The Possible Earliest Allopolyploidization in Tracheophytes Revealed by Phylotranscriptomics and Morphology of Selaginellaceae.

Selaginellaceae, originated in the Carboniferous and survived the Permian-Triassic mass extinction, is the largest family of lycophyte, which is sister to other tracheophytes. It stands out from tracheophytes by exhibiting extraordinary habitat diversity and lacking polyploidization. The organelle genome-based phylogenies confirmed the monophyly of Selaginella, with six or seven subgenera grouped into two superclades, but the phylogenetic positions of the enigmatic Selaginella sanguinolenta clade remained problematic. Here, we conducted a phylogenomic study on Selaginellaceae utilizing large-scale nuclear gene data from RNA-seq to elucidate the phylogeny and explore the causes of the phylogenetic incongruence of the S. sanguinolenta clade. Our phylogenetic analyses resolved three different positions of the S. sanguinolenta clade, which were supported by the sorted three nuclear gene sets, respectively. The results from the gene flow test, species network inference, and plastome-based phylogeny congruently suggested a probable hybrid origin of the S. sanguinolenta clade involving each common ancestor of the two superclades in Selaginellaceae. The hybrid hypothesis is corroborated by the evidence from rhizophore morphology and spore micromorphology. The chromosome observation and Ks distributions further suggested hybridization accompanied by polyploidization. Divergence time estimation based on independent datasets from nuclear gene sets and plastid genome data congruently inferred that allopolyploidization occurred in the Early Triassic. To our best knowledge, the allopolyploidization in the Mesozoic reported here represents the earliest record of tracheophytes. Our study revealed a unique triad of phylogenetic positions for a hybrid-originated group with comprehensive evidence and proposed a hypothesis for retaining both parental alleles through gene conversion.

The evolution of extremely diverged plastomes in Selaginellaceae (lycophyte) is driven by repeat patterns and the underlying DNA maintenance machinery.

Two factors are proposed to account for the unusual features of organellar genomes: the disruptions of organelle-targeted DNA replication, repair, and recombination (DNA-RRR) systems in the nuclear genome and repetitive elements in organellar genomes. Little is known about how these factors affect organellar genome evolution. The deep-branching vascular plant family Selaginellaceae is known to have a deficient DNA-RRR system and convergently evolved organellar genomes. However, we found that the plastid genome (plastome) of Selaginella sinensis has extremely accelerated substitution rates, a low GC content, pervasive repeat elements, a dynamic network structure, and it lacks direct or inverted repeats. Unexpectedly, its organelle DNA-RRR system is short of a plastid-targeted Recombinase A1 (RecA1) and a mitochondrion-targeted RecA3, in line with other explored Selaginella species. The plastome contains a large collection of short- and medium-sized repeats. Given the absence of RecA1 surveillance, we propose that these repeats trigger illegitimate recombination, accelerated mutation rates, and structural instability. The correlations between repeat quantity and architectural complexity in the Selaginella plastomes support these conclusions. We, therefore, hypothesize that the interplay of the deficient DNA-RRR system and the high repeat content has led to the extraordinary divergence of the S. sinensis plastome. Our study not only sheds new light on the mechanism of plastome divergence by emphasizing the power of cytonuclear integration, but it also reconciles the longstanding contradiction on the effects of DNA-RRR system disruption on genome structure evolution.

The first homosporous lycophyte genome revealed the association between the recent dynamic accumulation of LTR-RTs and genome size variation.

