long repeats and 68 simple sequence repeats were detected, which could be used as potential markers in the genetic studies of G. orientalis and related species. We found that the Ka/Ks values of three genes petL, rpl20, and ycf4 were higher than one in the pairwise comparation of G. officinalis and other three Galegeae species (Calophaca sinica, Caragana jubata, Caragana korshinskii), which indicated those three genes were under positive selection. A comparative genomic analysis of 15 Galegeae species showed that most conserved non-coding sequence regions and two genic regions (ycf1 and clpP) were highly divergent, which could be used as DNA barcodes for rapid and accurate species identification. Phylogenetic trees constructed based on the ycf1 and clpP genes confirmed the evolutionary relationships among Galegeae species. In addition, among the 15 Galegeae species analyzed, Galega orientalis had a unique 30-bp intron in the ycf1 gene and Tibetia liangshanensis lacked two introns in the clpP gene, which is contrary to existing conclusion that only Glycyrrhiza species in the IR lacking clade (IRLC) lack two introns. In conclusion, for the first time, the complete chloroplast genome of G. orientalis was determined and annotated, which could provide insights into the unsolved evolutionary relationships within the genus Galegeae.

Plastid phylogenomics uncovers multiple species in Medicago truncatula (Fabaceae) germplasm accessions

Medicago truncatula is a model legume that has been extensively investigated in diverse subdisciplines of plant science. Medicago littoralis can interbreed with M. truncatula and M. italica; these three closely related species form a clade, i.e. TLI clade. Genetic studies have indicated that M. truncatula accessions are heterogeneous but their taxonomic identities have not been verified. To elucidate the phylogenetic position of diverse M. truncatula accessions within the genus, we assembled 54 plastid genomes (plastomes) using publicly available next-generation sequencing data and conducted phylogenetic analyses using maximum likelihood. Five accessions showed high levels of plastid DNA polymorphism. Three of these highly polymorphic accessions contained sequences from both M. truncatula and M. littoralis. Phylogenetic analyses of sequences placed some accessions closer to distantly related species suggesting misidentification of source material. Most accessions were placed within the TLI clade and maximally supported the interrelationships of three subclades. Two Medicago accessions were placed within a M. italica subclade of the TLI clade. Plastomes with a 45-kb (rpl20-ycf1) inversion were placed within the M. littoralis subclade. Our results suggest that the M. truncatula accession genome pool represents more than one species due to possible mistaken identities and gene flow among closely related species.

Does IR-loss promote plastome structural variation and sequence evolution?

Plastids are one of the main distinguishing characteristics of the plant cell. The plastid genome (plastome) of most autotrophic seed plants possesses a highly conserved quadripartite structure containing a large single-copy (LSC) and a small single-copy (SSC) region separated by two copies of the inverted repeat (termed as IR_A and IR_B). The IRs have been inferred to stabilize the plastid genome via homologous recombination-induced repair mechanisms. IR loss has been documented in seven autotrophic flowering plant lineages and two autotrophic gymnosperm lineages, and the plastomes of these species (with a few exceptions) are rearranged to a great extent. However, some plastomes containing normal IRs also show high structural variation. Therefore, the role of IRs in maintaining plastome stability is still controversial. In this study, we first integrated and compared genome structure and sequence evolution of representative plastomes of all nine reported IR-lacking lineages and those of their closest relative(s) with canonical inverted repeats (CRCIRs for short) to explore the role of the IR in maintaining plastome structural stability and sequence evolution. We found the plastomes of most IR-lacking lineages have experienced significant structural rearrangement, gene loss and duplication, accumulation of novel small repeats, and acceleration of synonymous substitution compared with those of their CRCIRs. However, the IR-lacking plastomes show similar structural variation and sequence evolution rate, and even less rearrangement distance, dispersed repeat number, tandem repeat number, indels frequency and GC3 content than those of IR-present plastomes with variation in Geraniaceae. We argue that IR loss is not a driver of these changes but is instead itself a consequence of other processes that more broadly shape both structural and sequence-level plastome evolution.

Phylogenetic analysis based on single-copy orthologous proteins in highly variable chloroplast genomes of Corydalis

Corydalis is one of the few lineages that have been reported to have extensive large-scale chloroplast genome (cp-genome) rearrangements. In this study, novel cp-genome rearrangements of Corydalis pinnata, C. mucronate, and C. sheareri are described. C. pinnata is a narrow endemic species only distributed at Qingcheng Mountain in southwest China. Two independent relocations of the same four genes (trnM-CAU-rbcL) were found relocated from the typically posterior part of the large single-copy region to the front of it. A uniform inversion of an 11-14-kb segment (ndhB-trnR-ACG) was found in the inverted repeat region; and extensive losses of accD, clpP, and trnV-UAC genes were detected in all cp-genomes of all three species of Corydalis. In addition, a phylogenetic tree was reconstructed based on 31 single-copy orthologous proteins in 27 cp-genomes. This study provides insights into the evolution of cp-genomes throughout the genus Corydalis and also provides a reference for further studies on the taxonomy, identification, phylogeny, and genetic transformation of other lineages with extensive rearrangements in cp-genomes.

