The evolutionary significance of polyploidy

A gain-of-function allele of a DREB transcription factor gene ameliorates drought tolerance in wheat

Drought is a major environmental factor limiting wheat production worldwide. However, the genetic components underlying wheat drought tolerance are largely unknown. Here, we identify a DREB transcription factor gene (TaDTG6-B) by genome-wide association study that is tightly associated with drought tolerance in wheat. Candidate gene association analysis revealed that a 26-bp deletion in the TaDTG6-B coding region induces a gain-of-function for TaDTG6-BDel574, which exhibits stronger transcriptional activation, protein interactions, and binding activity to dehydration-responsive elements (DRE)/CRT cis-elements than the TaDTG6-BIn574 encoded by the allele lacking the deletion, thus conferring greater drought tolerance in wheat seedlings harboring this variant. Knockdown of TaDTG6-BDel574 transcripts attenuated drought tolerance in transgenic wheat, whereas its overexpression resulted in enhanced drought tolerance without accompanying phenotypic abnormalities. Furthermore, the introgression of the TaDTG6-BDel574 elite allele into drought-sensitive cultivars improved their drought tolerance, thus providing a valuable genetic resource for wheat breeding. We also identified 268 putative target genes that are directly bound and transcriptionally regulated by TaDTG6-BDel574. Further analysis showed that TaDTG6-BDel574 positively regulates TaPIF1 transcription to enhance wheat drought tolerance. These results describe the genetic basis and accompanying mechanism driving phenotypic variation in wheat drought tolerance, and provide a novel genetic resource for crop breeding programs.

Glutamate 1-semialdehyde aminotransferase is connected to GluTR by GluTR-binding protein and contributes to the rate-limiting step of 5-aminolevulinic acid synthesis

Tetrapyrroles play fundamental roles in crucial processes including photosynthesis, respiration, and catalysis. In plants, 5-aminolevulinic acid (ALA) is the common precursor of tetrapyrroles. ALA is synthesized from activated glutamate by the enzymes glutamyl-tRNA reductase (GluTR) and glutamate-1-semialdehyde aminotransferase (GSAAT). ALA synthesis is recognized as the rate-limiting step in this pathway. We aimed to explore the contribution of GSAAT to the control of ALA synthesis and the formation of a protein complex with GluTR. In Arabidopsis thaliana, two genes encode GSAAT isoforms: GSA1 and GSA2. A comparison of two GSA knockout mutants with the wild-type revealed the correlation of reduced GSAAT activity and ALA-synthesizing capacity in leaves with lower chlorophyll content. Growth and green pigmentation were more severely impaired in gsa2 than in gsa1, indicating the predominant role of GSAAT2 in ALA synthesis. Interestingly, GluTR accumulated to higher levels in gsa2 than in the wild-type and was mainly associated with the plastid membrane. We propose that the GSAAT content modulates the amount of soluble GluTR available for ALA synthesis. Several different biochemical approaches revealed the GSAAT-GluTR interaction through the assistance of GluTR-binding protein (GBP). A modeled structure of the tripartite protein complex indicated that GBP mediates the stable association of GluTR and GSAAT for adequate ALA synthesis.

Transcriptional network underpinning ploidy-related elevated leaf potassium in neo-tetraploids

Whole-genome duplication generates a tetraploid from a diploid. Newly created tetraploids (neo-tetraploids) of Arabidopsis (Arabidopsis thaliana) have elevated leaf potassium (K), compared to their diploid progenitor. Micro-grafting has previously established that this elevated leaf K is driven by processes within the root. Here, mutational analysis revealed that the K+-uptake transporters K+ TRANSPORTER 1 (AKT1) and HIGH AFFINITY K+ TRANSPORTER 5 (HAK5) are not necessary for the difference in leaf K caused by whole-genome duplication. However, the endodermis and salt overly sensitive and abscisic acid-related signaling were necessary for the elevated leaf K in neo-tetraploids. Contrasting the root transcriptomes of neo-tetraploid and diploid wild-type and mutants that suppress the neo-tetraploid elevated leaf K phenotype allowed us to identify a core set of 92 differentially expressed genes associated with the difference in leaf K between neo-tetraploids and their diploid progenitor. This core set of genes connected whole-genome duplication with the difference in leaf K between neo-tetraploids and their diploid progenitors. The set of genes is enriched in functions such as cell wall and Casparian strip development and ion transport in the endodermis, root hairs, and procambium. This gene set provides tools to test the intriguing idea of recreating the physiological effects of whole-genome duplication within a diploid genome.

Hybridization order is not the driving factor behind biases in duplicate gene losses among the hexaploid Solanaceae

We model the post-hexaploidy evolution of four genomes from the Solanaceae, a group of flowering plants comprising tomatoes, potatoes and their relatives. The hexaploidy that these genomes descend from occurred through two sequential allopolyploidy events and was marked by the unequal losses of duplicated genes from the different progenitor subgenomes. In contrast with the hexaploid Brassiceae (broccoli and its relatives), where the subgenome with the most surviving genes arrived last in the hexaploidy, among the Solanaceae the most preserved subgenome descends from one of the original two tetraploid progenitors. In fact, the last-arriving subgenome in these plants actually has the fewest surviving genes in the modern genomes. We explore whether the distribution of repetitive elements (REs) in these genomes can explain the biases in gene losses, but while the signals we find are broadly consistent with a role for high RE density in driving gene losses, the REs turn over so quickly that little signal of the RE condition at the time of paleopolyploidy is extant in the modern genomes.

Sublinear scaling of the cellular proteome with ploidy

Ploidy changes are frequent in nature and contribute to evolution, functional specialization and tumorigenesis. Analysis of model organisms of different ploidies revealed that increased ploidy leads to an increase in cell and nuclear volume, reduced proliferation, metabolic changes, lower fitness, and increased genomic instability, but the underlying mechanisms remain poorly understood. To investigate how gene expression changes with cellular ploidy, we analyzed isogenic series of budding yeasts from 1N to 4N. We show that mRNA and protein abundance scales allometrically with ploidy, with tetraploid cells showing only threefold increase in protein abundance compared to haploids. This ploidy-dependent sublinear scaling occurs via decreased rRNA and ribosomal protein abundance and reduced translation. We demonstrate that the activity of Tor1 is reduced with increasing ploidy, which leads to diminished rRNA gene repression via a Tor1-Sch9-Tup1 signaling pathway. mTORC1 and S6K activity are also reduced in human tetraploid cells and the concomitant increase of the Tup1 homolog Tle1 downregulates the rDNA transcription. Our results suggest that the mTORC1-Sch9/S6K-Tup1/TLE1 pathway ensures proteome remodeling in response to increased ploidy.

Alteration of synonymous codon usage bias accompanies polyploidization in wheat

The diploidization of polyploid genomes is accompanied by genomic variation, including synonymous nucleotide substitutions that may lead to synonymous codon usage bias (SCUB). SCUB can mirror the evolutionary specialization of plants, but its effect on the formation of polyploidies is not well documented. We explored this issue here with hexaploid wheat and its progenitors. Synonymous codons (SCs) ending in either cytosine (NNC) or guanidine (NNG) were more frequent than those ending in either adenosine (NNA) or thymine (NNT), and the preference for NNC/G codons followed the increase in genome ploidy. The ratios between NNC/G and NNA/T codons gradually decreased in genes with more introns, and the difference in these ratios between wheat and its progenitors diminished with increasing ploidy. SCUB frequencies were heterogeneous among exons, and the bias preferred to NNA/T in more internal exons, especially for genes with more exons; while the preference did not appear to associate with ploidy. The SCUB alteration of the progenitors was different during the formation of hexaploid wheat, so that SCUB was the homogeneous among A, B and D subgenomes. DNA methylation-mediated conversion from cytosine to thymine weakened following the increase of genome ploidy, coinciding with the stronger bias for NNC/G SCs in the genome as a function of ploidy, suggesting that SCUB contribute to the epigenetic variation in hexaploid wheat. The patterns in SCUB mirrored the formation of hexaploid wheat, which provides new insight into genome shock-induced genetic variation during polyploidization. SCs representing non-neutral synonymous mutations can be used for genetic dissection and improvement of agricultural traits of wheat and other polyploidies.

