Unambiguous chromosome identification reveals the factors impacting irregular chromosome behaviors in allotriploid AAC Brassica

KEY MESSAGE

The major irregular chromosome pairing and mis-segregation were detected during meiosis through unambiguous chromosome identification and found that allotriploid Brassica can undergo meiosis successfully and produce mostly viable aneuploid gametes. Triploids have played a crucial role in the evolution of species by forming polyploids and facilitating interploidy gene transfer. It is widely accepted that triploids cannot undergo meiosis normally and predominantly produce nonfunctional aneuploid gametes, which restricts their role in species evolution. In this study, we demonstrated that natural and synthetic allotriploid Brassica (AAC), produced by crossing natural and synthetic Brassica napus (AACC) with Brassica rapa (AA), exhibits basically normal chromosome pairing and segregation during meiosis. Homologous A chromosomes paired faithfully and generally segregated equally. Monosomic C chromosomes were largely retained as univalents and randomly entered daughter cells. The primary irregular meiotic behaviors included associations of homoeologs and 45S rDNA loci at diakinesis, as well as homoeologous chromosome replacement and premature sister chromatid separation at anaphase I. Preexisting homoeologous arrangements altered meiotic behaviors in both chromosome irregular pairing and mis-segregation by increasing the formation of A-genomic univalents and A-C bivalents, as well as premature sister chromatid separation and homologous chromosome nondisjunction. Meiotic behaviors depended significantly on the genetic background and heterozygous homoeologous rearrangement. AAC triploids mainly generated aneuploid gametes, most of which were viable. These results demonstrate that allotriploid Brassica containing an intact karyotype can proceed through meiosis successfully, broadening our current understanding of the inheritance and role in species evolution of allotriploid.

Local adaptation, recombination, and the fate of neopolyploids

Polyploidy is widely recognized as an important speciation mechanism because it isolates tetraploids from their diploid progenitors. Polyploidy also provides new genetic material that may facilitate adaptive evolution. However, new mutations are more likely to arise after a neopolyploid has already successfully invaded a population. Thus, the role of adaptive forces in establishing a polyploid remains unclear. One solution to this apparent paradox may lie in the capacity of polyploids to suppress recombination among preexisting locally adapted alleles. The local adaptation mechanism requires that spatially heterogeneous selection acts on multiple loci and that gene flow introduces maladapted alleles to the population where the polyploid forms. The mechanism requires neither strong genetic drift nor any intrinsic benefit of genome doubling and can accommodate any mode of gene action. A unique prediction of the mechanism is that adaptive alleles should predate polyploidization, a pattern consistent with observations from a few well-studied polyploids. The mechanism is also consistent with the coexistence of both diploid and tetraploid cytotypes, fitness heterogeneity among independently derived polyploids, and the prevalence of outcrossing among older polyploids. The local adaptation mechanism also makes novel predictions about circumstances favoring polyploid invasions that can be tested using molecular genetic or comparative approaches.

Friends without benefits: Extensive cytotype sympatry and polyploid persistence in an African geophyte

PREMISE

Polyploidy is a major factor in plant adaptation and speciation. Multiple mechanisms contribute to autopolyploid frequency within populations, but uncertainties remain regarding mechanisms that facilitate polyploid establishment and persistence. Here we aimed to document and predict cytotype distributions of Oxalis obliquifolia Steud. ex A. Rich. across Gauteng, South Africa, and test for evidence of possible mechanisms, including morphological, phenological, and reproductive traits, that may potentially facilitate polyploid persistence.

METHODS

Over 320 O. obliquifolia plants from 25 sites were cytotyped using flow cytometry, and DNA ploidy was confirmed using meiotic chromosome squashes. Cytotypes were mapped and correlations with abiotic variables assessed using ordinations. To assess morphological and phenological associations with cytotype, we grew multiple cytotypes in a common garden, measured phenotypic traits and compared them using linear models and discriminant analyses. Intercytotype reproductive isolation was assessed using crossing experiments, and AMOVAs based on ITS DNA sequences tested for cytogeographic structure.

RESULTS

Six cytotypes were identified, and most sites had multiple cytotypes. Abiotic variables were not predictive of cytotype distribution. A clear gigas effect was present. Differences in flower size and phenology suggested pollinator interactions could play a role in polyploid persistence. Intercytotype crosses produced seed at low frequency. DNA data suggested diploids and polyploids were largely reproductively isolated in situ, and polyploidization events were not frequent enough to explain high cytotype sympatry.

CONCLUSIONS

Diploids and polyploids are behaving as separate species, despite little observable niche differentiation and non-zero potential intercytotype seed set. Tests on biotic interactions and intercytotype F1 fitness may provide insights into diploid and polyploid coexistence.

A metabolic perspective on polyploid invasion and the emergence of life histories: Insights from a mechanistic model

PREMISE

Whole-genome duplication (WGD, polyploidization) has been identified as a driver of genetic and phenotypic novelty, having pervasive consequences for the evolution of lineages. While polyploids are widespread, especially among plants, the long-term establishment of polyploids is exceedingly rare. Genome doubling commonly results in increased cell sizes and metabolic expenses, which may be sufficient to modulate polyploid establishment in environments where their diploid ancestors thrive.

METHODS

We developed a mechanistic simulation model of photosynthetic individuals to test whether changes in size and metabolic efficiency allow autopolyploids to coexist with, or even invade, ancestral diploid populations. Central to the model is metabolic efficiency, which determines how energy obtained from size-dependent photosynthetic production is allocated to basal metabolism as opposed to somatic and reproductive growth. We expected neopolyploids to establish successfully if they have equal or higher metabolic efficiency as diploids or to adapt their life history to offset metabolic inefficiency.

RESULTS

Polyploid invasion was observed across a wide range of metabolic efficiency differences between polyploids and diploids. Polyploids became established in diploid populations even when they had a lower metabolic efficiency, which was facilitated by recurrent formation. Competition for nutrients is a major driver of population dynamics in this model. Perenniality did not qualitatively affect the relative metabolic efficiency from which tetraploids tended to establish.

CONCLUSIONS

Feedback between size-dependent metabolism and energy allocation generated size and age differences between plants with different ploidies. We demonstrated that even small changes in metabolic efficiency are sufficient for the establishment of polyploids.