The contrasting genome size between homosporous and heterosporous plants is fascinating. Different from the heterosporous seed plants and mainly homosporous ferns, the lycophytes are either heterosporous (Isoetales and Selaginellales) or homosporous (Lycopodiales). Many lycophytes are the resource plants of Huperzine A (HupA) which is invaluable for treating Alzheimer's disease. For the seed-free vascular plants, several high-quality genomes of heterosporous Selaginella, homosporous ferns (maidenhair fern, monkey spider tree fern), and heterosporous ferns (Azolla) have been published and provided important insights into the origin and evolution of early land plants. However, the homosporous lycophyte genome has not been decoded. Here, we assembled the first homosporous lycophyte genome and conducted comparative genomic analyses by applying a reformed pipeline for filtering out non-plant sequences. The obtained genome size of Lycopodium clavatum is 2.30 Gb, distinguished in more than 85% repetitive elements of which 62% is long terminal repeat (LTR). This study disclosed a high birth rate and a low death rate of the LTR-RTs in homosporous lycophytes, but the opposite occurs in heterosporous lycophytes. we propose that the recent activity of LTR-RT is responsible for the immense genome size variation between homosporous and heterosporous lycophytes. By combing Ks analysis with a phylogenetic approach, we discovered two whole genome duplications (WGD). Morover, we identified all the five recognized key enzymes for the HupA biosynthetic pathway in the L. clavatum genome, but found this pathway incomplete in other major lineages of land plants. Overall, this study is of great importance for the medicinal utilization of lycophytes and the decoded genome data will be a key cornerstone to elucidate the evolution and biology of early vascular land plants.

Genome sequence and population genomics provide insights into chromosomal evolution and phytochemical innovation of Hippophae rhamnoides.

Plants of the Elaeagnaceae family are widely used to treat various health disorders owing to their natural phytochemicals. Seabuckthorn (Hippophae rhamnoides L.) is an economically and ecologically important species within the family with richness of biologically and pharmacologically active substances. Here, we present a chromosome-level genome assembly of seabuckthorn (http://hipp.shengxin.ren/), the first genome sequence of Elaeagnaceae, which has a total length of 849.04 Mb with scaffold N50 of 69.52 Mb and 30 864 annotated genes. Two sequential tetraploidizations with one occurring ~36-41 million years ago (Mya) and the last ~24-27 Mya were inferred, resulting in expansion of genes related to ascorbate and aldarate metabolism, lipid biosynthesis, and fatty acid elongation. Comparative genomic analysis reconstructed the evolutionary trajectories of the seabuckthorn genome with the predicted ancestral genome of 14 proto-chromosomes. Comparative transcriptomic and metabonomic analyses identified some key genes contributing to high content of polyunsaturated fatty acids and ascorbic acid (AsA). Additionally, we generated and analysed 55 whole-genome sequences of diverse accessions, and identified 9.80 million genetic variants in the seabuckthorn germplasms. Intriguingly, genes in selective sweep regions identified through population genomic analysis appeared to contribute to the richness of AsA and fatty acid in seabuckthorn fruits, among which GalLDH, GMPase and ACC, TER were the potentially major-effect causative genes controlling AsA and fatty acid content of the fruit, respectively. Our research offers novel insights into the molecular basis underlying phytochemical innovation of seabuckthorn, and provides valuable resources for exploring the evolution of the Elaeagnaceae family and molecular breeding.

Conversion between 100-million-year-old duplicated genes contributes to rice subspecies divergence.

BACKGROUND: Duplicated gene pairs produced by ancient polyploidy maintain high sequence similarity over a long period of time and may result from illegitimate recombination between homeologous chromosomes. The genomes of Asian cultivated rice Oryza sativa ssp. indica (XI) and Oryza sativa ssp. japonica (GJ) have recently been updated, providing new opportunities for investigating ongoing gene conversion events and their impact on genome evolution. RESULTS: Using comparative genomics and phylogenetic analyses, we evaluated gene conversion rates between duplicated genes produced by polyploidization 100 million years ago (mya) in GJ and XI. At least 5.19-5.77% of genes duplicated across the three rice genomes were affected by whole-gene conversion after the divergence of GJ and XI at ~ 0.4 mya, with more (7.77-9.53%) showing conversion of only portions of genes. Independently converted duplicates surviving in the genomes of different subspecies often use the same donor genes. The ongoing gene conversion frequency was higher near chromosome termini, with a single pair of homoeologous chromosomes, 11 and 12, in each rice genome being most affected. Notably, ongoing gene conversion has maintained similarity between very ancient duplicates, provided opportunities for further gene conversion, and accelerated rice divergence. Chromosome rearrangements after polyploidization are associated with ongoing gene conversion events, and they directly restrict recombination and inhibit duplicated gene conversion between homeologous regions. Furthermore, we found that the converted genes tended to have more similar expression patterns than nonconverted duplicates. Gene conversion affects biological functions associated with multiple genes, such as catalytic activity, implying opportunities for interaction among members of large gene families, such as NBS-LRR disease-resistance genes, contributing to the occurrence of the gene conversion. CONCLUSION: Duplicated genes in rice subspecies generated by grass polyploidization ~ 100 mya remain affected by gene conversion at high frequency, with important implications for the divergence of rice subspecies.

