Widespread Whole Genome Duplications Contribute to Genome Complexity and Species Diversity in Angiosperms

Gene duplications provide evolutionary potentials for generating novel functions, while polyploidization or whole genome duplication （WGD） doubles the chromosomes initially and results in hundreds to thousands of retained duplicates. WGDs are strongly supported by evidence commonly found in many species-rich lineages of eukaryotes, and thus are considered as a major driving force in species diversification. We per- formed comparative genomic and phylogenomic analyses of 59 public genomes/transcriptomes and 46 newly sequenced transcriptomes covering major lineages of angiosperms to detect large-scale gene dupli- cation events by surveying tens of thousands of gene family trees. These analyses confirmed most of the previously reported WGDs and provided strong evidence for novel ones in many lineages. The detected WGDs supported a model of exponential gene loss during evolution with an estimated half-life of approx- imately 21.6 million years, and were correlated with both the emergence of lineages with high degrees of diversification and periods of global climate changes. The new datasets and analyses detected many novel WGDs widely spread during angiosperm evolution, uncovered preferential retention of gene functions in essential cellular metabolisms, and provided clues for the roles of WGD in promoting angiosperm radiation and enhancing their adaptation to environmental changes.

Functional Divergence between Subgenomes anc Gene Pairs after Whole Genome Duplications

Gene loss following whole genome duplication （WGD） is often biased, with one subgenome retaining more ancestral genes and the other sustaining more gene deletions. While bias toward the greater expression of gene copies on one subgenome can explain bias in gene loss, this raises the question to what drives differences in gene expression levels between subgenomes. Differences in chromatin modifications and epigenetic markers between subgenomes in several model species are now being identified, providing an explanation for bias in gene expression between subgenomes. WGDs can be classified into duplications with higher, biased gene loss and bias in gene expression between subgenomes versus those with lower, unbiased rates of gene loss and an absence of detectable bias between subgenomes; however, the origi- nally proposed link between these two classes and whether WGD results from an allo- or autopolyploid event is inconsistent with recent data from the allopolyploid Capsella bursa-pastoris. The gene balance hypothesis can explain bias in the functional categories of genes retained following WGD, the difference in gene loss rates between unbiased and biased WGDs, and how plant genomes have avoided being overrun with genes encoding dose-sensitive subunits of multiprotein complexes. Comparisons of gene expression patterns between retained transcription factor pairs in maize suggest the high degree of retention for WGD-derived pairs of transcription factors may instead be explained by the older duplication-degeneration-complementation model.

Lycophyte transcriptomes reveal two whole-genome duplications in Lycopodiaceae:Insights into the polyploidization of Phlegmariurus

Lycophytes are an ancient clade of the non-flowering vascular plants with chromosome numbers that vary from tens to hundreds.They are an excellent study system for examining whole-genome duplications(WGDs),or polyploidization,in spore-dispersed vascular plants.However,a lack of genome sequence data limits the reliable detection of very ancient WGDs,small-scale duplications(SSDs),and recent WGDs.Here,we integrated phylogenomic analysis and the distribution of synonymous substitutions per synonymous sites(Ks)of the transcriptomes of 13 species of lycophytes to identify,locate,and date multiple WGDs in the lycophyte family Lycopodiaceae.Additionally,we examined the genus Phlegmariurus for signs of genetic discordance,which can provide valuable insight into the underlying causes of such conflict(e.g.,hybridization,incomplete lineage sorting,or horizontal gene transfer).We found strong evidence that two WGD events occurred along the phylogenetic backbone of Lycopodiaceae,with one occurring in the common ancestor of extant Phlegmariurus(Lycopodiaceae)approximately 22-23 million years ago(Mya)and the other occurring in the common ancestor of Lycopodiaceae around 206-214 Mya.Interestingly,we found significant genetic discordance in the genus Phlegmariurus,indicating that the genus has a complex evolutionary history.This study provides molecular evidence for multiple WGDs in Lycopodiaceae and offers phylogenetic clues to the evolutionary history of Lycopodiaceae.

Whole Genome Duplication of Intra- and Inter-chromosomes in the Tomato Genome

Whole genome duplication （WGD） events have been proven to occur in the evolutionary history of most angiosperms. Tomato is considered a model species of the Solanaceae family. In this study, we describe the details of the evolutionary process of the tomato genome by detecting collinearity blocks and dating the WGD events on the tree of life by combining two different methods： synonymous substitution rates （Ks） and phylogenetic trees. In total, 593 collinearity blocks were discovered out of 12 pseudo-chromosomes con- structed. It was evident that chromosome 2 had experienced an intra-chromosomat duplication event. Major inter-chromosomal dupli- cation occurred among all the pseudo-chromosome. We calculated the Ks value of these collinearity blocks. Two peaks of Ks distribution were found, corresponding to two WGD events occurring approximately 36-82 million years ago （MYA） and 148--205 MYA. Additionally, the results of phylogenetic trees suggested that the more recent WGD event may have occurred after the divergence of the rosidasterid clade, but before the major diversification in Solanaceae. The older WGD event was shown to have occurred before the divergence of the rosid-asterid clade and after the divergence of rice-Arabidopsis （monocot-dicot）.