Defining autopolyploidy: Cytology, genetics, and taxonomy

Autopolyploidy is taxonomically defined as the presence of more than two copies of each genome within an organism or species, where the genomes present must all originate within the same species. Alternatively, "genetic" or "cytological" autopolyploidy is defined by polysomic inheritance: random pairing and segregation of the four (or more) homologous chromosomes present, with no preferential pairing partners. In this review, we provide an overview of methods used to categorize species as taxonomic and cytological autopolyploids, including both modern and obsolete cytological methods, marker-segregation-based and genomics methods. Subsequently, we also investigated how frequently polysomic inheritance has been reliably documented in autopolyploids. Pure or predominantly polysomic inheritance was documented in 39 of 43 putative autopolyploid species where inheritance data was available (91%) and in seven of eight synthetic autopolyploids, with several cases of more mixed inheritance within species. We found no clear cases of autopolyploids with disomic inheritance, which was likely a function of our search methodology. Interestingly, we found seven species with purely polysomic inheritance and another five species with partial or predominant polysomic inheritance that appear to be taxonomic allopolyploids. Our results suggest that observations of polysomic inheritance can lead to relabeling of taxonomically allopolyploid species as autopolyploid and highlight the need for further cytogenetic and genomic investigation into polyploid origins and inheritance types.

Doubling down on polyploid discoveries: Global advances in genomics and ecological impacts of polyploidy

All flowering plants are now recognized as diploidized paleopolyploids (Jiao et al., 2011; One Thousand Plant Transcriptomes Initiative, 2019), and polyploid species comprise approximately 30% of contemporary plant species (Wood et al., 2009; Barker et al., 2016a). A major implication of these discoveries is that, to appreciate the evolution of plant diversity, we need to understand the fundamental biology of polyploids and diploidization. This need is broadly recognized by our community as there is a continued, growing interest in polyploidy as a research topic. Over the past 25 years, the sequencing and analysis of plant genomes has revolutionized our understanding of the importance of polyploid speciation to the evolution of land plants.

Investigating historical drivers of latitudinal gradients in polyploid plant biogeography: A multiclade perspective

PREMISE

The proportion of polyploid plants in a community increases with latitude, and different hypotheses have been proposed about which factors drive this pattern. Here, we aimed to understand the historical causes of the latitudinal polyploidy gradient using a combination of ancestral state reconstruction methods. Specifically, we assessed whether (1) polyploidization enables movement to higher latitudes (i.e., polyploidization precedes occurrences in higher latitudes) or (2) higher latitudes facilitate polyploidization (i.e., occurrence in higher latitudes precedes polyploidization).

METHODS

We reconstructed the ploidy states and ancestral niches of 1032 angiosperm species at four paleoclimatic time slices ranging from 3.3 million years ago to the present, comprising taxa from four well-represented clades: Onagraceae, Primulaceae, Solanum (Solanaceae), and Pooideae (Poaceae). We used ancestral niche reconstruction models alongside a customized discrete character evolution model to allow reconstruction of states at specific time slices. Patterns of latitudinal movement were reconstructed and compared in relation to inferred ploidy shifts.

RESULTS

No single hypothesis applied equally well across all analyzed clades. While significant differences in median latitudinal occurrence were detected in the largest clade, Poaceae, no significant differences were detected in latitudinal movement in any clade.

CONCLUSIONS

Our preliminary study is the first to attempt to connect ploidy changes to continuous latitudinal movement, but we cannot favor one hypothesis over another. Given that patterns seem to be clade-specific, more clades must be analyzed in future studies for generalities to be drawn.

Neopolyploidy has variable effects on the diversity and composition of the wild strawberry microbiome

PREMISE

Whole-genome duplication (neopolyploidy) can instantly differentiate the phenotype of neopolyploids from their diploid progenitors. These phenotypic shifts in organs such as roots and leaves could also differentiate the way neopolyploids interact with microbial species. While some studies have addressed how specific microbial interactions are affected by neopolyploidy, we lack an understanding of how genome duplication affects the diversity and composition of microbial communities.

METHODS

We performed a common garden experiment with multiple clones of artificially synthesized autotetraploids and their ancestral diploids, derived from 13 genotypes of wild strawberry, Fragaria vesca. We sequenced epiphytic bacteria and fungi from roots and leaves and characterized microbial communities and leaf functional traits.

RESULTS

Autotetraploidy had no effect on bacterial alpha diversity of either organ, but it did have a genotype-dependent effect on the diversity of fungi on leaves. In contrast, autotetraploidy restructured the community composition of leaf bacteria and had a genotype-dependent effect on fungal community composition in both organs. The most differentially abundant bacterial taxon on leaves belonged to the Sphingomonas, while a member of the Trichoderma was the most differentially abundant fungal taxon on roots. Ploidy-induced change in leaf size was strongly correlated with a change in bacterial but not fungal leaf communities.

CONCLUSIONS

Genome duplication can immediately alter aspects of the plant microbiome, but this effect varies by host genotype and bacterial and fungal community. Expanding these studies to wild settings where plants are exposed continuously to microbes are needed to confirm the patterns observed here.

Complex Polyploids: Origins, Genomic Composition, and Role of Introgressed Alleles

Introgression allows polyploid species to acquire new genomic content from diploid progenitors or from other unrelated diploid or polyploid lineages, contributing to genetic diversity and facilitating adaptive allele discovery. In some cases, high levels of introgression elicit the replacement of large numbers of alleles inherited from the polyploid's ancestral species, profoundly reshaping the polyploid's genomic composition. In such complex polyploids, it is often difficult to determine which taxa were the progenitor species and which taxa provided additional introgressive blocks through subsequent hybridization. Here, we use population-level genomic data to reconstruct the phylogenetic history of Betula pubescens (downy birch), a tetraploid species often assumed to be of allopolyploid origin and which is known to hybridize with at least four other birch species. This was achieved by modeling polyploidization and introgression events under the multispecies coalescent and then using an approximate Bayesian computation rejection algorithm to evaluate and compare competing polyploidization models. We provide evidence that B. pubescens is the outcome of an autoploid genome doubling event in the common ancestor of B. pendula and its extant sister species, B. platyphylla, that took place approximately 178,000-188,000 generations ago. Extensive hybridization with B. pendula, B. nana, and B. humilis followed in the aftermath of autopolyploidization, with the relative contribution of each of these species to the B. pubescens genome varying markedly across the species' range. Functional analysis of B. pubescens loci containing alleles introgressed from B. nana identified multiple genes involved in climate adaptation, while loci containing alleles derived from B. humilis revealed several genes involved in the regulation of meiotic stability and pollen viability in plant species.

Polyploids of Brassicaceae: Genomic Insights and Assembly Strategies

The Brassicaceae family is distinguished by its inclusion of high-value crops such as cabbage, broccoli, mustard, and wasabi, all noted for their glucosinolates. In this family, many polyploidy species are distributed and shaped by numerous whole-genome duplications, independent genome doublings, and hybridization events. The evolutionary trajectory of the family is marked by enhanced diversification and lineage splitting after paleo- and meso-polyploidization, with discernible remnants of whole-genome duplications within their genomes. The recent neopolyploidization events notably increased the proportion of polyploid species within the family. Although sequencing efforts for the Brassicaceae genome have been robust, accurately distinguishing sub-genomes remains a significant challenge, frequently complicating the assembly process. Assembly strategies include comparative analyses with ancestral species and examining k-mers, long terminal repeat retrotransposons, and pollen sequencing. This review comprehensively explores the unique genomic characteristics of the Brassicaceae family, with a particular emphasis on polyploidization events and the latest strategies for sequencing and assembly. This review will significantly improve our understanding of polyploidy in the Brassicaceae family and assist in future genome assembly methods.