Sequential Paleotetraploidization shaped the carrot genome.

BACKGROUND: Carrot (Daucus carota subsp. carota L.) is an important root crop with an available high-quality genome. The carrot genome is thought to have undergone recursive paleo-polyploidization, but the extent, occurrences, and nature of these events are not clearly defined. RESULTS: Using a previously published comparative genomics pipeline, we reanalysed the carrot genome and characterized genomic fractionation, as well as gene loss and retention, after each of the two tetraploidization events and inferred a dominant and sensitive subgenome for each event. In particular, we found strong evidence of two sequential tetraploidization events, with one (Dc-α) approximately 46-52 million years ago (Mya) and the other (Dc-ß) approximately 77-87 Mya, both likely allotetraploidization in nature. The Dc-ß event was likely common to all Apiales plants, occurring around the divergence of Apiales-Bruniales and after the divergence of Apiales-Asterales, likely playing an important role in the derivation and divergence of Apiales species. Furthermore, we found that rounds of polyploidy events contributed to the expansion of gene families responsible for plastidial methylerythritol phosphate (MEP), the precursor of carotenoid accumulation, and shaped underlying regulatory pathways. The alignment of orthologous and paralogous genes related to different events of polyploidization and speciation constitutes a comparative genomics platform for studying Apiales, Asterales, and many other related species. CONCLUSIONS: Hierarchical inference of homology revealed two tetraploidization events that shaped the carrot genome, which likely contributed to the successful establishment of Apiales plants and the expansion of MEP, upstream of the carotenoid accumulation pathway.

Deciphering the high-quality genome sequence of coriander that causes controversial feelings.

Coriander (Coriandrum sativum L. 2n = 2x = 22), a plant from the Apiaceae family, also called cilantro or Chinese parsley, is a globally important crop used as vegetable, spice, fragrance and traditional medicine. Here, we report a high-quality assembly and analysis of its genome sequence, anchored to 11 chromosomes, with total length of 2118.68 Mb and N50 scaffold length of 160.99 Mb. We found that two whole-genome duplication events, respectively, dated to ~45-52 and ~54-61 million years ago, were shared by the Apiaceae family after their split from lettuce. Unbalanced gene loss and expression are observed between duplicated copies produced by these two events. Gene retention, expression, metabolomics and comparative genomic analyses of terpene synthase (TPS) gene family, involved in terpenoid biosynthesis pathway contributing to coriander's special flavour, revealed that tandem duplication contributed to coriander TPS gene family expansion, especially compared to their carrot counterparts. Notably, a TPS gene highly expressed in all 4 tissues and 3 development stages studied is likely a major-effect gene encoding linalool synthase and myrcene synthase. The present genome sequencing, transcriptome, metabolome and comparative genomic efforts provide valuable insights into the genome evolution and spice trait biology of Apiaceae and other related plants, and facilitated further research into important gene functions and crop improvement.

Recursive Paleohexaploidization Shaped the Durian Genome.