The impact of tandem duplication on gene evolution in Solanaceae species

Whole genome duplication(WGD) and tandem duplication(TD) are important modes of gene amplification and functional innovation, and they are common in plant genome evolution. We analyzed the genomes of three Solanaceae species(Solanum lycopersicum, Capsicum annuum, and Petunia inflata), which share a common distant ancestor with Vitis vinifera, Theobroma cacao, and Coffea canephora but have undergone an extra whole genome triplication(WGT) event. The analysis was used to investigate the phenomenon of tandem gene evolution with(S. lycopersicum) or without WGT(V. vinifera). Among the tandem gene arrays in these genomes, we found that V. vinifera, which has not experienced the WGT event, retained relatively more and larger tandem duplicated gene(TDG) clusters than the Solanaceae species that experienced the WGT event. Larger TDG clusters tend to be derived from older TD events, so this indicates that continuous TDGs(absolute dosage) accumulated during long-term evolution. In addition, WGD and TD show a significant bias in the functional categories of the genes retained. WGD tends to retain dose-sensitive genes related to biological processes, including DNA-binding and transcription factor activity, while TD tends to retain genes involved in stress resistance. WGD and TD also provide more possibilities for gene functional innovation through gene fusion and fission. The TDG cluster containing the tomato fusarium wilt resistance gene I3 contains 15 genes, and one of these genes, Solyc07g055560, has undergone a fusion event after the duplication events. These data provide evidence that helps explain the new functionalization of TDGs in adapting to environmental changes.

Comparative DNA-methylome and transcriptome analysis reveals heterosis-and polyploidy-associated epigenetic changes in rice

Heterosis and polyploidy have an overwhelming influence on plant evolution.Recently,polyploid rice hybrids have been used to breed new rice varieties because they combine the advantages of both heterosis and polyploidy.In this study,we generated six rice lines:autotetraploid rice hybrids and their autotetraploid parents,diploid donors,and hybrids of the diploid donors.To investigate the molecular mechanism controlling the effects of both hybridization and polyploidization,we performed bisulfite and RNA sequencing on young panicles at the pollen meiosis stage to compare the DNA metabolomes and transcriptomes among the six rice lines.The hybrids lines were hypermethylated compared to their corresponding parents and the autotetraploid lines showed globally increased DNA methylation of their transposable elements compared to the diploid donors.The alteration in DNA methylation level corresponded to the differential gene expressions among the rice genotypes,suggesting that methylation changes induced by polyploidization and hybridization may affect gene expression.Groups of gene candidates were identified that may be associated with heterosis and polyploidy.Our results provide DNA information that can be used to investigate epigenetic modification during heterosis and polyploidy in rice.

Upgraded durian genome reveals the role of chromosome reshuffling during ancestral karyotype evolution,lignin biosynthesis regulation,and stress tolerance

Durian(Durio zibethinus)is a tropical fruit that has a unique flavor and aroma.It occupies a significant phylogenetic position within the Malvaceae family.Extant core-eudicot plants are reported to share seven ancestral karyotypes that have undergone reshuffling,resulting in an abundant genomic diversity.However,the ancestral karyotypes of the Malvaceae family,as well as the evolution trajectory leading to the28 chromosomes in durian,remain poorly understood.Here,we report the high-quality assembly of the durian genome with comprehensive comparative genomic analyses.By analyzing the collinear blocks between cacao and durian,we inferred 11 Malvaceae ancestral karyotypes.These blocks were present in a single-copy form in cacao and mainly in triplicates in durian,possibly resulting from a recent whole genome triplication(WGT)event that led to hexaploidization of the durian genome around 20(17–24)million years ago.A large proportion of the duplicated genes in durian,such as those involved in the lignin biosynthesis module for phenylpropane biosynthesis,are derived directly from whole genome duplication,which makes it an important force in reshaping its genomic architecture.Transcriptome studies have revealed that genes involved in feruloyl-Co A formations were highly preferentially expressed in fruit peels,indicating that the thorns produced on durian fruit may comprise guaiacyl and syringyl lignins.Among all the analyzed transcription factors(TFs),members of the heat shock factor family(HSF)were the most significantly upregulated under heat stress.All subfamilies of genes encoding heat shock proteins(HSPs)in the durian genome appear to have undergone expansion.The potential interactions between HSF Dzi05.397 and HSPs were examined and experimentally verified.Our study provides a high-quality durian genome and reveals the reshuffling mechanism of ancestral Malvaceae chromosomes to produce the durian genome.We also provide insights into the mechanism underlying lignin biosynthesis and heat stress tolerance.