Multiple origins, one evolutionary trajectory: gradual evolution characterizes distinct lineages of allotetraploid Brachypodium

The "genomic shock" hypothesis posits that unusual challenges to genome integrity such as whole genome duplication may induce chaotic genome restructuring. Decades of research on polyploid genomes have revealed that this is often, but not always the case. While some polyploids show major chromosomal rearrangements and derepression of transposable elements in the immediate aftermath of whole genome duplication, others do not. Nonetheless, all polyploids show gradual diploidization over evolutionary time. To evaluate these hypotheses, we produced a chromosome-scale reference genome for the natural allotetraploid grass Brachypodium hybridum, accession "Bhyb26." We compared 2 independently derived accessions of B. hybridum and their deeply diverged diploid progenitor species Brachypodium stacei and Brachypodium distachyon. The 2 B. hybridum lineages provide a natural timecourse in genome evolution because one formed 1.4 million years ago, and the other formed 140 thousand years ago. The genome of the older lineage reveals signs of gradual post-whole genome duplication genome evolution including minor gene loss and genome rearrangement that are missing from the younger lineage. In neither B. hybridum lineage do we find signs of homeologous recombination or pronounced transposable element activation, though we find evidence supporting steady post-whole genome duplication transposable element activity in the older lineage. Gene loss in the older lineage was slightly biased toward 1 subgenome, but genome dominance was not observed at the transcriptomic level. We propose that relaxed selection, rather than an abrupt genomic shock, drives evolutionary novelty in B. hybridum, and that the progenitor species' similarity in transposable element load may account for the subtlety of the observed genome dominance.

Systematic and functional analysis of non-specific lipid transfer protein family genes in sugarcane under Xanthomonas albilineans infection and salicylic acid treatment

Plant non-specific lipid transfer proteins (nsLTPs) are small basic proteins that play a significant regulatory role in a wide range of physiological processes. To date, no genome-wide survey and expression analysis of this gene family in sugarcane has been performed. In this study we identified the nsLTP gene family in Saccharum spontaneum and carried out expression profiling of nsLTPs in two sugarcane cultivars (Saccharum spp.) that have different resistance to leaf scald caused by Xanthomonas albilineans (Xa) infection. The effect of stress related to exogenous salicylic acid (SA) treatment was also examined. At a genome-wide level, S. spontaneum AP85-441 had 71 SsnsLTP genes including 66 alleles. Tandem (9 gene pairs) and segmental (36 gene pairs) duplication events contributed to SsnsLTP gene family expansion. Five SsnsLTP proteins were predicted to interact with five other proteins. Expression of ShnsLTPI.8/10/Gb.1 genes was significantly upregulated in LCP85-384 (resistant cultivar), but downregulated in ROC20 (susceptible cultivar), suggesting that these genes play a positive regulatory role in response of sugarcane to Xa infection. Conversely, ShnsLTPGa.4/Ge.3 appears to act as a negative regulator in response Xa infection. The majority (16/17) of tested genes were positively induced in LCP85-384 72 h after SA treatment. In both cultivars, but particularly in LCP85-384, ShnsLTPIV.3/VIII.1 genes were upregulated at all time-points, suggesting that the two genes might act as positive regulators under SA stress. Meanwhile, both cultivars showed downregulated ShnsLTPGb.1 gene expression, indicating its potential negative role in SA treatment responses. Notably, the ShnsLTPGb.1 gene had contrasting effects, with positive regulation of gene expression in response to Xa infection and negative regulation induced by SA stress. Together, our results provide valuable information for elucidating the function of ShnsLTP family members under two stressors and identified novel gene sources for development of sugarcane that are tolerant of environmental stimuli.

A high-quality Buxus austro-yunnanensis (Buxales) genome provides new insights into karyotype evolution in early eudicots

BACKGROUND

Eudicots are the most diverse group of flowering plants that compromise five well-defined lineages: core eudicots, Ranunculales, Proteales, Trochodendrales, and Buxales. However, the phylogenetic relationships between these five lineages and their chromosomal evolutions remain unclear, and a lack of high-quality genome analyses for Buxales has hindered many efforts to address this knowledge gap.

RESULTS

Here, we present a high-quality chromosome-level genome of Buxus austro-yunnanensis (Buxales). Our phylogenomic analyses revealed that Buxales and Trochodendrales are genetically similar and classified as sisters. Additionally, both are sisters to the core eudicots, while Ranunculales was found to be the first lineage to diverge from these groups. Incomplete lineage sorting and hybridization were identified as the main contributors to phylogenetic discordance (34.33%) between the lineages. In fact, B. austro-yunnanensis underwent only one whole-genome duplication event, and collinear gene phylogeny analyses suggested that separate independent polyploidizations occurred in the five eudicot lineages. Using representative genomes from these five lineages, we reconstructed the ancestral eudicot karyotype (AEK) and generated a nearly gapless karyotype projection for each eudicot species. Within core eudicots, we recovered one common chromosome fusion event in asterids and malvids, respectively. Further, we also found that the previously reported fused AEKs in Aquilegia (Ranunculales) and Vitis (core eudicots) have different fusion positions, which indicates that these two species have different karyotype evolution histories.

CONCLUSIONS

Based on our phylogenomic and karyotype evolution analyses, we revealed the likely relationships and evolutionary histories of early eudicots. Ultimately, our study expands genomic resources for early-diverging eudicots.

The wild allotetraploid sesame genome provides novel insights into evolution and lignan biosynthesis

INTRODUCTION

The wild tetraploid sesame (Sesamum schinzianum), an ancestral relative of diploid cultivated sesame, grows in the tropical desert of the African Plateau. As a valuable seed resource, wild sesame has several advantageous traits, such as strong environmental adaptability and an extremely high content of sesamolin in its seeds. High-quality genome assembly is essential for a detailed understanding of genome structure, genome evolution and crop improvement.

OBJECTIVES

Here, we generated two high-quality chromosome-scale genomes from S. schinzianum and a cultivated diploid elite sesame (Sesamum indicum L.) to investigate the potential genetic basis underlying these traits of wild sesame.

METHODS

The long-read data from PacBio Sequel II platform and high-throughput chromosome conformation capture (Hi-C) data were used to construct high-quality sesame genome. Then dissecting the molecular mechanisms of sesame evolution and lignan biosynthesis through comparative genomics and transcriptomics.

RESULTS

We found evidence of divergent evolution that involved differences in the number, sequence and expression level of homologous genes between the two sets of subgenomes from allotetraploids in S. schinzianum, all of which might be driven by subfunctionalization after polyploidization. Furthermore, it was found that a great number of genes involved in the stress response have undergone positive selection and resulted from gene family expansion in the wild sesame genome compared with the cultivated sesame genome, which, overall, is associated with adaptative evolution to the environment. We hypothesized that the sole functional member CYP92B14 (SscC22g35272) could be associated with high content of sesamolin in wild sesame seeds.

CONCLUSION

This study provides high-quality wild allotetraploid sesame and cultivated sesame genomes, reveals evolutionary features of the allotetraploid genome and provides novel insights into lignan synthesis pathways. Meanwhile, the wild sesame genome will be an important resource to conduct comparative genomic and evolutionary studies and plant improvement programmes.

Polyploidy before and after domestication of crop species

Recent advances in the genomics of polyploid species answer some of the long-standing questions about the role of polyploidy in crop species. Here, we summarize the current literature to reexamine scenarios in which polyploidy played a role both before and after domestication. The prevalence of polyploidy can help to explain environmental robustness in agroecosystems. This review also clarifies the molecular basis of some agriculturally advantageous traits of polyploid crops, including yield increments in polyploid cotton via subfunctionalization, modification of a separated sexuality to selfing in polyploid persimmon via neofunctionalization, and transition to a selfing system via nonfunctionalization combined with epistatic interaction between duplicated S-loci. The rapid progress in genomics and genetics is discussed along with how this will facilitate functional studies of understudied polyploid crop species.

Robustness and the generalist niche of polyploid species: Genome shock or gradual evolution?

The prevalence of polyploidy in wild and crop species has stimulated debate over its evolutionary advantages and disadvantages. Previous studies have focused on changes occurring at the polyploidization events, including genome-wide changes termed "genome shock," as well as ancient polyploidy. Recent bioinformatics advances and empirical studies of Arabidopsis and wheat relatives are filling a research gap: the functional evolutionary study of polyploid species using RNA-seq, DNA polymorphism, and epigenomics. Polyploid species can become generalists in natura through environmental robustness by inheriting and merging parental stress responses. Their evolvability is enhanced by mutational robustness working on inherited standing variation. The identification of key genes responsible for gradual adaptive evolution will encourage synthetic biological approaches to transfer polyploid advantages to other species.

Do water and soil nutrient scarcities differentially impact the performance of diploid and tetraploid Solidago gigantea (Giant Goldenrod, Asteraceae)?