Polyploidy in maize: from evolution to breeding

Polyploidy played an important role in the evolution of the three most important crops: wheat, maize and rice, each of them providing a unique model for studying allopolyploidy, segmental alloploidy or paleopolyploidy. However, its genetic and evolutionary role is still vague. The undelying mechanisms and consequences of polyploidy remain fundamental objectives in the study of eukaryotes. Maize is one of the underutilized crops at the polyploid level. This species has no stable natural polyploids, the existing ones being artificially obtained. From the experimental polyploid series of maize, only the tetraploid forms (4n = 40) are of interest. They are characterized by some valuable morphological, physiological and biochemical features, superior to the diploid forms from which they originated, but also by some drawbacks such as: reduced fertility, slower development, longer vegetation period, low productivity and adaptedness. Due to these barriers to using tetraploids in field production, maize tetraploids primarily found utility in scientific studies regarding genetic variability, inbreeding, heterosis and gene dosage effect. Since the first mention of a triploid maize plant to present, many scientists and schools, devoted their efforts to capitalize on the use of polyploidy in maize. Despite its common disadvantages as a crop, significant progress in developing tetraploid maize with good agronomic performance was achieved leading to registered tetraploid maize varieties. In this review we summarize and discuss the different aspects of polyploidy in maize, such as evolutionary context, methods of induction, morphology, fertility issue, inheritance patterns, gene expression and potential use.

Genome-wide data reveal bi-direction and asymmetrical hybridization origin of a fern species Microlepia matthewii

Introduction

Natural hybridization is common and plays a crucial role in driving biodiversity in nature. Despite its significance, the understanding of hybridization in ferns remains inadequate. Therefore, it is imperative to study fern hybridization to gain a more comprehensive understanding of fern biodiversity. Our study delves into the role of hybridization in shaping fern species, employing Microlepia matthewii as a case study to investigate its origins of hybridization.

Methods

We performed double digest Genotyping-by-sequencing (dd-GBS) on M. matthewii and its potential parent species, identifying nuclear and chloroplast SNPs. Initially, nuclear SNPs were employed to construct the three cluster analysis: phylogenetic tree, principal component analysis, and population structure analysis. Subsequently, to confirm whether the observed genetic mixture pattern resulted from hybridization, we utilized two methods: ABBA-BABA statistical values in the D-suite program and gene frequency covariance in the Treemix software to detect gene flow. Finally, we employed chloroplast SNPs to construct a phylogenetic tree, tracing the maternal origin.

Results and discussion

The analysis of the nuclear SNP cluster revealed that M. matthewii possesses a genetic composition that is a combination of M. hancei and M. calvescens. Furthermore, the analysis provided strong evidence of significant gene flow signatures from the parental species to the hybrid, as indicated by the two gene flow analyses. The samples of M. matthewii cluster separately with M. hancei or M. calvescens on the chloroplast systematic tree. However, the parentage ratio significantly differs from 1:1, suggesting that M. matthewii is a bidirectional and asymmetrical hybrid offspring of M. hancei and M. calvescens.

Clonal growth characteristics and diversity patterns of different Clintonia udensis (Liliaceae) diploid and tetraploid cytotypes in the Hualongshan Mountains

Polyploidization plays an important role in plant evolution and biodiversity. However, intraspecific polyploidy compared to interspecific polyploidy received less attention. Clintonia udensis (Liliaceae) possess diploid (2n = 2x = 14) and autotetraploid (2n = 4x = 28) cytotypes. In the Hualongshan Mountains, the autotetraploids grew on the northern slope, while the diploids grew on the southern slopes. The clonal growth characteristics and clonal architecture were measured and analyzed by field observations and morphological methods. The diversity level and differentiation patterns for two different cytotypes were investigated using SSR markers. The results showed that the clonal growth parameters, such as the bud numbers of each rhizome node and the ratio of rhizome branches in the autotetraploids were higher than those in the diploids. Both the diploids and autotetraploids appeared phalanx clonal architectures with short internodes between ramets. However, the ramets or genets of the diploids had a relatively scattered distribution, while those of the autotetraploids were relatively clumping. The diploids and autotetraploids all allocated more biomass to their vegetative growth. The diploids had a higher allocation to reproductive organs than that of autotetraploids, which indicated that the tetraploids invested more resources in clonal reproduction than diploids. The clone diversity and genetic diversity of the autotetraploids were higher than that of the diploids. Significant genetic differentiation between two different cytotypes was observed (P < 0.01). During establishment and evolution, C. udensis autotetraploids employed more clumping phalanx clonal architecture and exhibited more genetic variation than the diploids.

Shared single copy genes are generally reliable for inferring phylogenetic relationships among polyploid taxa

Polyploidy, or whole-genome duplication, is expected to confound the inference of species trees with phylogenetic methods for two reasons. First, the presence of retained duplicated genes requires the reconciliation of the inferred gene trees to a proposed species tree. Second, even if the analyses are restricted to shared single copy genes, the occurrence of reciprocal gene loss, where the surviving genes in different species are paralogs from the polyploidy rather than orthologs, will mean that such genes will not have evolved under the corresponding species tree and may not produce gene trees that allow inference of that species tree. Here we analyze three different ancient polyploidy events, using synteny-based inferences of orthology and paralogy to infer gene trees from nearly 17,000 sets of homologous genes. We find that the simple use of single copy genes from polyploid organisms provides reasonably robust phylogenetic signals, despite the presence of reciprocal gene losses. Such gene trees are also most often in accord with the inferred species relationships inferred from maximum likelihood models of gene loss after polyploidy: a completely distinct phylogenetic signal present in these genomes. As seen in other studies, however, we find that methods for inferring phylogenetic confidence yield high support values even in cases where the underlying data suggest meaningful conflict in the phylogenetic signals.

Genome-wide patterns of homoeologous gene flow in allotetraploid coffee

Premise

Allopolyploidy-a hybridization-induced whole-genome duplication event-has been a major driver of plant diversification. The extent to which chromosomes pair with their proper homolog vs. with their homoeolog in allopolyploids varies across taxa, and methods to detect homoeologous gene flow (HGF) are needed to understand how HGF has shaped polyploid lineages.

Methods

The ABBA-BABA test represents a classic method for detecting introgression between closely related species, but here we developed a modified use of the ABBA-BABA test to characterize the extent and direction of HGF in allotetraploid Coffea arabica.