The durian (Durio zibethinus) genome has recently become available, and analysis of this genome reveals two paleopolyploidization events previously inferred as shared with cotton (Gossypium spp.). Here, we reanalyzed the durian genome in comparison with other well-characterized genomes. We found that durian and cotton were actually affected by different polyploidization events: hexaploidization in durian ∼19-21 million years ago (mya) and decaploidization in cotton ∼13-14 mya. Previous interpretations of shared polyploidization events may have resulted from the elevated evolutionary rates in cotton genes due to the decaploidization and insufficient consideration of the complexity of plant genomes. The decaploidization elevated evolutionary rates of cotton genes by ∼64% compared to durian and explained a previous ∼4-fold over dating of the event. In contrast, the hexaploidization in durian did not prominently elevate gene evolutionary rates, likely due to its long generation time. Moreover, divergent evolutionary rates probably explain 98.4% of reconstructed phylogenetic trees of homologous genes being incongruent with expected topology. The findings provide further insight into the roles played by polypoidization in the evolution of genomes and genes, and they suggest revisiting existing reconstructed phylogenetic trees.

Reconstruction of evolutionary trajectories of chromosomes unraveled independent genomic repatterning between Triticeae and Brachypodium.

BACKGROUND: After polyploidization, a genome may experience large-scale genome-repatterning, featuring wide-spread DNA rearrangement and loss, and often chromosome number reduction. Grasses share a common tetraploidization, after which the originally doubled chromosome numbers reduced to different chromosome numbers among them. A telomere-centric reduction model was proposed previously to explain chromosome number reduction. With Brachpodium as an intermediate linking different major lineages of grasses and a model plant of the Pooideae plants, we wonder whether it mediated the evolution from ancestral grass karyotype to Triticeae karyotype. RESULTS: By inferring the homology among Triticeae, rice, and Brachpodium chromosomes, we reconstructed the evolutionary trajectories of the Triticeae chromosomes. By performing comparative genomics analysis with rice as a reference, we reconstructed the evolutionary trajectories of Pooideae plants, including Ae. Tauschii (2n = 14, DD), barley (2n = 14), Triticum turgidum (2n = 4x = 28, AABB), and Brachypodium (2n = 10). Their extant Pooidea and Brachypodium chromosomes were independently produced after sequential nested chromosome fusions in the last tens of millions of years, respectively, after their split from rice. More frequently than would be expected by chance, in Brachypodium, the 'invading' and 'invaded' chromosomes are homoeologs, originating from duplication of a common ancestral chromosome, that is, with more extensive DNA-level correspondence to one another than random chromosomes, nested chromosome fusion events between homoeologs account for three of seven cases in Brachypodium (P-value≈0.00078). However, this phenomenon was not observed during the formation of other Pooideae chromosomes. CONCLUSIONS: Notably, we found that the Brachypodium chromosomes formed through exclusively distinctive trajectories from those of Pooideae plants, and were well explained by the telomere-centric model. Our work will contribute to understanding the structural and functional innovation of chromosomes in different Pooideae lineages and beyond.

An Overlooked Paleotetraploidization in Cucurbitaceae.

Cucurbitaceae plants are of considerable biological and economic importance, and genomes of cucumber, watermelon, and melon have been sequenced. However, a comparative genomics exploration of their genome structures and evolution has not been available. Here, we aimed at performing a hierarchical inference of genomic homology resulted from recursive paleopolyploidizations. Unexpectedly, we found that, shortly after a core-eudicot-common hexaploidy, a cucurbit-common tetraploidization (CCT) occurred, overlooked by previous reports. Moreover, we characterized gene loss (and retention) after these respective events, which were significantly unbalanced between inferred subgenomes, and between plants after their split. The inference of a dominant subgenome and a sensitive one suggested an allotetraploid nature of the CCT. Besides, we found divergent evolutionary rates among cucurbits, and after doing rate correction, we dated the CCT to be 90-102 Ma, likely common to all Cucurbitaceae plants, showing its important role in the establishment of the plant family.

Hierarchically Aligning 10 Legume Genomes Establishes a Family-Level Genomics Platform.