Plants require water and nutrients for survival, although the effects of their availabilities on plant fitness differ amongst species. Genome size variation, within and across species, is suspected to influence plant water and nutrient requirements, but little is known about how variations in these resources concurrently affect plant fitness based on genome size. We examined how genome size variation between autopolyploid cytotypes influences plant morphological and physiological traits, and whether cytotype-specific trait responses differ based on water and/or nutrient availability. Diploid and autotetraploid Solidago gigantea (Giant Goldenrod) were grown in a greenhouse under four soil water:N+P treatments (L:L, L:H, H:L, H:H), and stomata characteristics (size, density), growth (above- and belowground biomass, R/S), and physiological (A_net , E, WUE) responses were measured. Resource availabilities and cytotype identity influenced some plant responses but their effects were independent of each other. Plants grown in high-water and nutrient treatments were larger, plants grown in low-water or high-nutrient treatments had higher WUE but lower E, and A_net and E rates decreased as plants aged. Autotetraploids also had larger and fewer stomata, higher biomass and larger A_net than diploids. Nutrient and water availability could influence intra- and interspecific competitive outcomes. Although S. gigantea cytotypes were not differentially affected by resource treatments, genome size may influence cytogeographic range patterning and population establishment likelihood. For instance, the larger size of autotetraploid S. gigantea might render them more competitive for resources and niche space than diploids.

Evolutionary divergence of duplicated genomes in newly described allotetraploid cottons

Wild relatives of domesticated plants provide a rich resource for crop improvement and a valuable comparative perspective for understanding genomic, physiological, and agricultural traits. Here, we provide high-quality reference genomes of one early domesticated form of the economically most important cotton species, Gossypium hirsutum, and two other wild species, to clarify evolutionary relationships and understand the genomic changes that characterize these species and their close relatives. We document abundant gene resources involved in adaptation to environmental challenges, highlighting the potential for introgression of favorable genes into domesticated cotton and for increasing resilience to climate variability. Our study complements other recent genomic analyses in the cotton genus and provides a valuable foundation for breeding improved cotton varieties.

Allotetraploid cotton (Gossypium) species represents a model system for the study of plant polyploidy, molecular evolution, and domestication. Here, chromosome-scale genome sequences were obtained and assembled for two recently described wild species of tetraploid cotton, Gossypium ekmanianum [(AD)₆, Ge] and Gossypium stephensii [(AD)₇, Gs], and one early form of domesticated Gossypium hirsutum, race punctatum [(AD)₁, Ghp]. Based on phylogenomic analysis, we provide a dated whole-genome level perspective for the evolution of the tetraploid Gossypium clade and resolved the evolutionary relationships of Gs, Ge, and domesticated G. hirsutum. We describe genomic structural variation that arose during Gossypium evolution and describe its correlates—including phenotypic differentiation, genetic isolation, and genetic convergence—that contributed to cotton biodiversity and cotton domestication. Presence/absence variation is prominent in causing cotton genomic structural variations. A presence/absence variation-derived gene encoding a phosphopeptide-binding protein is implicated in increasing fiber length during cotton domestication. The relatively unimproved Ghp offers the potential for gene discovery related to adaptation to environmental challenges. Expanded gene families enoyl-CoA δ isomerase 3 and RAP2-7 may have contributed to abiotic stress tolerance, possibly by targeting plant hormone-associated biochemical pathways. Our results generate a genomic context for a better understanding of cotton evolution and for agriculture.

Genome evolution: On the nature of trade-offs with polyploidy and endopolyploidy

A new study uncovers a novel trade-off in polyploid plants. While the larger cells of polyploids benefit from increased cell storage and water retention, this comes at the cost of reduced structural stability, potentially impacting survival.

Does polyploidy inhibit sex chromosome evolution in angiosperms?

Dioecy is rare in flowering plants (5-6% of species), but is often controlled genetically by sex-linked regions (SLRs). It has so far been unclear whether, polyploidy affects sex chromosome evolution, as it does in animals, though polyploidy is quite common in angiosperms, including in dioecious species. Plants could be different, as, unlike many animal systems, degenerated sex chromosomes, are uncommon in plants. Here we consider sex determination in plants and plant-specific factors, and propose that constraints created at the origin of polyploids limit successful polyploidization of species with SLRs. We consider the most likely case of a polyploid of a dioecious diploid with an established SLR, and discuss the outcome in autopolyploids and allopolyploids. The most stable system possibly has an SLR on just one chromosome, with a strongly dominant genetic factor in the heterogametic sex (e.g., xxxY male in a tetraploid). If recombination occurs with its homolog, this will prevent Y chromosome degeneration. Polyploidy may also allow for reversibility of multiplied Z or X chromosomes into autosomes. Otherwise, low dosage of Y-linked SLRs compared to their multiple homologous x copies may cause loss of reliable sex-determination at higher ploidy levels. We discuss some questions that can be studied using genome sequencing, chromosome level-assemblies, gene expression studies and analysis of loci under selection.

Transposons and non-coding regions drive the intrafamily differences of genome size in insects

Genome size (GS) can vary considerably between phylogenetically close species, but the landscape of GS changes in insects remain largely unclear. To better understand the specific evolutionary factors that determine GS in insects, we examined flow cytometry-based published GS data from 1,326 insect species, spanning 700 genera, 155 families, and 21 orders. Model fitting showed that GS generally followed an Ornstein-Uhlenbeck adaptive evolutionary model in Insecta overall. Ancestral reconstruction indicated a likely GS of 1,069 Mb, suggesting that most insect clades appeared to undergo massive genome expansions or contractions. Quantification of genomic components in 56 species from nine families in four insect orders revealed that the proliferation of transposable elements contributed to high variation in GS between close species, such as within Coleoptera. This study sheds lights on the pattern of GS variation in insects and provides a better understanding of insect GS evolution.

Genome-wide characterization of ovate family protein gene family associated with number of seeds per silique in Brassica napus

Ovate family proteins (OFPs) were firstly identified in tomato as proteins controlling the pear shape of the fruit. Subsequent studies have successively proved that OFPs are a class of negative regulators of plant development, and are involved in the regulation of complex traits in different plants. However, there has been no report about the functions of OFPs in rapeseed growth to date. Here, we identified the OFPs in rapeseed at the genomic level. As a result, a total of 67 members were obtained. We then analyzed the evolution from Arabidopsis thaliana to Brassica napus, illustrated their phylogenetic and syntenic relationships, and compared the gene structure and conserved domains between different copies. We also analyzed their expression patterns in rapeseed, and found significant differences in the expression of different members and in different tissues. Additionally, we performed a GWAS for the number of seeds per silique (NSPS) in a rapeseed population consisting of 204 natural accessions, and identified a new gene BnOFP13_2 significantly associated with NSPS, which was identified as a novel function of OFPs. Haplotype analysis revealed that the accessions with haplotype 3 had a higher NSPS than other accessions, suggesting that BnOFP13_2 is associated with NSPS. Transcript profiling during the five stages of silique development demonstrated that BnOFP13_2 negatively regulates NSPS. These findings provide evidence for functional diversity of OFP gene family and important implications for oilseed rape breeding.

Symmetric subgenomes and balanced homoeolog expression stabilize the establishment of allopolyploidy in cyprinid fish

BACKGROUND

Interspecific postzygotic reproduction isolation results from large genetic divergence between the subgenomes of established hybrids. Polyploidization immediately after hybridization may reset patterns of homologous chromosome pairing and ameliorate deleterious genomic incompatibility between the subgenomes of distinct parental species in plants and animals. However, the observation that polyploidy is less common in vertebrates raises the question of which factors restrict its emergence. Here, we perform analyses of the genome, epigenome, and gene expression in the nascent allotetraploid lineage (2.95 Gb) derived from the intergeneric hybridization of female goldfish (Carassius auratus, 1.49 Gb) and male common carp (Cyprinus carpio, 1.42 Gb), to shed light on the changes leading to the stabilization of hybrids.

RESULTS

We firstly identify the two subgenomes derived from the parental lineages of goldfish and common carp. We find variable unequal homoeologous recombination in somatic and germ cells of the intergeneric F1 and allotetraploid (F22 and F24) populations, reflecting high plasticity between the subgenomes, and rapidly varying copy numbers between the homoeolog genes. We also find dynamic changes in transposable elements accompanied by genome merger and duplication in the allotetraploid lineage. Finally, we observe the gradual decreases in cis-regulatory effects and increases in trans-regulatory effects along with the allotetraploidization, which contribute to increases in the symmetrical homoeologous expression in different tissues and developmental stages, especially in early embryogenesis.

CONCLUSIONS

Our results reveal a series of changes in transposable elements, unequal homoeologous recombination, cis- and trans-regulations (e.g. DNA methylation), and homoeologous expression, suggesting their potential roles in mediating adaptive stabilization of regulatory systems of the nascent allotetraploid lineage. The symmetrical subgenomes and homoeologous expression provide a novel way of balancing genetic incompatibilities, providing a new insight into the early stages of allopolyploidization in vertebrate evolution.