The Tartary Buckwheat Genome Provides Insights into Rutin Biosynthesis and Abiotic Stress Tolerance

Tartary buckwheat （Fagopyrum tataricum） is an important pseudocereal crop that is strongly adapted to growth in adverse environments. Its gluten-free grain contains complete proteins with a well-balanced composition of essential amino acids and is a rich source of beneficial phytochemicals that provide significant health benefits. Here, we report a high-quality, chromosome-scale Tartary buckwheat genome sequence of- 489.3 Mb that is assembled by combining whole-genome shotgun sequencing of both Illumina short reads and single-molecule real-time long reads, sequence tags of a large DNA insert fosmid library, Hi-C sequencing data, and BioNano genome maps. We annotated 33 366 high-confidence protein-coding genes based on expression evidence. Comparisons of the intra-genome with the sugar beet genome revealed an independent whole-genome duplication that occurred in the buckwheat lineage after they diverged from the common ancestor, which was not shared with rosids or asterids. The reference genome facilitated the identification of many new genes predicted to be involved in rutin biosynthesis and regulation, aluminum stress resistance, and in drought and cold stress responses. Our data suggest that Tartary buckwheat＇s ability to tolerate high levels of abiotic stress is attributed to the expansion of several gene families involved in signal transduction, gene regulation, and membrane transport. The availability of these genomic resources will facilitate the discovery of agronomically and nutritionally important genes and genetic improvement of Tartary buckwheat.

Hox Gene Clusters of Early Vertebrates:Do They Serve as Reliable Markers for Genome Evolution?

Hox genes, responsible for regional specification along the anteroposterior axis in embryogenesis, are found as clusters in most eumetazoan genomes sequenced to date. Invertebrates possess a single Hox gene cluster with some exceptions of secondary cluster breakages, while osteichthyans （bony vertebrates） have multiple Hox clusters. In tetrapods, four Hox clusters, derived from the so-called two-round whole genome duplications （2R-WGDs）, are observed. Overall, the number of Hox gene clusters has been regarded as a reliable marker of ploidy levels in animal genomes. In fact, this scheme also fits the situations in teleost fishes that experienced an additional WGD. In this review, I focus on cyclostomes and cartilaginous fishes as lineages that would fill the gap between invertebrates and osteichthyans. A recent study highlighted a possible loss of the HoxC cluster in the galeomorph shark lineage, while other aspects of cartilaginous fish Hox clusters usually mark their conserved nature. In contrast, existing resources suggest that the cyclostomes exhibit a different mode of Hox cluster organization. For this group of species, whose genomes could have differently responded to the 2R-WGDs from jawed vertebrates, therefore the number of Hox clusters may not serve as a good indicator of their ploidy level.

Innovations and stepwise evolution of CBFs/DREB1s and their regulatory networks in angiosperms

The C-repeat binding factors/dehydrationresponsive element binding protein 1 s(CBFs/DREB1 s)have been identified as major regulators of cold acclimation in many angiosperm plants.However,their origin and evolutionary process associated to cold responsiveness are still lacking.By integrating multi-omics data of genomes,transcriptomes,and CBFs/DREB1 s genome-wide binding profiles,we unveil the origin and evolution of CBFs/DREB1 s and their regulatory network.Gene collinearity and phylogeny analyses show that CBF/DREB1 is an innovation evolved from tandem duplication-derived DREBⅢgene.A subsequent event of e-whole genome duplication led to two CBF/DREB1 archetypes(CladesⅠandⅡ)in ancient angiosperms.In contrast to cold-insensitivity of Clade I and their parent DREBⅢgenes,CladeⅡevolved a further innovation in cold-sensitive response and was stepwise expanded in eudicots and monocots by independent duplications.In geological time,the duplication events were mainly enriched around the Cretaceous-Paleogene(K-Pg)boundary and/or in the Late Cenozoic Ice Age,when the global average temperature significantly decreased.Consequently,the duplicated CBF/DREB1 genes contributed to the rewiring of CBFs/DREB1 s-regulatory network for cold tolerance.Altogether,our results highlight an origin and convergent evolution of CBFs/DREB1 s and their regulatory network probably for angiosperms adaptation to global cooling.