Mainly due to their economic importance, genomes of 10 legumes, including soybean (Glycine max), wild peanut (Arachis duranensis and Arachis ipaensis), and barrel medic (Medicago truncatula), have been sequenced. However, a family-level comparative genomics analysis has been unavailable. With grape (Vitis vinifera) and selected legume genomes as outgroups, we managed to perform a hierarchical and event-related alignment of these genomes and deconvoluted layers of homologous regions produced by ancestral polyploidizations or speciations. Consequently, we illustrated genomic fractionation characterized by widespread gene losses after the polyploidizations. Notably, high similarity in gene retention between recently duplicated chromosomes in soybean supported the likely autopolyploidy nature of its tetraploid ancestor. Moreover, although most gene losses were nearly random, largely but not fully described by geometric distribution, we showed that polyploidization contributed divergently to the copy number variation of important gene families. Besides, we showed significantly divergent evolutionary levels among legumes and, by performing synonymous nucleotide substitutions at synonymous sites correction, redated major evolutionary events during their expansion. This effort laid a solid foundation for further genomics exploration in the legume research community and beyond. We describe only a tiny fraction of legume comparative genomics analysis that we performed; more information was stored in the newly constructed Legume Comparative Genomics Research Platform (www.legumegrp.org).

Genome structure and evolutionary history of frankincense producing Boswellia sacra.

Boswellia sacra Flueck (family Burseraceae) tree is wounded to produce frankincense. We report its de novo assembled genome (667.8 Mb) comprising 18,564 high-confidence protein-encoding genes. Comparing conserved single-copy genes across eudicots suggest >97% gene space assembly of B. sacra genome. Evolutionary history shows B. sacra gene-duplications derived from recent paralogous events and retained from ancient hexaploidy shared with other eudicots. The genome indicated a major expansion of Gypsy retroelements in last 2 million years. The B. sacra genetic diversity showed four clades intermixed with a primary genotype-dominating most resin-productive trees. Further, the stem transcriptome revealed that wounding concurrently activates phytohormones signaling, cell wall fortification, and resin terpenoid biosynthesis pathways leading to the synthesis of boswellic acid-a key chemotaxonomic marker of Boswellia. The sequence datasets reported here will serve as a foundation to investigate the genetic determinants of frankincense and other resin-producing species in Burseraceae.

Chromosome-length genome assemblies of six legume species provide insights into genome organization, evolution, and agronomic traits for crop improvement.

INTRODUCTION: Legume crops are an important source of protein and oil for human health and in fixing atmospheric N2 for soil enrichment. With an objective to accelerate much-needed genetic analyses and breeding applications, draft genome assemblies were generated in several legume crops; many of them are not high quality because they are mainly based on short reads. However, the superior quality of genome assembly is crucial for a detailed understanding of genomic architecture, genome evolution, and crop improvement. OBJECTIVES: Present study was undertaken with an objective of developing improved chromosome-length genome assemblies in six different legumes followed by their systematic investigation to unravel different aspects of genome organization and legume evolution. METHODS: We employed in situ Hi-C data to improve the existing draft genomes and performed different evolutionary and comparative analyses using improved genome assemblies. RESULTS: We have developed chromosome-length genome assemblies in chickpea, pigeonpea, soybean, subterranean clover, and two wild progenitor species of cultivated groundnut (A. duranensis and A. ipaensis). A comprehensive comparative analysis of these genome assemblies offered improved insights into various evolutionary events that shaped the present-day legume species. We highlighted the expansion of gene families contributing to unique traits such as nodulation in legumes, gravitropism in groundnut, and oil biosynthesis in oilseed legume crops such as groundnut and soybean. As examples, we have demonstrated the utility of improved genome assemblies for enhancing the resolution of "QTL-hotspot" identification for drought tolerance in chickpea and marker-trait associations for agronomic traits in pigeonpea through genome-wide association study. Genomic resources developed in this study are publicly available through an online repository, 'Legumepedia'. CONCLUSION: This study reports chromosome-length genome assemblies of six legume species and demonstrates the utility of these assemblies in crop improvement. The genomic resources developed here will have significant role in accelerating genetic improvement applications of legume crops.

Cotton Duplicated Genes Produced by Polyploidy Show Significantly Elevated and Unbalanced Evolutionary Rates, Overwhelmingly Perturbing Gene Tree Topology.