Chromosome-scale assembly of the Moringa oleifera Lam. genome uncovers polyploid history and evolution of secondary metabolism pathways through tandem duplication

The African Orphan Crops Consortium (AOCC) selected the highly nutritious, fast growing and drought tolerant tree crop moringa (Moringa oleifera Lam.) as one of the first of 101 plant species to have its genome sequenced and a first draft assembly was published in 2019. Given the extensive uses and culture of moringa, often referred to as the multipurpose tree, we generated a significantly improved new version of the genome based on long-read sequencing into 14 pseudochromosomes equivalent to n = 14 haploid chromosomes. We leveraged this nearly complete version of the moringa genome to investigate main drivers of gene family and genome evolution that may be at the origin of relevant biological innovations including agronomical favorable traits. Our results reveal that moringa has not undergone any additional whole-genome duplication (WGD) or polyploidy event beyond the gamma WGD shared by all core eudicots. Moringa duplicates retained following that ancient gamma events are also enriched for functions commonly considered as dosage balance sensitive. Furthermore, tandem duplications seem to have played a prominent role in the evolution of specific secondary metabolism pathways including those involved in the biosynthesis of bioactive glucosinolate, flavonoid, and alkaloid compounds as well as of defense response pathways and might, at least partially, explain the outstanding phenotypic plasticity attributed to this species. This study provides a genetic roadmap to guide future breeding programs in moringa, especially those aimed at improving secondary metabolism related traits.

Adaptive responses drive the success of polyploid yellowcresses (Rorippa, Brassicaceae) in the Hengduan Mountains, a temperate biodiversity hotspot

Polyploids contribute substantially to plant evolution and biodiversity; however, the mechanisms by which they succeed are still unclear. According to the polyploid adaptation hypothesis, successful polyploids spread by repeated adaptive responses to new environments. Here, we tested this hypothesis using two tetraploid yellowcresses (Rorippa), the endemic Rorippa elata and the widespread Rorippa palustris, in the temperate biodiversity hotspot of the Hengduan Mountains. Speciation modes were resolved by phylogenetic modeling using 12 low-copy nuclear loci. Phylogeographical patterns were then examined using haplotypes phased from four plastid and ITS markers, coupled with historical niche reconstruction by ecological niche modeling. We inferred the time of hybrid origins for both species as the mid-Pleistocene, with shared glacial refugia within the southern Hengduan Mountains. Phylogeographic and ecological niche reconstruction indicated recurrent northward colonization by both species after speciation, possibly tracking denuded habitats created by glacial retreat during interglacial periods. Common garden experiment involving perennial R. elata conducted over two years revealed significant changes in fitness-related traits across source latitudes or altitudes, including latitudinal increases in survival rate and compactness of plant architecture, suggesting gradual adaptation during range expansion. These findings support the polyploid adaptation hypothesis and suggest that the spread of polyploids was aided by adaptive responses to environmental changes during the Pleistocene. Our results thus provide insight into the evolutionary success of polyploids in high-altitude environments.

Untying Gordian knots: unraveling reticulate polyploid plant evolution by genomic data using the large Ranunculus auricomus species complex

Speciation via hybridization and polyploidization is a major evolutionary force in plant evolution but is still poorly understood for neopolyploid groups. Challenges are attributed to high heterozygosity, low genetic divergence, and missing information on progenitors, ploidy, and reproduction. We study the large Eurasian Ranunculus auricomus species complex and use a comprehensive workflow integrating reduced-representation sequencing (RRS) genomic data to unravel reticulate evolution, genome diversity and composition of polyploids. We rely on 97 312 restriction site-associated DNA sequencing (RAD-Seq) loci, 576 targeted nuclear genes (48 phased), and 71 plastid regions derived from 78 polyploid apomictic taxa and four diploid and one tetraploid putative sexual progenitor species. We applied (phylo)genomic structure, network, and single nucleotide polymorphism (SNP)-origin analyses. Results consistently showed only 3-5 supported and geographically structured polyploid genetic groups, each containing extant sexual and one unknown progenitor species. Combined analyses demonstrated predominantly allopolyploid origins, each involving 2-3 different diploid sexual progenitor species. Young allotetraploids were characterized by subgenome dominance and nonhybrid SNPs, suggesting substantial post-origin but little lineage-specific evolution. The biodiversity of neopolyploid complexes can result from multiple hybrid origins involving different progenitors and substantial post-origin evolution (e.g. homoeologous exchanges, hybrid segregation, gene flow). Reduced-representation sequencing genomic data including multi-approach information is efficient to delimit shallow reticulate relationships.

Induction of 2n pollen with colchicine during microsporogenesis in Phalaenopsis

The induction of 2n pollen is an important technique for breeding polyploid plants. Here, we observed meiosis in the pollen mother cells (PMCs) of six Phalaenopsis cultivars and attempted to induce 2n pollen. The meiotic stage was related to flower bud length. During meiosis, Phalaenopsis cultivars with flower widths of approximately 20-40 mm and 50-60 mm had bud lengths of approximately 3-8 mm and 5-13 mm, respectively. The duration of meiosis ranged from 4.2 to 14 d. This was the first study to characterize meiosis of the PMCs of Phalaenopsis. The natural generation frequency of 2n pollen varied from 0.68% to 1.78%. Meiotic stage and colchicine concentration significantly affected the induction of 2n pollen. The most effective treatment for obtaining 2n pollen was 0.05% colchicine in the leptotene to zygotene stage for 3 d, which achieved a 2n pollen frequency of 10.04%.

Establishment of an allotetraploid fern species, Lepisorus yamaokae Seriz., between two highly niche-differentiated parental species

PREMISE

The successful establishment of polyploid species is hypothesized to be promoted by niche differentiation from the parental species or by range shifts during climate oscillations. However, few studies have considered both of these factors simultaneously. We resolved the origin of a tetraploid fern, Lepisorus yamaokae, and explored a pattern of niche differentiation among the allotetraploid and parental species in past and current climates.

METHODS

We reconstructed phylogenetic trees based on plastid marker and single-copy nuclear genes to resolve the allopolyploid origin of L. yamaokae. We also evaluated climatic niche differentiation among L. yamaokae and its two parental species using species distribution models in geographic space and principal component analysis.

RESULTS

We infer that L. yamaokae had a single allotetraploid origin from L. annuifrons and L. uchiyamae. Climatic niche analyses show that the parental species currently occupy different niche spaces. The predicted distribution of the parental species at the Last Glacial Maximum (LGM) suggests more opportunities for hybridization during the LGM or during other recent temporary range shifts. Lepisorus yamaokae has a narrower niche than the additive niche of the parental species. We also observed niche conservatism in L. yamaokae.

CONCLUSIONS

Range shifts of the parental species during climatic oscillations in the Quaternary likely facilitated the formation and establishment of L. yamaokae. Further, the genetic intermediacy of L. yamaokae may have enabled a niche shift in its microenvironment, resulting in its successful establishment without a macroclimatic niche shift in L. yamaokae.

DupScan: predicting and visualizing vertebrate genome duplication database

Duplicated genes prevail in vertebrates and are important in the acquisition of new genes and novelties. Whole genome duplication (WGD) is one of the sources of duplicated genes. It can provide raw materials for natural selection by increasing the flexibility and complexity of the genome. WGDs are the driving force for the evolution of vertebrates and contribute greatly to their species diversity, especially in fish species with complicated WGD patterns. Here, we constructed the DupScan database (https://dupscan.sysumeg.com/) by integrating 106 chromosomal-level genomes, which can analyze and visualize synteny at both the gene and genome scales, visualize the Ka, Ks, and 4DTV values, and browse genomes. DupScan was used to perform functional adaptation for the intricate WGD investigation based on synteny matching. DupScan supports the analysis of five WGD rounds (R): VGD2 (vertebrate genome duplication 2), Ars3R (Acipenser-ruthenus-specific 3R), Pss3R (Polyodon-spathula-specific 3R), Ts3R (teleost-specific duplication 3R), Ss4R (salmonid-specific 4R), and Cs4R (carp-specific 4R). DupScan serves as one-stop analysis platform for synteny and WGD research in which users can analyze and predict synteny and WGD patterns across 106 species of whole genome sequences. This further aided us in elucidating genome evolutionary patterns across over 60,000 vertebrate species with synteny and WGD events.