Results

We found that HGF is abundant in the C. arabica genome, with both subgenomes serving as donors and recipients of variation. We also found that HGF is highly maternally biased in plastid-targeted-but not mitochondrial-targeted-genes, as would be expected if plastid-nuclear incompatibilities exist between the two parent species.

Discussion

Together, our analyses provide a simple framework for detecting HGF and new evidence consistent with selection favoring overwriting of paternally derived alleles by maternally derived alleles to ameliorate plastid-nuclear incompatibilities. Natural selection therefore appears to shape the direction and intensity of HGF in allopolyploid coffee, indicating that cytoplasmic inheritance has long-term consequences for polyploid lineages.

A locus-dependent mixed inheritance in the segmental allohexaploid sweetpotato (Ipomoea batatas [L.] Lam)

Two interrelated aspects of the sweetpotato genome, its polyploid origin and inheritance type, remain uncertain. We recently proposed a segmental allohexaploid sweetpotato and thus sought to clarify its inheritance type by direct analyses of homoeolog segregations at selected single-copy loci. For such analyses, we developed a digital quantitative PCR genotyping method using one nondiscriminatory and three discriminatory probes for each selected locus to discriminate and quantify three homoeolog-differentiating variation types (homoeolog-types) in genomic DNA samples for genotype fitting and constructed a F2 population for segregation analyses. We confirmed inter-subgenomic distinctions of three identified homoeolog-types at each of five selected loci by their interspecific differentiations among 14 species in Ipomoea section batatas and genotyped the loci in 549 F2 lines, selected F1 progenies, and their founding parents. Segregation and genotype analyses revealed a locus-dependent mixed inheritance (disomic, polysomic, and intermediate types) of the homoeolog-types at 4 loci in the F2 population, displaying estimated disomic-inheritance frequencies of 0, 2.72%, 14.52%, and 36.92%, and probably in the F1 population too. There were also low-frequency non-hexaploid F1 and F2 genotypes that were probably derived from double-reduction recombination or partially unreduced gametes, and F2 genotypes of apparent aneuploids/dysploids with neopolyploid-like frequencies. Additional analyses of homoeolog-type genotypes at the 5 loci in 46 lines from various regions revealed locus-dependent selection biases, favoring genotypes having more of one homoeolog-type, i.e. more of di- or homogenized homoeolog-type composition, and one-direction ploidy trending among apparent aneuploids/dysploids. These inheritance features pointed to an evolving segmental allohexaploid sweetpotato impacted by selection biases.

Characterization of the cryptic interspecific hybrid Lemna×mediterranea by an integrated approach provides new insights into duckweed diversity

Lemnaceae taxonomy is challenged by the particular morphology of these tiny free-floating angiosperms. Although molecular taxonomy has helped clarify the phylogenetic history of this family, some inconsistency with morphological data leads to frequent misclassifications in the genus Lemna. Recently, the finding that Lemna japonica is an interspecific hybrid between Lemna minor and Lemna turionifera provided a clear explanation for one such taxonomic question. Here we demonstrated that L. minor is also capable of hybridizing with Lemna gibba, generating a cryptic but widespread taxon in the Mediterranean area. The nothotaxon Lemna ×mediterranea is described and compared with clones of the putative parental species L. minor and L. gibba. Genetic analysis by nuclear and plastid markers, as well as genome size measurement, revealed that two different cytotypes, diploid and triploid, originated by at least two independent hybridization events. Despite high overall similarity, morphometrical, physiological, and biochemical analyses showed an intermediate position of L. ×mediterranea between its parental species in most qualitative and quantitative characters, and also separation of the two hybrid cytotypes by some criteria. These data provide evidence that hybridization and polyploidization, driving forces of terrestrial plant evolution, contribute to duckweed genetic diversity and may have shaped the phylogenetic history of these mainly asexual, aquatic plants.

Evolutionary Dynamics of Chromatin Structure and Duplicate Gene Expression in Diploid and Allopolyploid Cotton

Polyploidy is a prominent mechanism of plant speciation and adaptation, yet the mechanistic understandings of duplicated gene regulation remain elusive. Chromatin structure dynamics are suggested to govern gene regulatory control. Here, we characterized genome-wide nucleosome organization and chromatin accessibility in allotetraploid cotton, Gossypium hirsutum (AADD, 2n = 4X = 52), relative to its two diploid parents (AA or DD genome) and their synthetic diploid hybrid (AD), using DNS-seq. The larger A-genome exhibited wider average nucleosome spacing in diploids, and this intergenomic difference diminished in the allopolyploid but not hybrid. Allopolyploidization also exhibited increased accessibility at promoters genome-wide and synchronized cis-regulatory motifs between subgenomes. A prominent cis-acting control was inferred for chromatin dynamics and demonstrated by transposable element removal from promoters. Linking accessibility to gene expression patterns, we found distinct regulatory effects for hybridization and later allopolyploid stages, including nuanced establishment of homoeolog expression bias and expression level dominance. Histone gene expression and nucleosome organization are coordinated through chromatin accessibility. Our study demonstrates the capability to track high-resolution chromatin structure dynamics and reveals their role in the evolution of cis-regulatory landscapes and duplicate gene expression in polyploids, illuminating regulatory ties to subgenomic asymmetry and dominance.

Deep genome skimming reveals the hybrid origin of Pseudosasa gracilis (Poaceae: Bambusoideae)

Pseudosasa gracilis (Poaceae: Bambusoideae) is a temperate woody bamboo species endemic to South-central China with a narrow distribution. Previous phylogenetic studies revealed an unexpected, isolated phylogenetic position of Ps. gracilis. Here we conducted phylogenomic analysis by sampling populations of Ps. gracilis and its sympatric species Ps. nanunica and Sinosasa polytricha reflecting different genomic signals, by deep genome skimming. Integrating molecular evidence from chloroplast genes and genome-wide SNPs, we deciphered the phylogenetic relationships of Ps. gracilis. Both plastid and nuclear data indicate that Ps. gracilis is more closely related to Sinosasa, which is discordant with the taxonomic treatment. To further explore this molecular-morphological conflict, we screened 411 "perfect-copy" syntenic genes to reconstruct phylogenies using both the concatenation and coalescent methods. We observed extensive discordance between gene trees and the putative species tree. A significant hybridization event was detected based on 411 genes from the D subgenome, showing Ps. gracilis was a hybrid descendant between Sinosasa longiligulata and Ps. nanunica, with 63.56% and 36.44% inheritance probabilities of each parent. Moreover, introgression events were detected in the C subgenome between Ps. gracilis and S. polytricha in the same distribution region. Our findings suggest that sympatric hybridization and introgression play a crucial role in the origin of Ps. gracilis. By providing an empirical example of bamboo of hybrid origin using comprehensive analyses based on genomic data from different inheritance systems and morphological characters, our study represents a step forward in understanding of reticulate evolution of bamboos.