A phylogenetic tree can be used to illustrate the evolutionary relationship between a group of genes, especially duplicated genes, which are sources of genetic innovation and are often a hotspot of research. However, duplicated genes may have complex phylogenetic topologies due to changes in their evolutionary rates. Here, by constructing phylogenetic trees using different methods, we evaluated the phylogenetic relationships of duplicated genes produced by polyploidization in cotton. We found that at least 83.2% of phylogenetic trees did not conform the expected topology. Moreover, cotton homologous gene copy number has little effect on the topology of duplicated genes. Compared with their cacao orthologs, elevated evolutionary rates of cotton genes are responsible for distorted tree topology. Furthermore, as to both branch and site models, we inferred that positive natural selection during the divergence of fiber-development-related MYB genes was likely, and found that the reconstructed tree topology may often overestimate natural selection, as compared to the inference with the expected trees. Therefore, we emphasize the importance of borrowing precious information from gene collinearity in tree construction and evaluation, and have evidence to alert the citation of thousands of previous reports of adaptivity and functional innovation of duplicated genes.

Illegitimate Recombination Between Homeologous Genes in Wheat Genome.

Polyploidies produce a large number of duplicated regions and genes in genomes, which have a long-term impact and stimulate genetic innovation. The high similarity between homeologous chromosomes, forming different subgenomes, or homologous regions after genome repatterning, may permit illegitimate DNA recombination. Here, based on gene colinearity, we aligned the (sub)genomes of common wheat (Triticum aestivum, AABBDD genotype) and its relatives, including Triticum urartu (AA), Aegilops tauschii (DD), and T. turgidum ssp. dicoccoides (AABB) to detect the homeologous (paralogous or orthologous) colinear genes within and between (sub)genomes. Besides, we inferred more ancient paralogous regions produced by a much ancient grass-common tetraploidization. By comparing the sequence similarity between paralogous and orthologous genes, we assumed abnormality in the topology of constructed gene trees, which could be explained by gene conversion as a result of illegitimate recombination. We found large numbers of inferred converted genes (>2,000 gene pairs) suggested long-lasting genome instability of the hexaploid plant, and preferential donor roles by DD genes. Though illegitimate recombination was much restricted, duplicated genes produced by an ancient whole-genome duplication, which occurred millions of years ago, also showed evidence of likely gene conversion. As to biological function, we found that ~40% catalytic genes in colinearity, including those involved in starch biosynthesis, were likely affected by gene conversion. The present study will contribute to understanding the functional and structural innovation of the common wheat genome.

Paleo-polyploidization in Lycophytes.

Lycophytes and seed plants constitute the typical vascular plants. Lycophytes have been thought to have no paleo-polyploidization although the event is known to be critical for the fast expansion of seed plants. Here, genomic analyses including the homologous gene dot plot analysis detected multiple paleo-polyploidization events, with one occurring approximately 13-15 million years ago (MYA) and another about 125-142 MYA, during the evolution of the genome of Selaginella moellendorffii, a model lycophyte. In addition, comparative analysis of reconstructed ancestral genomes of lycophytes and angiosperms suggested that lycophytes were affected by more paleo-polyploidization events than seed plants. Results from the present genomic analyses indicate that paleo-polyploidization has contributed to the successful establishment of both lineages-lycophytes and seed plants-of vascular plants.

Preferential gene retention increases the robustness of cold regulation in Brassicaceae and other plants after polyploidization.

Cold stress profoundly affects plant growth and development and is a key factor affecting the geographic distribution and evolution of plants. Plants have evolved adaptive mechanisms to cope with cold stress. Here, through the genomic analysis of Arabidopsis, three Brassica species and 17 other representative species, we found that both cold-related genes (CRGs) and their collinearity were preferentially retained after polyploidization followed by genome instability, while genome-wide gene sets exhibited a variety of other expansion mechanisms. The cold-related regulatory network was increased in Brassicaceae genomes, which were recursively affected by polyploidization. By combining our findings regarding the selective retention of CRGs from this ecological genomics study with the available knowledge of cold-induced chromosome doubling, we hypothesize that cold stress may have contributed to the success of polyploid plants through both increasing polyploidization and selectively maintaining CRGs during evolution. This hypothesis requires further biological and ecological exploration to obtain solid supporting evidence, which will potentially contribute to understanding the generation of polyploids and to the field of ecological genomics.