Polyploidy and Myc Proto-Oncogenes Promote Stress Adaptation via Epigenetic Plasticity and Gene Regulatory Network Rewiring

Polyploid cells demonstrate biological plasticity and stress adaptation in evolution; development; and pathologies, including cardiovascular diseases, neurodegeneration, and cancer. The nature of ploidy-related advantages is still not completely understood. Here, we summarize the literature on molecular mechanisms underlying ploidy-related adaptive features. Polyploidy can regulate gene expression via chromatin opening, reawakening ancient evolutionary programs of embryonality. Chromatin opening switches on genes with bivalent chromatin domains that promote adaptation via rapid induction in response to signals of stress or morphogenesis. Therefore, stress-associated polyploidy can activate Myc proto-oncogenes, which further promote chromatin opening. Moreover, Myc proto-oncogenes can trigger polyploidization de novo and accelerate genome accumulation in already polyploid cells. As a result of these cooperative effects, polyploidy can increase the ability of cells to search for adaptive states of cellular programs through gene regulatory network rewiring. This ability is manifested in epigenetic plasticity associated with traits of stemness, unicellularity, flexible energy metabolism, and a complex system of DNA damage protection, combining primitive error-prone unicellular repair pathways, advanced error-free multicellular repair pathways, and DNA damage-buffering ability. These three features can be considered important components of the increased adaptability of polyploid cells. The evidence presented here contribute to the understanding of the nature of stress resistance associated with ploidy and may be useful in the development of new methods for the prevention and treatment of cardiovascular and oncological diseases.

Genome-wide characterization of cys-tathionine-β-synthase domain-containing proteins in sugarcane reveals their role in defense responses under multiple stressors

Cys-tathionine-β-synthase (CBS) domain-containing proteins (CDCPs) are essential for regulating plant responses to various biotic and abiotic stressors. This study describes the systematic identification and characterization of CDCP family genes in Saccharum spontaneum. A total of 95 SsCDCP genes and eight phylogenetic groups were identified that were distributed over 29 chromosomes of the AP85-441 genome. Most (78/95) SsCDCPs underwent fragment duplication events, and 64 gene pairs were located in synteny blocks. Expression profiling of nine ShCDCPs was also carried out in the Saccharum spp. cultivars ROC22 and MT11-611 that are resistant and susceptible to red stripe, respectively, in response to: (i) Infection by the bacterial pathogen Acidovorax avenue subsp. avenae (Aaa); (ii) abiotic stressors (drought and salinity); and (iii) exogenous salicylic acid (SA) treatment. Members of one gene pair (ShCBSD-PB1-5A and ShCBSD-PB1-7A-1) with a fragment duplication event acted as negative regulators in sugarcane under four stresses, as supported by the significantly decreased expression levels of ShCBSD-PB1-5A (23-83%) and ShCBSD-PB1-7A-1 (15-75%) at all-time points, suggesting that they have functional redundancy. Genes in another pair, ShCBS-4C and ShCBS-4D-1, which have a fragment duplication event, play opposing regulatory roles in sugarcane exposed to multiple stresses, particularly Aaa and NaCl treatments. ShCBS-4C expression was significantly decreased by 32-77%, but ShCBS-4D-1 expression was dramatically upregulated by 1.2-6.2-fold in response to Aaa treatment of both cultivars across all-time points. This result suggested that both genes exhibited functional divergence. Meanwhile, the expression of SsCBSDCBS-5A was significantly upregulated in ROC22 by 1.4-4.6-fold in response to the four stressors. These findings provide important clues for further elucidating the function of ShCDCP genes in sugarcane responding to a diverse range of stresses.

A temporal gradient of cytonuclear coordination of chaperonins and chaperones during RuBisCo biogenesis in allopolyploid plants

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo), consisting of subunits encoded by nuclear and cytoplasmic genes, is a model for cytonuclear evolution in plant allopolyploids. To date, coordinated cytonuclear evolutionary responses of auxiliary cofactors involved in RuBisCo biogenesis remain unexplored. This study characterized and compared genomic and transcriptional cytonuclear coevolutionary responses of chaperonin/chaperones in RuBisCo folding and assembly processes across different allopolyploids. We discovered significant cytonuclear evolutionary responses in folding cofactors, with diminishing or attenuated responses later during assembly. Our results have general significance for understanding the unrecognized cytonuclear evolution of chaperonin/chaperone genes, structural and functional features of intermediate complexes, and the functioning stage of the Raf2 cofactor. Generally, the results reveal a hitherto unexplored dimension of allopolyploidy in plants.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) has long been studied from many perspectives. As a multisubunit (large subunits [LSUs] and small subunits[SSUs]) protein encoded by genes residing in the chloroplast (rbcL) and nuclear (rbcS) genomes, RuBisCo also is a model for cytonuclear coevolution following allopolyploid speciation in plants. Here, we studied the genomic and transcriptional cytonuclear coordination of auxiliary chaperonin and chaperones that facilitate RuBisCo biogenesis across multiple natural and artificially synthesized plant allopolyploids. We found similar genomic and transcriptional cytonuclear responses, including respective paternal-to-maternal conversions and maternal homeologous biased expression, in chaperonin/chaperon-assisted folding and assembly of RuBisCo in different allopolyploids. One observation is about the temporally attenuated genomic and transcriptional cytonuclear evolutionary responses during early folding and later assembly process of RuBisCo biogenesis, which were established by long-term evolution and immediate onset of allopolyploidy, respectively. Our study not only points to the potential widespread and hitherto unrecognized features of cytonuclear evolution but also bears implications for the structural interaction interface between LSU and Cpn60 chaperonin and the functioning stage of the Raf2 chaperone.

Chromosomal-level genome of velvet bean (Mucuna pruriens) provides resources for L-DOPA synthetic research and development

Mucuna pruriens, commonly called velvet bean, is the main natural source of levodopa (L-DOPA), which has been marketed as a psychoactive drug for the clinical management of Parkinson’s disease and dopamine-responsive dystonia. Although velvet bean is a very important plant species for food and pharmaceutical manufacturing, the lack of genetic and genomic information about this species severely hinders further molecular research thereon and biotechnological development. Here, we reported the first velvet bean genome, with a size of 500.49 Mb and 11 chromosomes encoding 28,010 proteins. Genomic comparison among legume species indicated that velvet bean speciated ∼29 Ma from soybean clade, without specific genome duplication. Importantly, we identified 21 polyphenol oxidase coding genes that catalyse l-tyrosine to L-DOPA in velvet bean, and two subfamilies showing tandem expansion on Chr3 and Chr7 after speciation. Interestingly, disease-resistant and anti-pathogen gene families were found contracted in velvet bean, which might be related to the expansion of polyphenol oxidase. Our study generated a high-quality genomic reference for velvet bean, an economically important agricultural and medicinal plant, and the newly reported L-DOPA biosynthetic genes could provide indispensable information for the biotechnological and sustainable development of an environment-friendly L-DOPA biosynthesis processing method.

Chromosomal instability and phenotypic variation in a specific lineage derived from a synthetic allotetraploid wheat

Newly formed plant allopolyploids usually have meiosis defect, resulting in chromosomal instability manifested as variation in chromosome number and/or structure. However, not all nascent allopolyploids are equally unstable. The wheat group (Aegilops/Triticum) contains 13 diploid species with distinct genome types. Many of these species can be artificially hybridized to produce viable but sterile inter-specific/intergeneric F1 hybrids, which can generate fertile synthetic allotetraploid wheats after whole genome doubling. Compared with synthetic allotetraploid wheats that contain genome combinations of AADD and S*S*DD (S* refers to related S genomes of a different species), those containing an S*S*AA genome are significantly more stable. However, robustness of the relative stability of S*S*AA genomes is unknown, nor are the phenotypic and fitness consequences during occurrences of secondary chromosomal instability. Here, we report a specific lineage originated from a single individual plant of a relatively stable synthetic allotetraploid wheat with genomes S ^l S ^l AA (S ^l and A subgenomes were from Ae. longissima and T. urartu, respectively) that showed a high degree of transgenerational chromosomal instability. Both numerical chromosome variation (NCV) and structural chromosome variation (SCV) occurred widely. While substantial differences in frequencies of both NCV and SCV were detected across the different chromosomes, only NCV frequencies were significantly different between the two subgenomes. We found that NCVs and SCVs occurred primarily due to perturbed meiosis, allowing formation of multivalents and univalents as well as homoeologous exchanges. Thus, the combination of NCVs and SCVs affected multiple phenotypic traits, particularly those related to reproductive fitness.