Plastid Phylogenomic Analysis of Tordylieae Tribe (Apiaceae, Apioideae)

Based on the nrDNA ITS sequence data, the Tordylieae tribe is recognized as monophyletic with three major lineages: the subtribe Tordyliinae, the Cymbocarpum clade, and the Lefebvrea clade. Recent phylogenomic investigations showed incongruence between the nuclear and plastid genome evolution in the tribe. To assess phylogenetic relations and structure evolution of plastomes in Tordylieae, we generated eleven complete plastome sequences using the genome skimming approach and compared them with the available data from this tribe and close relatives. Newly assembled plastomes had lengths ranging from 141,148 to 150,103 base pairs and contained 122-127 genes, including 79-82 protein-coding genes, 35-37 tRNAs, and 8 rRNAs. We observed substantial differences in the inverted repeat length and gene content, accompanied by a complex picture of multiple JLA and JLB shifts. In concatenated phylogenetic analyses, Tordylieae plastomes formed at least three not closely related lineages with plastomes of the Lefebvrea clade as a sister group to plastomes from the Selineae tribe. The newly obtained data have increased our knowledge on the range of plastome variability in Apiaceae.

The plastome of the arctic Oxytropis arctobia (Fabaceae) has large differences compared with that of O. splendens and those of related species

Anatomical and physiological specializations for plant adaptation to harsh climates are the results of molecular mechanisms that can be nuclear or organellar encoded. In this study, the complete plastomes of an arctic species, Oxytropis arctobia Bunge (Fabaceae,) and a closely related temperate species, O. splendens Douglas ex Hook., were assembled, annotated and analyzed to search for differences that might help explain their adaptation to different environments. Consistently with the previously sequenced O. bicolor DC. and O. glabra plastomes, the O. arctobia and O. splendens plastomes both have the common features of the inverted repeat-lacking clade (IRLC), as well as the atpF intron loss, which is unique to the genus. However, significant differences distinguishes the O. arctobia from O. splendens and other closely related plastomes (Oxytropis spp. and Astragalus spp.), including a 3 kb inversion, two large insertions (>1 kb), significant modifications of the accD gene, and an overall larger size.

Highly Resolved Papilionoid Legume Phylogeny Based on Plastid Phylogenomics

Comprising 501 genera and around 14,000 species, Papilionoideae is not only the largest subfamily of Fabaceae (Leguminosae; legumes), but also one of the most extraordinarily diverse clades among angiosperms. Papilionoids are a major source of food and forage, are ecologically successful in all major biomes, and display dramatic variation in both floral architecture and plastid genome (plastome) structure. Plastid DNA-based phylogenetic analyses have greatly improved our understanding of relationships among the major groups of Papilionoideae, yet the backbone of the subfamily phylogeny remains unresolved. In this study, we sequenced and assembled 39 new plastomes that are covering key genera representing the morphological diversity in the subfamily. From 244 total taxa, we produced eight datasets for maximum likelihood (ML) analyses based on entire plastomes and/or concatenated sequences of 77 protein-coding sequences (CDS) and two datasets for multispecies coalescent (MSC) analyses based on individual gene trees. We additionally produced a combined nucleotide dataset comprising CDS plus matK gene sequences only, in which most papilionoid genera were sampled. A ML tree based on the entire plastome maximally supported all of the deep and most recent divergences of papilionoids (223 out of 236 nodes). The Swartzieae, ADA (Angylocalyceae, Dipterygeae, and Amburaneae), Cladrastis, Andira, and Exostyleae clades formed a grade to the remainder of the Papilionoideae, concordant with nine ML and two MSC trees. Phylogenetic relationships among the remaining five papilionoid lineages (Vataireoid, Dermatophyllum, Genistoid s.l., Dalbergioid s.l., and Baphieae + Non-Protein Amino Acid Accumulating or NPAAA clade) remained uncertain, because of insufficient support and/or conflicting relationships among trees. Our study fully resolved most of the deep nodes of Papilionoideae, however, some relationships require further exploration. More genome-scale data and rigorous analyses are needed to disentangle phylogenetic relationships among the five remaining lineages.

The complete chloroplast genome of Onobrychis gaubae (Fabaceae-Papilionoideae): comparative analysis with related IR-lacking clade species

BACKGROUND

Plastome (Plastid genome) sequences provide valuable markers for surveying evolutionary relationships and population genetics of plant species. Papilionoideae (papilionoids) has different nucleotide and structural variations in plastomes, which makes it an ideal model for genome evolution studies. Therefore, by sequencing the complete chloroplast genome of Onobrychis gaubae in this study, the characteristics and evolutionary patterns of plastome variations in IR-loss clade were compared.

RESULTS

In the present study, the complete plastid genome of O. gaubae, endemic to Iran, was sequenced using Illumina paired-end sequencing and was compared with previously known genomes of the IRLC species of legumes. The O. gaubae plastid genome was 122,688 bp in length and included a large single-copy (LSC) region of 81,486 bp, a small single-copy (SSC) region of 13,805 bp and one copy of the inverted repeat (IR_b) of 29,100 bp. The genome encoded 110 genes, including 76 protein-coding genes, 30 transfer RNA (tRNA) genes and four ribosome RNA (rRNA) genes and possessed 83 simple sequence repeats (SSRs) and 50 repeated structures with the highest proportion in the LSC. Comparative analysis of the chloroplast genomes across IRLC revealed three hotspot genes (ycf1, ycf2, clpP) which could be used as DNA barcode regions. Moreover, seven hypervariable regions [trnL(UAA)-trnT(UGU), trnT(GGU)-trnE(UUC), ycf1, ycf2, ycf4, accD and clpP] were identified within Onobrychis, which could be used to distinguish the Onobrychis species. Phylogenetic analyses revealed that O. gaubae is closely related to Hedysarum. The complete O. gaubae genome is a valuable resource for investigating evolution of Onobrychis species and can be used to identify related species.

CONCLUSIONS

Our results reveal that the plastomes of the IRLC are dynamic molecules and show multiple gene losses and inversions. The identified hypervariable regions could be used as molecular markers for resolving phylogenetic relationships and species identification and also provide new insights into plastome evolution across IRLC.