Phenotypic variation seems not to be associated with the genetic profile in Zygopetalum (Orchidaceae): a case study of a high-elevation rocky complex

BACKGROUND

Hybridization associated with polyploidy studies is rare in the tropics. The genus Zygopetalum (Orchidaceae) was investigated here as a case study of Neotropical plants. In the rocky highlands of the Ibitipoca State Park (ISP), southeast Brazil, individuals with intermediate colors and forms between the species Z. maculatum and Z. triste were commonly identified.

METHODS AND RESULTS

Chromosomal analysis and DNA quantity showed a uniform population. Regardless of the aspects related to the color and shape of floral structures, all individuals showed 2n = 96 chromosomes and an average of 14.05 pg of DNA. Irregularities in meiosis associated with chromosome number and C value suggest the occurrence of polyploidy. The genetic distance estimated using ISSR molecular markers revealed the existence of genetic variability not related to morphological clusters. Morphometric measurements of the flower pieces revealed that Z. maculatum shows higher variation than Z. triste although lacking a defined circumscription.

CONCLUSION

The observed variation can be explained by the polyploid and phenotypic plasticity resulting from the interaction of the genotypes with the heterogeneous environments observed in this habitat.

Pangenome analysis reveals transposon-driven genome evolution in cotton

BACKGROUND

Transposable elements (TEs) have a profound influence on the trajectory of plant evolution, driving genome expansion and catalyzing phenotypic diversification. The pangenome, a comprehensive genetic pool encompassing all variations within a species, serves as an invaluable tool, unaffected by the confounding factors of intraspecific diversity. This allows for a more nuanced exploration of plant TE evolution.

RESULTS

Here, we constructed a pangenome for diploid A-genome cotton using 344 accessions from representative geographical regions, including 223 from China as the main component. We found 511 Mb of non-reference sequences (NRSs) and revealed the presence of 5479 previously undiscovered protein-coding genes. Our comprehensive approach enabled us to decipher the genetic underpinnings of the distinct geographic distributions of cotton. Notably, we identified 3301 presence-absence variations (PAVs) that are closely tied to gene expression patterns within the pangenome, among which 2342 novel expression quantitative trait loci (eQTLs) were found residing in NRSs. Our investigation also unveiled contrasting patterns of transposon proliferation between diploid and tetraploid cotton, with long terminal repeat (LTR) retrotransposons exhibiting a synchronized surge in polyploids. Furthermore, the invasion of LTR retrotransposons from the A subgenome to the D subgenome triggered a substantial expansion of the latter following polyploidization. In addition, we found that TE insertions were responsible for the loss of 36.2% of species-specific genes, as well as the generation of entirely new species-specific genes.

CONCLUSIONS

Our pangenome analyses provide new insights into cotton genomics and subgenome dynamics after polyploidization and demonstrate the power of pangenome approaches for elucidating transposon impacts and genome evolution.

The genetic consequences of range expansion and its influence on diploidization in polyploids

Despite newly formed polyploids being subjected to myriad fitness consequences, the relative prevalence of polyploidy both contemporarily and in ancestral branches of the tree of life suggests alternative advantages that outweigh these consequences. One proposed advantage is that polyploids may more easily colonize novel habitats such as deglaciated areas. However, previous research conducted in diploids suggests that range expansion comes with a fitness cost as deleterious mutations may fix rapidly on the expansion front. Here, we interrogate the potential consequences of expansion in polyploids by conducting spatially explicit forward-in-time simulations to investigate how ploidy and inheritance patterns impact the relative ability of polyploids to expand their range. We show that under realistic dominance models, autopolyploids suffer greater fitness reductions than diploids as a result of range expansion due to the fixation of increased mutational load that is masked in the range core. Alternatively, the disomic inheritance of allopolyploids provides a shield to this fixation resulting in minimal fitness consequences. In light of this advantage provided by disomy, we investigate how range expansion may influence cytogenetic diploidization through the reversion to disomy in autotetraploids. We show that under a wide range of parameters investigated for two models of diploidization, disomy frequently evolves more rapidly on the expansion front than in the range core, and that this dynamic inheritance model has additional effects on fitness. Together our results point to a complex interaction between dominance, ploidy, inheritance, and recombination on fitness as a population spreads across a geographic range.

Traits-based approach: leveraging genome size in plant-microbe interactions

Trait-based approaches have gained growing interest in studying plant-microbe interactions. However, current traits normally considered (e.g., morphological, physiological, or chemical traits) are biased towards those showing large intraspecific variations, necessitating the identification of fewer plastic traits that differ between species. Here, we propose using genome size (the amount of DNA in the nucleus of a cell) as a suitable trait for studying plant-microbiome interactions due to its relatively stable nature, minimally affected by external environmental variations. Emerging evidence suggests that plant genome size affects the plant-associated microbial community, and tissue-specific environments select microbes based on their genome size. These findings pinpoint environmental selection in genome size as an emerging driver of plant-microbiome interactions, potentially impacting ecosystem functions and productivity.

Exploring the patterns of evolution: Core thoughts and focus on the saltational model

The Modern Synthesis, a pillar in biological thought, united Darwin's species origin concepts with Mendel's laws of character heredity, providing a comprehensive understanding of evolution within species. Highlighting phenotypic variation and natural selection, it elucidated the environment's role as a selective force, shaping populations over time. This framework integrated additional mechanisms, including genetic drift, random mutations, and gene flow, predicting their cumulative effects on microevolution and the emergence of new species. Beyond the Modern Synthesis, the Extended Evolutionary Synthesis expands perspectives by recognizing the role of developmental plasticity, non-genetic inheritance, and epigenetics. We suggest that these aspects coexist in the plant evolutionary process; in this context, we focus on the saltational model, emphasizing how saltation events, such as dichotomous saltation, chromosomal mutations, epigenetic phenomena, and polyploidy, contribute to rapid evolutionary changes. The saltational model proposes that certain evolutionary changes, such as the rise of new species, may result suddenly from single macromutations rather than from gradual changes in DNA sequences and allele frequencies within a species over time. These events, observed in domesticated and wild higher plants, provide well-defined mechanistic bases, revealing their profound impact on plant diversity and rapid evolutionary events. Notably, next-generation sequencing exposes the likely crucial role of allopolyploidy and autopolyploidy (saltational events) in generating new plant species, each characterized by distinct chromosomal complements. In conclusion, through this review, we offer a thorough exploration of the ongoing dissertation on the saltational model, elucidating its implications for our understanding of plant evolutionary processes and paving the way for continued research in this intriguing field.