The chromosome-level holly (Ilex latifolia) genome reveals key enzymes in triterpenoid saponin biosynthesis and fruit color change

The Ilex L. (hollies) genus of Aquifoliaceae shows high species diversity in tropical and subtropical regions of Asia and South America. Throughout the range of the genus, Ilex species have been widely used in beverage and medicine production and as ornamentals. Here, we assembled a high-quality, chromosome-level genome of Ilex latifolia, which has extremely high economic value because of its useful secondary metabolite production and the high ornamental value of its decorative red berries. The 99.8% genome sequence was anchored to 20 pseudochromosomes, with a total length of 766.02 Mb and a scaffold N50 of 33.45 Mb. Based on the comparative genomic analysis of 14 angiosperm species, we recovered I. latifolia as the sister group to all other campanulids. Two whole-genome duplication (WGD) events were identified in hollies: one shared ancient WGD in the ancestor of all eudicots and a recent and independent WGD in hollies. We performed a genome-wide search to screen candidate genes involved in the biosynthesis of pentacyclic triterpenoid saponins in I. latifolia. Three subfamilies of CYP450 (CYP71A, CYP72A, and CYP716A) appear to have expanded. The transcriptomic analysis of I. latifolia leaves at five developmental stages revealed that two CYP716A genes and one CYP72A gene probably play important roles in this biosynthetic pathway. In addition, we totally identified 12 genes in the biosynthesis pathways of pelargonidin and cyanidin and observed their differential expression in green and red fruit pericarps, suggesting an association between pelargonidin and cyanidin biosynthesis and fruit pericarp color change. The accumulation of pelargonidin and cyanidin is expected to play an important role in the ornamental value of I. latifolia. Altogether, this study elucidated the molecular basis of the medicinal and ornamental value of I. latifolia, providing a data basis and promising clues for further applications.

Pulmonate slug evolution is reflected in the de novo genome of Arion vulgaris Moquin-Tandon, 1855

Stylommatophoran pulmonate land slugs and snails successfully completed the water-to-land transition from an aquatic ancestor and flourished on land. Of the 30,000 estimated species, very few genomes have so far been published. Here, we assembled and characterized a chromosome-level genome of the "Spanish" slug, Arion vulgaris Moquin-Tandon, 1855, a notorious pest land slug in Europe. Using this reference genome, we conclude that a whole-genome duplication event occurred approximately 93-109 Mya at the base of Stylommatophora and might have promoted land invasion and adaptive radiation. Comparative genomic analyses reveal that genes related to the development of kidney, blood vessels, muscle, and nervous systems had expanded in the last common ancestor of land pulmonates, likely an evolutionary response to the terrestrial challenges of gravity and water loss. Analyses of A. vulgaris gene families and positively selected genes show the slug has evolved a stronger ability to counteract the greater threats of external damage, radiation, and water loss lacking a protective shell. Furthermore, a recent burst of long interspersed elements in the genome of A. vulgaris might affect gene regulation and contribute to rapid phenotype changes in A. vulgaris, which might be conducive to its rapid adaptation and invasiveness.

Induced polyploidy deeply influences reproductive life cycles, related phytochemical features, and phytohormonal activities in blackberry species

In some cases, polyploidy is an important phenomenon in the evolution of fruit crops. Polyploidy can be used in fruit breeding programs to develop varieties with higher yields and better fruit quality, as well as better adaptation to adverse environmental conditions. In this study, three wild species of blackberry were subjected to different degrees of induced polyploidy, and the effects of which were evaluated on morphological, physiological, and phytohormonal traits. With the aim of gaining a deep insight into the generative phase of plant growth and development, different levels of induced polyploidy were evaluated on the three blackberry species, i.e., Rubus persicus Bioss. (2x, 4x, and 8x), R. caesius L. (2x and 4x), and R. hirtus Schreb. (2x and 4x). The results showed that the polyploid plants performed significantly better than their diploid counterparts in terms of morphological traits such as flower count per spike and berry weight, as well as biochemical traits such as total soluble solids in the leaves. Induced polyploidy increased berry weight and drupe count per fruit. Microscopic examinations revealed a smaller number of viable pollen in the polyploids, compared to the diploids. Electron microscopy showed that the octaploid R. persicus had larger conical cells on the flower surface, compared to the diploid R. persicus. Correlation analysis showed that the ratio of indoleacetic acid to jasmonic acid changed synergistically with the total soluble solids in the leaves during the fruit set. The ploidy level correlated significantly with the number of pistils, leaf green index, total soluble solids in the leaves, and glucose content in floral nectar. Overall, induced polyploidy allowed Rubus to develop advantageous traits that can benefit future breeding programs and expand reproductive research in blackberries.

Repeat Dynamics across Timescales: A Perspective from Sibling Allotetraploid Marsh Orchids (Dactylorhiza majalis s.l.)

To provide insights into the fate of transposable elements (TEs) across timescales in a post-polyploidization context, we comparatively investigate five sibling Dactylorhiza allotetraploids (Orchidaceae) formed independently and sequentially between 500 and 100K generations ago by unidirectional hybridization between diploids D. fuchsii and D. incarnata. Our results first reveal that the paternal D. incarnata genome shows a marked increased content of LTR retrotransposons compared to the maternal species, reflected in its larger genome size and consistent with a previously hypothesized bottleneck. With regard to the allopolyploids, in the youngest D. purpurella both genome size and TE composition appear to be largely additive with respect to parents, whereas for polyploids of intermediate ages we uncover rampant genome expansion on a magnitude of multiple entire genomes of some plants such as Arabidopsis. The oldest allopolyploids in the series are not larger than the intermediate ones. A putative tandem repeat, potentially derived from a non-autonomous miniature inverted-repeat TE (MITE) drives much of the genome dynamics in the allopolyploids. The highly dynamic MITE-like element is found in higher proportions in the maternal diploid, D. fuchsii, but is observed to increase in copy number in both subgenomes of the allopolyploids. Altogether, the fate of repeats appears strongly regulated and therefore predictable across multiple independent allopolyploidization events in this system. Apart from the MITE-like element, we consistently document a mild genomic shock following the allopolyploidizations investigated here, which may be linked to their relatively large genome sizes, possibly associated with strong selection against further genome expansions.

De novo genes in Arachis hypogaea cv. Tifrunner: systematic identification, molecular evolution, and potential contributions to cultivated peanut

De novo genes are derived from non-coding sequences, and they can play essential roles in organisms. Cultivated peanut (Arachis hypogaea) is a major oil and protein crop derived from a cross between Arachis duranensis and Arachis ipaensis. However, few de novo genes have been documented in Arachis. Here, we identified 381 de novo genes in A. hypogaea cv. Tifrunner based on comparison with five closely related Arachis species. There are distinct differences in gene expression patterns and gene structures between conserved and de novo genes. The identified de novo genes originated from ancestral sequence regions associated with metabolic and biosynthetic processes, and they were subsequently integrated into existing regulatory networks. De novo paralogs and homoeologs were identified in A. hypogaea cv. Tifrunner. De novo paralogs and homoeologs with conserved expression have mismatching cis-acting elements under normal growth conditions. De novo genes potentially have pluripotent functions in responses to biotic stresses as well as in growth and development based on quantitative trait locus data. This work provides a foundation for future research examining gene birth processes and gene function in Arachis and related taxa.

Reference genome assemblies reveal the origin and evolution of allohexaploid oat

Common oat (Avena sativa) is an important cereal crop serving as a valuable source of forage and human food. Although reference genomes of many important crops have been generated, such work in oat has lagged behind, primarily owing to its large, repeat-rich polyploid genome. Here, using Oxford Nanopore ultralong sequencing and Hi-C technologies, we have generated a reference-quality genome assembly of hulless common oat, comprising 21 pseudomolecules with a total length of 10.76 Gb and contig N50 of 75.27 Mb. We also produced genome assemblies for diploid and tetraploid Avena ancestors, which enabled the identification of oat subgenomes and provided insights into oat chromosomal evolution. The origin of hexaploid oat is inferred from whole-genome sequencing, chloroplast genomes and transcriptome assemblies of different Avena species. These findings and the high-quality reference genomes presented here will facilitate the full use of crop genetic resources to accelerate oat improvement.

Idahoa and Subularia: Hidden polyploid origins of two enigmatic genera of crucifers

PREMISE

The monotypic Idahoa (I. scapigera) and the bispecific Subularia (S. aquatica and S. monticola) belong to Brassicaceae with unclear phylogenetic relationships and no tribal assignment. To fill this knowledge gap, we investigated these species and their closest relatives by combining cytogenomic and phylogenomic methods.

METHODS

We used whole plastome sequences in maximum likelihood and Bayesian inference analyses. We tested the phylogenetic informativeness of shared genomic repeats. We combined nuclear gene tree reconciliation and comparative chromosome painting (CCP) to examine the occurrence of past whole-genome duplications (WGDs).

RESULTS

The plastid data set corroborated the sister relationship between Idahoa and Subularia within the crucifer Lineage V but failed to resolve consistent topologies using both inference methods. The shared repetitive sequences provided conflicting pwhylogenetic signals. CCP analysis unexpectedly revealed that Idahoa (2n = 16) has a diploidized mesotetraploid genome, whereas two Subularia species (2n = 28 and 30) have diploidized mesoctoploid genomes. Several ancient allopolyploidy events have also been detected in closely related taxa (Chamira circaeoides, Cremolobeae, Eudemeae, and Notothlaspideae).