Alignment of Rutaceae Genomes Reveals Lower Genome Fractionation Level Than Eudicot Genomes Affected by Extra Polyploidization.

Owing to their nutritional and commercial values, the genomes of several citrus plants have been sequenced, and the genome of one close relative in the Rutaceae family, atalantia (Atalantia buxifolia), has also been sequenced. Here, we show a family-level comparative analysis of Rutaceae genomes. By using grape as the outgroup and checking cross-genome gene collinearity, we systematically performed a hierarchical and event-related alignment of Rutaceae genomes, and produced a gene list defining homologous regions based on ancestral polyploidization or speciation. We characterized genome fractionation resulting from gene loss or relocation, and found that erosion of gene collinearity could largely be described by a geometric distribution. Moreover, we found that well-assembled Rutaceae genomes retained significantly more genes (65-82%) than other eudicots affected by recursive polyploidization. Additionally, we showed divergent evolutionary rates among Rutaceae plants, with sweet orange evolving faster than others, and by performing evolutionary rate correction, re-dated major evolutionary events during their evolution. We deduced that the divergence between the Rutaceae family and grape occurred about 81.15-91.74 million years ago (mya), while the split between citrus and atalantia plants occurred <10 mya. In addition, we showed that polyploidization led to a copy number expansion of key gene families contributing to the biosynthesis of vitamin C. Overall, the present effort provides an important comparative genomics resource and lays a foundation to understand the evolution and functional innovation of Rutaceae genomes.

The wax gourd genomes offer insights into the genetic diversity and ancestral cucurbit karyotype.

The botanical family Cucurbitaceae includes a variety of fruit crops with global or local economic importance. How their genomes evolve and the genetic basis of diversity remain largely unexplored. In this study, we sequence the genome of the wax gourd (Benincasa hispida), which bears giant fruit up to 80 cm in length and weighing over 20 kg. Comparative analyses of six cucurbit genomes reveal that the wax gourd genome represents the most ancestral karyotype, with the predicted ancestral genome having 15 proto-chromosomes. We also resequence 146 lines of diverse germplasm and build a variation map consisting of 16 million variations. Combining population genetics and linkage mapping, we identify a number of regions/genes potentially selected during domestication and improvement, some of which likely contribute to the large fruit size in wax gourds. Our analyses of these data help to understand genome evolution and function in cucurbits.

Polyploidy Index and Its Implications for the Evolution of Polyploids.

Polyploidy has contributed to the divergence and domestication of plants; however, estimation of the relative roles that different types of polyploidy have played during evolution has been difficult. Unbalanced and balanced gene removal was previously related to allopolyploidies and autopolyploidies, respectively. Here, to infer the types of polyploidies and evaluate their evolutionary effects, we devised a statistic, the Polyploidy-index or P-index, to characterize the degree of divergence between subgenomes of a polyploidy, to find whether there has been a balanced or unbalanced gene removal from the homoeologous regions. Based on a P-index threshold of 0.3 that distinguishes between known or previously inferred allo- or autopolyploidies, we found that 87.5% of 24 angiosperm paleo-polyploidies were likely produced by allopolyploidizations, responsible for establishment of major tribes such as Poaceae and Fabaceae, and large groups such as monocots and eudicots. These findings suggest that >99.7% of plant genomes likely derived directly from allopolyploidies, with autopolyploidies responsible for the establishment of only a few small genera, including Glycine, Malus, and Populus, each containing tens of species. Overall, these findings show that polyploids with high divergence between subgenomes (presumably allopolyploids) established the major plant groups, possibly through secondary contact between previously isolated populations and hybrid vigor associated with their re-joining.