The emerging importance of cross-ploidy hybridisation and introgression

Natural hybridisation is now recognised as pervasive in its occurrence across the Tree of Life. Resurgent interest in natural hybridisation fuelled by developments in genomics has led to an improved understanding of the genetic factors that promote or prevent species cross-mating. Despite this body of work overturning many widely held assumptions about the genetic barriers to hybridisation, it is still widely thought that ploidy differences between species will be an absolute barrier to hybridisation and introgression. Here, we revisit this assumption, reviewing findings from surveys of polyploidy and hybridisation in the wild. In a case study in the British flora, 203 hybrids representing 35% of hybrids with suitable data have formed via cross-ploidy matings, while a wider literature search revealed 59 studies (56 in plants and 3 in animals) in which cross-ploidy hybridisation has been confirmed with genetic data. These results show cross-ploidy hybridisation is readily overlooked, and potentially common in some groups. General findings from these studies include strong directionality of hybridisation, with introgression usually towards the higher ploidy parent, and cross-ploidy hybridisation being more likely to involve allopolyploids than autopolyploids. Evidence for adaptive introgression across a ploidy barrier and cases of cross-ploidy hybrid speciation shows the potential for important evolutionary outcomes.

Formation of Different Polyploids Through Disrupting Meiotic Crossover Frequencies Based on cntd1 Knockout in Zebrafish

Polyploidy, a significant catalyst for speciation and evolutionary processes in both plant and animal kingdoms, has been recognized for a long time. However, the exact molecular mechanism that leads to polyploid formation, especially in vertebrates, is not fully understood. Our study aimed to elucidate this phenomenon using the zebrafish model. We successfully achieved an effective knockout of the cyclin N-terminal domain containing 1 (cntd1) using CRISPR/Cas9 technology. This resulted in impaired formation of meiotic crossovers, leading to cell-cycle arrest during meiotic metaphase and triggering apoptosis of spermatocytes in the testes. Despite these defects, the mutant (cntd1-/-) males were still able to produce a limited amount of sperm with normal ploidy and function. Interestingly, in the mutant females, it was the ploidy not the capacity of egg production that was altered. This resulted in the production of haploid, aneuploid, and unreduced gametes. This alteration enabled us to successfully obtain triploid and tetraploid zebrafish from cntd1-/- and cntd1-/-/- females, respectively. Furthermore, the tetraploid-heterozygous zebrafish produced reduced-diploid gametes and yielded all-triploid or all-tetraploid offspring when crossed with wild-type (WT) or tetraploid zebrafish, respectively. Collectively, our findings provide direct evidence supporting the crucial role of meiotic crossover defects in the process of polyploidization. This is particularly evident in the generation of unreduced eggs in fish and, potentially, other vertebrate species.

Incorrect recombination partner associations contribute to meiotic instability of neo-allopolyploid Arabidopsis suecica

Combining two or more related homoeologous genomes in a single nucleus, newly formed allopolyploids must rapidly adapt meiosis to restore balanced chromosome segregation, production of euploid gametes and fertility. The poor fertility of such neo-allopolyploids thus strongly selects for the limitation or avoidance of genetic crossover formation between homoeologous chromosomes. In this study, we have reproduced the interspecific hybridization between Arabidopsis thaliana and Arabidopsis arenosa leading to the allotetraploid Arabidopsis suecica and have characterized the first allopolyploid meioses. First-generation neo-allopolyploid siblings vary considerably in fertility, meiotic behavior and levels of homoeologous recombination. We show that centromere dynamics at early meiosis is altered in synthetic neo-allopolyploids compared with evolved A. suecica, with a significant increase in homoeologous centromere interactions at zygotene. At metaphase I, the presence of multivalents involving homoeologous chromosomes confirms that homoeologous recombination occurs in the first-generation synthetic allopolyploid plants and this is associated with a significant reduction in homologous recombination, compared to evolved A. suecica. Together, these data strongly suggest that the fidelity of recombination partner choice, likely during the DNA invasion step, is strongly impaired during the first meiosis of neo-allopolyploids and requires subsequent adaptation.

Identification and Validation of Reference Genes for qRT-PCR Analysis of Petal-Color-Related Genes in Rosa praelucens

The flower's color is regarded as one of the most outstanding features of the rose. Rosa praelucens Byhouwer, an endemic and critically endangered decaploid wild rose species, is abundant in phenotypic diversity, especially in flower color variation, from white to different degrees of pink. The mechanism underlying this variation, e.g., the level of petal-color-related genes, is worth probing. Seven candidate reference genes for qRT-PCR analysis, including tubulin α chain (TUBA), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), histone H2B (Histone2A), eukaryotic translation elongation factor 1-α (EEF1A), 60S ribosomal protein (RPL37), eukaryotic translation initiation factor 1-α (EIF1A), and aquaporins (AQP), were detected from the transcriptome datasets of full blooming flowers of white-petaled and pink-petaled individuals, and their expression stabilities were evaluated through qRT-PCR analysis. According to stability rankings analysis, EEF1A showed the highest stability and could be chosen as the most suitable reference gene. Moreover, the reliability of EEF1A was demonstrated via qRT-PCR analysis of six petal-color-related target genes, the expression patterns of which, through EEF1A normalization, were found to be consistent with the findings of transcriptome analysis. The result provides an optimal reference gene for exploring the expression level of petal-color-related genes in R. praelucens, which will accelerate the dissection of petal-color-variation mechanisms in R. praelucens.

Phenotypic characterization, plant growth and development, genome methylation, and mineral elements composition of neotetraploid lettuce (Lactuca sativa L.)

Stable neotetraploid lines of lettuce (Lactuca sativa L.) were produced from three phenotypically distinct cultivars (Annapolis, Eruption, Merlot) and an advanced breeding line (SM13-L2) using colchicine treatment of seeds or young seedlings. When tested under the greenhouse and field conditions, neotetraploids initially grew more rapidly than their diploid progenitors, however they reached their reproductive stage (bolting, flower bud formation, and flowering) substantially later. Seeds production on neotetraploids was delayed by more than 30 days compared to diploids. Tetraploid plants had fewer, but larger stomata and leaves, less chlorophyll per area, higher photosystem II photochemical efficiency, generally lighter root system, and produced less than 1% of seeds in comparison with diploids. Field-grown neotetraploids of all lines displayed a significant reduction in tipburn (1.8% vs. 22.2%, respectively), a highly undesirable physiological disorder. Changes in leaf and root mineral composition were detected in neotetraploids. Several elements were found in lower abundance than in diploids, most notably iron, calcium, and silicon. Whole genome bisulfite sequencing (WGBS) revealed 498 differentially methylated regions (DMR), with 106 of these regions having at least 50% difference in the level of methylation between neotetraploids and their diploid progenitors. At least 18 of the most prominent DMR were detected in proximity to genes predicted to be involved in plant development or reaction to biotic and abiotic stressors. Because neotetraploid lines have low seed production, they are not suitable for commercial cultivation. They can be used, however, in research to study the factors contributing to tipburn, traits affected by stomata size or density, and the effect of ploidy on resistance to environmental stressors.