CONCLUSIONS

Our results suggest that the contentious phylogenetic placement of Idahoa and Subularia is best explained by two WGDs involving one or more shared parental genomes. The newly identified mesopolyploid genomes highlight the challenges of studying plant clades with complex polyploidy histories and provide a better framework for understanding genome evolution in the crucifer family.

Morphological, ecological and geographic differences between diploids and tetraploids of Symphytum officinale (Boraginaceae) justify both cytotypes as separate species

Polyploidization is generally considered to be an important evolutionary driver affecting the genetic diversity, that can alter the morphology, phenology, physiology or ecology of plants, which in turn may make the taxonomy of polyploids more difficult. One such example is the Symphytum officinale complex, a polyploid species group represented by three major cytotypes: tetraploids (2n = 48), less common, geographically restricted diploids (2n = 24) and hypotetraploids (2n = 40). In most European floras only one polymorphic species, S. officinale, is widely recognized, while the particular cytotypes are usually considered conspecific. Our study provided a thorough evaluation of the ploidy level diversity, morphological and ecological variation, with a special attempt to clarify the status of 'white-flowered' diploids. Using flow cytometry, we identified three cytotypes: widespread tetraploids (76.1 %); less frequent diploids (23.6 %) with scattered distribution across the range of tetraploids and confined only to several areas of Europe; and extremely rare triploids (0.3 %). Diploids and tetraploids showed diffuse parapatric pattern of distribution, with only four mixed-cytotype populations (2.7 %) found, but almost entirely without triploids, suggesting reproductive isolation between di- and tetraploids. Niche of diploids falls nearly completely within the niche of tetraploids that showed niche expansion. Tetraploids also showed a shift in niche optimum towards a less continental and colder climate, coupled with expansion to more disturbance-prone sites with higher nutrient availability. Diploids were clearly distinguishable morphologically from tetraploids. The morphological differentiation of studied cytotypes appears to be taxonomically significant, especially in combination with ecological differences and the apparent presence of hybridization barriers. Both cytotypes should be treated as separate species (i.e. S. bohemicum and S. officinale s. str.).

Parental legacy versus regulatory innovation in salt stress responsiveness of allopolyploid cotton (Gossypium) species

Polyploidy provides an opportunity for evolutionary innovation and species diversification, especially under stressful conditions. In allopolyploids, the conditional dynamics of homoeologous gene expression can be either inherited from ancestral states pre-existing in the parental diploids or novel upon polyploidization, the latter potentially permitting a wider range of phenotypic responses to stresses. To gain insight into regulatory mechanisms underlying the diversity of salt resistance in Gossypium species, we compared global transcriptomic responses to modest salinity stress in two allotetraploid (AD-genome) cotton species, Gossypium hirsutum and G. mustelinum, relative to their model diploid progenitors (A-genome and D-genome). Multivariate and pairwise analyses of salt-responsive changes revealed a profound alteration of gene expression for about one third of the transcriptome. Transcriptional responses and associated functional implications of salt acclimation varied across species, as did species-specific coexpression modules among species and ploidy levels. Salt responsiveness in both allopolyploids was strongly biased toward the D-genome progenitor. A much lower level of transgressive downregulation was observed in the more salt-tolerant G. mustelinum than in the less tolerant G. hirsutum. By disentangling inherited effects from evolved responses, we show that expression biases that are not conditional upon salt stress approximately equally reflect parental legacy and regulatory novelty upon allopolyploidization, whereas stress-responsive biases are predominantly novel, or evolved, in allopolyploids. Overall, our work suggests that allopolyploid cottons acquired a wide range of stress response flexibility relative to their diploid ancestors, most likely mediated by complex suites of duplicated genes and regulatory factors.

Polyploidization: Consequences of genome doubling on the evolutionary potential of populations

Whole-genome duplication is common in plants and is considered to have a broad range of effects on individuals' phenotypes and genomes and to be an important driver of plant adaptation and speciation. Despite their increased capacity to cope with challenging environments, polyploid lineages are generally as prone to extinction, and sometimes more prone, than their diploid progenitors. Although several explanations have been proposed to explain the short- and long-term disadvantages of polyploidy on the survival probability of populations, the consequences of whole-genome doubling on the heritable variance remain poorly studied. Whole-genome doubling can have major effects not only on the genetics, but also on the ecology and life history of the populations. Modifications of other properties of populations can reverse the effects of polyploidization per se on heritable variance. In this synthesis, I summarize the empirical and theoretical knowledge about the multifarious consequences of genome doubling on the heritable variance of quantitative traits and on the evolutionary potential of polyploid populations compared to their diploid progenitors. I propose several ways to decipher the consequences of whole-genome doubling on survival probability and to study the further consequences of shifting the ecological niche and life-history traits of a population. I also highlight some practical considerations for comparing the heritable variance of a trait among different cytotypes. Such investigations appear to be timely and necessary to understand more about the paradoxical aspects of polyploidization and to understand the evolutionary potential of polyploid lineages in a global warming context.

Altered Developmental and Metabolic Gene Expression in Basil Interspecific Hybrids

To understand the altered developmental changes and associated gene expression in inter-genomic combinations, a study was planned in two diverse yet closely related species of Ocimum, targeting their hybrid F1 and amphidiploids. The existing developmental variations between F1 and amphidiploids was analyzed through phenotypical and anatomical assessments. The absence of 8330 transcripts of F1 in amphidiploids and the exclusive presence of two transcripts related to WNK lysine-deficient protein kinase and geranylgeranyl transferase type-2 subunit beta 1-like proteins in amphidiploids provided a set of genes to compare the suppressed and activated functions between F1 and amphidiploids. The estimation of eugenol and methyleugenol, flavonoid, lignin and chlorophyll content was correlated with the average FPKM and differential gene expression values and further validated through qRT-PCR. Differentially expressed genes of stomatal patterning and development explained the higher density of stomata in F1 and the larger size of stomata in amphidiploids. Gene expression study of several transcription factors putatively involved in the growth and developmental processes of plants clearly amalgamates the transcriptome data linking the phenotypic differences in F1 and amphidiploids. This investigation describes the influence of interspecific hybridization on genes and transcription factors leading to developmental changes and alleviation of intergenomic instability in amphidiploids.

Diversity, phylogeny, and adaptation of bryophytes: insights from genomic and transcriptomic data

Bryophytes including mosses, liverworts, and hornworts are among the earliest land plants, and occupy a crucial phylogenetic position to aid in the understanding of plant terrestrialization. Despite their small size and simple structure, bryophytes are the second largest group of extant land plants. They live ubiquitously in various habitats and are highly diversified, with adaptive strategies to modern ecosystems on Earth. More and more genomes and transcriptomes have been assembled to address fundamental questions in plant biology. Here, we review recent advances in bryophytes associated with diversity, phylogeny, and ecological adaptation. Phylogenomic studies have provided increasing supports for the monophyly of bryophytes, with hornworts sister to the Setaphyta clade including liverworts and mosses. Further comparative genomic analyses revealed that multiple whole-genome duplications might have contributed to the species richness and morphological diversity in mosses. We highlight that the biological changes through gene gain or neofunctionalization that primarily evolved in bryophytes have facilitated the adaptation to early land environments; among the strategies to adapt to modern ecosystems in bryophytes, desiccation tolerance is the most remarkable. More genomic information for bryophytes would shed light on key mechanisms for the ecological success of these 'dwarfs' in the plant kingdom.

Phylotranscriptomics Illuminates the Placement of Whole Genome Duplications and Gene Retention in Ferns

Ferns are the second largest clade of vascular plants with over 10,000 species, yet the generation of genomic resources for the group has lagged behind other major clades of plants. Transcriptomic data have proven to be a powerful tool to assess phylogenetic relationships, using thousands of markers that are largely conserved across the genome, and without the need to sequence entire genomes. We assembled the largest nuclear phylogenetic dataset for ferns to date, including 2884 single-copy nuclear loci from 247 transcriptomes (242 ferns, five outgroups), and investigated phylogenetic relationships across the fern tree, the placement of whole genome duplications (WGDs), and gene retention patterns following WGDs. We generated a well-supported phylogeny of ferns and identified several regions of the fern phylogeny that demonstrate high levels of gene tree-species tree conflict, which largely correspond to areas of the phylogeny that have been difficult to resolve. Using a combination of approaches, we identified 27 WGDs across the phylogeny, including 18 large-scale events (involving more than one sampled taxon) and nine small-scale events (involving only one sampled taxon). Most inferred WGDs occur within single lineages (e.g., orders, families) rather than on the backbone of the phylogeny, although two inferred events are shared by leptosporangiate ferns (excluding Osmundales) and Polypodiales (excluding Lindsaeineae and Saccolomatineae), clades which correspond to the majority of fern diversity. We further examined how retained duplicates following WGDs compared across independent events and found that functions of retained genes were largely convergent, with processes involved in binding, responses to stimuli, and certain organelles over-represented in paralogs while processes involved in transport, organelles derived from endosymbiotic events, and signaling were under-represented. To date, our study is the most comprehensive investigation of the nuclear fern phylogeny, though several avenues for future research remain unexplored.