Subgenomic Stability of Progenitor Genomes During Repeated Allotetraploid Origins of the Same Grass Brachypodium hybridum

Both homeologous exchanges and homeologous expression bias are generally found in most allopolyploid species. Whether homeologous exchanges and homeologous expression bias differ between repeated allopolyploid speciation events from the same progenitor species remains unknown. Here, we detected a third independent and recent allotetraploid origin for the model grass Brachypodium hybridum. Our homeologous exchange with replacement analyses indicated the absence of significant homeologous exchanges in any of the three types of wild allotetraploids, supporting the integrity of their progenitor subgenomes and the immediate creation of the amphidiploids. Further homeologous expression bias tests did not uncover significant subgenomic dominance in different tissues and conditions of the allotetraploids. This suggests a balanced expression of homeologs under similar or dissimilar ecological conditions in their natural habitats. We observed that the density of transposons around genes was not associated with the initial establishment of subgenome dominance; rather, this feature is inherited from the progenitor genome. We found that drought response genes were highly induced in the two subgenomes, likely contributing to the local adaptation of this species to arid habitats in the third allotetraploid event. These findings provide evidence for the consistency of subgenomic stability of parental genomes across multiple allopolyploidization events that led to the same species at different periods. Our study emphasizes the importance of selecting closely related progenitor species genomes to accurately assess homeologous exchange with replacement in allopolyploids, thereby avoiding the detection of false homeologous exchanges when using less related progenitor species genomes.

HOP1 and HAP2 are conserved components of the meiosis-related machinery required for successful mating in Leishmania

Whole genome analysis of Leishmania hybrids generated experimentally in sand flies supports a meiotic mechanism of genetic exchange, with Mendelian segregation of the nuclear genome. Here, we perform functional analyses through the generation of double drug-resistant hybrids in vitro and in vivo (during sand fly infections) to assess the importance of conserved meiosis-related genes in recombination and plasmogamy. We report that HOP1 and a HAP2-paralog (HAP2-2) are essential components of the Leishmania meiosis machinery and cell-to-cell fusion mechanism, respectively, since deletion of either gene in one or both parents significantly reduces or completely abrogates mating competence. These findings significantly advance our understanding of sexual reproduction in Leishmania, with likely relevance to other trypanosomatids, by formally demonstrating the involvement of a meiotic protein homolog and a distinct fusogen that mediates non-canonical, bilateral fusion in the hybridizing cells.

Cryptic invasion suggested by a cytogeographic analysis of the halophytic Puccinellia distans complex (Poaceae) in Central Europe

Introduction

Despite the wealth of studies dealing with the invasions of alien plants, invasions of alien genotypes of native species (cryptic invasions) have been vastly neglected. The impact of cryptic invasions on the biodiversity of plant communities can, however, be significant. Inland saline habitats and halophytes (i.e., salt-tolerant plant species) are especially threatened by this phenomenon as they inhabit fragmented remnants of largely destroyed habitats, but at the same time some of these halophytic species are rapidly spreading along salt-treated roads. To study potential cryptic invasion of halophytes, the patterns of genome size and ploidy variation in the Puccinellia distans complex (Poaceae), the most rapidly spreading roadside halophyte in Central Europe, were investigated.

Methods

DNA flow cytometry with confirmatory chromosome counts were employed to assess ploidy levels of 1414 individuals from 133 populations of the P. distans complex. In addition, climatic niche modelling was used to predict the distributions of selected cytotypes.

Results

Eight groups differing in ploidy level and/or genome size were discovered, one diploid (2x; 2n = 14), two tetraploid (4xA, 4xB; 2n = 28), one pentaploid (5x; 2n = 35), three hexaploid (6xA, 6xB, 6xC; 2n = 42), and one heptaploid (7x; 2n = 49). The hexaploids (mostly the 6xC cytotype) were widespread through the study area, spreading intensively in both anthropogenic and natural habitats and probably hybridizing with the natural habitat dwelling tetraploids. In contrast, the non-hexaploid cytotypes rarely spread and were predominantly confined to natural habitats.

Discussion

The extensive spread of the hexaploid cytotypes along roadsides has most likely facilitated their incursion into natural habitats. The colonization of new natural habitats by the hexaploids may pose a threat to the indigenous Puccinellia populations by compromising their genetic integrity and/or by outcompeting them.

The duplication of genomes and genetic networks and its potential for evolutionary adaptation and survival during environmental turmoil

The importance of whole-genome duplication (WGD) for evolution is controversial. Whereas some view WGD mainly as detrimental and an evolutionary dead end, there is growing evidence that polyploidization can help overcome environmental change, stressful conditions, or periods of extinction. However, despite much research, the mechanistic underpinnings of why and how polyploids might be able to outcompete or outlive nonpolyploids at times of environmental upheaval remain elusive, especially for autopolyploids, in which heterosis effects are limited. On the longer term, WGD might increase both mutational and environmental robustness due to redundancy and increased genetic variation, but on the short-or even immediate-term, selective advantages of WGDs are harder to explain. Here, by duplicating artificially generated Gene Regulatory Networks (GRNs), we show that duplicated GRNs-and thus duplicated genomes-show higher signal output variation than nonduplicated GRNs. This increased variation leads to niche expansion and can provide polyploid populations with substantial advantages to survive environmental turmoil. In contrast, under stable environments, GRNs might be maladaptive to changes, a phenomenon that is exacerbated in duplicated GRNs. We believe that these results provide insights into how genome duplication and (auto)polyploidy might help organisms to adapt quickly to novel conditions and to survive ecological uproar or even cataclysmic events.

Deciphering complex reticulate evolution of Asian Buddleja (Scrophulariaceae): insights into the taxonomy and speciation of polyploid taxa in the Sino-Himalayan region

BACKGROUND AND AIMS

Species of the genus Buddleja in Asia are mainly distributed in the Sino-Himalayan region and form a challenging taxonomic group, with extensive hybridization and polyploidization. A phylogenetic approach to unravelling the history of reticulation in this lineage will deepen our understanding of the speciation in biodiversity hotspots.

METHODS

For this study, we obtained 80 accessions representing all the species in the Asian Buddleja clade, and the ploidy level of each taxon was determined by flow cytometry analyses. Whole plastid genomes, nuclear ribosomal DNA, single nucleotide polymorphisms and a large number of low-copy nuclear genes assembled from genome skimming data were used to investigate the reticulate evolutionary history of Asian Buddleja. Complex cytonuclear conflicts were detected through a comparison of plastid and species trees. Gene tree incongruence was also analysed to detect any reticulate events in the history of this lineage.

KEY RESULTS

Six hybridization events were detected, which are able to explain the cytonuclear conflict in Asian Buddleja. Furthermore, PhyloNet analysis combining species ploidy data indicated several allopolyploid speciation events. A strongly supported species tree inferred from a large number of low-copy nuclear genes not only corrected some earlier misinterpretations, but also indicated that there are many Asian Buddleja species that have been lumped mistakenly. Divergent time estimation shows two periods of rapid diversification (8-10 and 0-3 Mya) in the Asian Buddleja clade, which might coincide with the final uplift of the Hengduan Mountains and Quaternary climate fluctuations, respectively.