Equilibrated evolution of the mixed auto-/allopolyploid haplotype-resolved genome of the invasive hexaploid Prussian carp

Understanding genome evolution of polyploids requires dissection of their often highly similar subgenomes and haplotypes. Polyploid animal genome assemblies so far restricted homologous chromosomes to a 'collapsed' representation. Here, we sequenced the genome of the asexual Prussian carp, which is a close relative of the goldfish, and present a haplotype-resolved chromosome-scale assembly of a hexaploid animal. Genome-wide comparisons of the 150 chromosomes with those of two ancestral diploid cyprinids and the allotetraploid goldfish and common carp revealed the genomic structure, phylogeny and genome duplication history of its genome. It consists of 25 syntenic, homeologous chromosome groups and evolved by a recent autoploid addition to an allotetraploid ancestor. We show that de-polyploidization of the alloploid subgenomes on the individual gene level occurred in an equilibrated fashion. Analysis of the highly conserved actinopterygian gene set uncovered a subgenome dominance in duplicate gene loss of one ancestral chromosome set.

Phylogenomics Resolves the Phylogeny of Theaceae by Using Low-Copy and Multi-Copy Nuclear Gene Makers and Uncovers a Fast Radiation Event Contributing to Tea Plants Diversity

Simple Summary

The Theaceae includes more than 300 species of great morphological diversity and has immense economic, cultural, and ornamental values. However, the evolutionary history of this family remains elusive. We integrated 91 genomes and transcriptome datasets of Theaceae and successfully resolved the phylogeny of Theaceae including relatives of cultivated tea plants from both extensive low-copy and multi-copy nuclear gene markers. Bayes-based molecular dating revealed that the ancestor of the tea family originated slightly earlier than the K-Pg boundary (Mass extinction events including the extinction of dinosaurs) with early diversification of three tribes associated with the Early Eocene Climatic Optimum. Further speciation analysis suggested a sole significant diversification shift rate in the common ancestor of Camellia associated with the Mid-Miocene Climatic Optimum. Collectively, polyploidy events, and key morphological innovation characters, such as pericarp with seed coat hardening, could possibly contribute to the Theaceae species diversity.

Abstract

Tea is one of the three most popular nonalcoholic beverages globally and has extremely high economic and cultural value. Currently, the classification, taxonomy, and evolutionary history of the tea family are largely elusive, including phylogeny, divergence, speciation, and diversity. For understanding the evolutionary history and dynamics of species diversity in Theaceae, a robust phylogenetic framework based on 1785 low-copy and 79,103 multi-copy nuclear genes from 91 tea plant genomes and transcriptome datasets had been reconstructed. Our results maximumly supported that the tribes Stewartieae and Gordonieae are successive sister groups to the tribe Theeae from both coalescent and super matrix ML tree analyses. Moreover, in the most evolved tribe, Theeae, the monophyletic genera Pyrenaria, Apterosperma, and Polyspora are the successive sister groups of Camellia. We also yield a well-resolved relationship of Camellia, which contains the vast majority of Theaceae species richness. Molecular dating suggests that Theaceae originated in the late L-Cretaceous, with subsequent early radiation under the Early Eocene Climatic Optimal (EECO) for the three tribes. A diversification rate shift was detected in the common ancestors of Camellia with subsequent acceleration in speciation rate under the climate optimum in the early Miocene. These results provide a phylogenetic framework and new insights into factors that likely have contributed to the survival of Theaceae, especially a successful radiation event of genus Camellia members to subtropic/tropic regions. These novel findings will facilitate the efficient conservation and utilization of germplasm resources for breeding cultivated tea and oil-tea. Collectively, these results provide a foundation for further morphological and functional evolutionary analyses across Theaceae.

Transcriptome-wide characterization, evolutionary analysis, and expression pattern analysis of the NF-Y transcription factor gene family and salt stress response in Panax ginseng

Jilin ginseng (Panax ginseng C. A. Meyer) has a long history of medicinal use worldwide. The quality of ginseng is governed by a variety of internal and external factors. Nuclear factor Y (NF-Y), an important transcription factor in eukaryotes, plays a crucial role in the plant response to abiotic stresses by binding to a specific promoter, the CCAAT box. However, the NF-Y gene family has not been reported in Panax ginseng. In this study, 115 PgNF-Y transcripts with 40 gene IDs were identified from the Jilin ginseng transcriptome database. These genes were classified into the PgNF-YA (13), PgNF-YB (14), and PgNF-YC (13) subgroups according to their subunit types, and their nucleotide sequence lengths, structural domain information, and amino acid sequence lengths were analyzed. The phylogenetic analysis showed that the 79 PgNF-Y transcripts with complete ORFs were divided into three subfamilies, NF-YA, NF-YB, and NF-YC. PgNF-Y was annotated to eight subclasses under three major functions (BP, MF, and CC) by GO annotation, indicating that these transcripts perform different functions in ginseng growth and development. Expression pattern analysis of the roots of 42 farm cultivars, 14 different tissues of 4-year-old ginseng plants, and the roots of 4 different-ages of ginseng plants showed that PgNF-Y gene expression differed across lineages and had spatiotemporal specificity. Coexpression network analysis showed that PgNF-Ys acted synergistically with each other in Jilin ginseng. In addition, the analysis of the response of PgNF-YB09, PgNF-YC02, and PgNF-YC07-04 genes to salt stress treatment was investigated by fluorescence quantitative PCR. The expression of these genes increased after salt stress treatment, indicating that they may be involved in the regulation of the response to salt stresses in ginseng. These results provide important functional genetic resources for the improvement and gene breeding of ginseng in the future.Conclusions: This study fills a knowledge gap regarding the NF-Y gene family in ginseng, provides systematic theoretical support for subsequent research on PgNF-Y genes, and provides data resources for resistance to salt stress in ginseng.

Genome doubling enabled the expansion of yeast vesicle traffic pathways

Vesicle budding and fusion in eukaryotes depend on a suite of protein types, such as Arfs, Rabs, coats and SNAREs. Distinct paralogs of these proteins act at distinct intracellular locations, suggesting a link between gene duplication and the expansion of vesicle traffic pathways. Genome doubling, a common source of paralogous genes in fungi, provides an ideal setting in which to explore this link. Here we trace the fates of paralog doublets derived from the 100-Ma-old hybridization event that gave rise to the whole genome duplication clade of budding yeast. We find that paralog doublets involved in specific vesicle traffic functions and pathways are convergently retained across the entire clade. Vesicle coats and adaptors involved in secretory and early-endocytic pathways are retained as doublets, at rates several-fold higher than expected by chance. Proteins involved in later endocytic steps and intra-Golgi traffic, including the entire set of multi-subunit and coiled-coil tethers, have reverted to singletons. These patterns demonstrate that selection has acted to expand and diversify the yeast vesicle traffic apparatus, across species and time.

Admixture of divergent genomes facilitates hybridization across species in the family Brassicaceae

Hybridization and polyploidization are pivotal to plant evolution. Genetic crosses between distantly related species are rare in nature due to reproductive barriers but how such hurdles can be overcome is largely unknown. Here we report the hybrid genome structure of xBrassicoraphanus, a synthetic allotetraploid of Brassica rapa and Raphanus sativus. We performed cytogenetic analysis and de novo genome assembly to examine chromosome behaviors and genome integrity in the hybrid. Transcriptome analysis was conducted to investigate expression of duplicated genes in conjunction with epigenome analysis to address whether genome admixture entails epigenetic reconfiguration. Allotetraploid xBrassicoraphanus retains both parental chromosomes without genome rearrangement. Meiotic synapsis formation and chromosome exchange are avoided between nonhomologous progenitor chromosomes. Reconfiguration of transcription network occurs, and less divergent cis-elements of duplicated genes are associated with convergent expression. Genome-wide DNA methylation asymmetry between progenitors is largely maintained but, notably, B. rapa-originated transposable elements are transcriptionally silenced in xBrassicoraphanus through gain of DNA methylation. Our results demonstrate that hybrid genome stabilization and transcription compatibility necessitate epigenome landscape adjustment and rewiring of cis-trans interactions. Overall, this study suggests that a certain extent of genome divergence facilitates hybridization across species, which may explain the great diversification and expansion of angiosperms during evolution.