CONCLUSIONS

This study presents a well-supported phylogenetic backbone for the Asian Buddleja species, elucidates their complex and reticulate evolutionary history and suggests that tectonic activity, climate fluctuations, polyploidization and hybridization together promoted the diversification of this lineage.

Genome-material costs and functional trade-offs in the autopolyploid Solidago gigantea (giant goldenrod) series

PREMISE

Increased genome-material costs of N and P atoms inherent to organisms with larger genomes have been proposed to limit growth under nutrient scarcities and to promote growth under nutrient enrichments. Such responsiveness may reflect a nutrient-dependent diploid versus polyploid advantage that could have vast ecological and evolutionary implications, but direct evidence that material costs increase with ploidy level and/or influence cytotype-dependent growth, metabolic, and/or resource-use trade-offs is limited.

METHODS

We grew diploid, autotetraploid, and autohexaploid Solidago gigantea plants with one of four ambient or enriched N:P ratios and measured traits related to material costs, primary and secondary metabolism, and resource-use.

RESULTS

Relative to diploids, polyploids invested more N and P into cells, and tetraploids grew more with N enrichments, suggesting that material costs increase with ploidy level. Polyploids also generally exhibited strategies that could minimize material-cost constraints over both long (reduced monoploid genome size) and short (more extreme transcriptome downsizing, reduced photosynthesis rates and terpene concentrations, enhanced N-use efficiencies) evolutionary time periods. Furthermore, polyploids had lower transpiration rates but higher water-use efficiencies than diploids, both of which were more pronounced under nutrient-limiting conditions.

CONCLUSIONS

N and P material costs increase with ploidy level, but material-cost constraints might be lessened by resource allocation/investment mechanisms that can also alter ecological dynamics and selection. Our results enhance mechanistic understanding of how global increases in nutrients might provide a release from material-cost constraints in polyploids that could impact ploidy (or genome-size)-specific performances, cytogeographic patterning, and multispecies community structuring.

Mutagenic effects of nitrogen protoxide and oryzalin on "41 B" and "Fercal" grapevine rootstocks seedlings

In this study, the mutagenic effects of different doses and exposure times of oryzalin and Nitrogen Protoxide (N2O) were tested for stimulating polyploid on 41 B and Fercal grapevine rootstocks seedlings. Ploidy changes were examined by morphological, cytological, macroscopic, and microscopic methods. Leaf thickness, chlorophyll contents, stomatal sizes, and chloroplast numbers of polyploid seedlings stimulated with mutagens increased but their stomatal densities decreased. Flow cytometry (FC) analyses were performed on 50 samples selected by morphological and microscopic preliminary determinations. In FC analyses, 1 tetraploid seedling and 4 mixoploid seedlings from Fercal offspring and 1 mixoploid seedling from 41 B offspring were verified. The nuclear DNA content of tetraploid and mixoploid seedlings were increased by 2.00 and 1.34-fold, respectively, when compared to their diploid parents. Chromosome counts in root tip samples propagated in vitro from the tetraploid Fercal offspring confirmed a 2-fold increase compared to the diploid parent. In polyploidy induction studies, it was deemed appropriate to use FC analysis and chromosome count together to confirm the ploidy levels of mutants. Oryzalin and N2O applications at different doses and exposure times were found to be effective for inducing polyploidy in 41 B and Fercal grapevine rootstocks.

Genetic mapping of the LOSS OF PARTHENOGENESIS locus in Pilosella piloselloides and the evolution of apomixis in the Lactuceae

Pilosella piloselloides var. praealta (syn. P. praealta; Hieracium praealtum) is a versatile model used to study gametophytic apomixis. In this system apomixis is controlled by three loci: one that controls the avoidance of meiosis (LOA), one that controls the avoidance of fertilization (LOP) and a third that controls autonomous endosperm formation (AutE). Using a unique polyhaploid mapping approach the LOP locus was mapped to a 654 kb genomic interval syntenic to linkage group 8 of Lactuca sativa. Polyhaploids form through the gametophytic action of a dominant determinant at LOP, so the mapped region represents both a functional and a physical domain for LOP in P. piloselloides. Allele sequence divergence (ASD) analysis of the PARTHENOGENESIS (PAR) gene within the LOP locus revealed that dominant PAR alleles in Pilosella remain highly similar across the genus, whilst the recessive alleles are more divergent. A previous report noted that dominant PAR alleles in both Pilosella and Taraxacum are modified by the presence of a class II transposable element (TE) in the promoter of the gene. This observation was confirmed and further extended to the related genus Hieracium. Sufficient differences were noted in the structure and location of the TE elements to conclude that TE insertional events had occurred independently in the three genera. Measures of allele crossover amongst the polyhaploids revealed that P. piloselloides is an autopolyploid species with tetrasomic inheritance. It was also noted that the dominant determinant of LOP in P. piloselloides could transmit via a diploid gamete (pollen or egg) but not via a haploid gamete. Using this information, a model is presented of how gametophytic apomixis may have evolved in several members of the Lactuceae, a tribe of the Asteraceae.

Genetic factors inherited from both diploid parents interact to affect genome stability and fertility in resynthesized allotetraploid Brassica napus

Established allopolyploids are known to be genomically stable and fertile. However, in contrast, most newly resynthesized allopolyploids are infertile and meiotically unstable. Identifying the genetic factors responsible for genome stability in newly formed allopolyploid is key to understanding how 2 genomes come together to form a species. One hypothesis is that established allopolyploids may have inherited specific alleles from their diploid progenitors which conferred meiotic stability. Resynthesized Brassica napus lines are often unstable and infertile, unlike B. napus cultivars. We tested this hypothesis by characterizing 41 resynthesized B. napus lines produced by crosses between 8 Brassica rapa and 8 Brassica oleracea lines for copy number variation resulting from nonhomologous recombination events and fertility. We resequenced 8 B. rapa and 5 B. oleracea parent accessions and analyzed 19 resynthesized lines for allelic variation in a list of meiosis gene homologs. SNP genotyping was performed using the Illumina Infinium Brassica 60K array for 3 individuals per line. Self-pollinated seed set and genome stability (number of copy number variants) were significantly affected by the interaction between both B. rapa and B. oleracea parental genotypes. We identified 13 putative meiosis gene candidates which were significantly associated with frequency of copy number variants and which contained putatively harmful mutations in meiosis gene haplotypes for further investigation. Our results support the hypothesis that allelic variants inherited from parental genotypes affect genome stability and fertility in resynthesized rapeseed.

The novel expression of clonality following whole-genome multiplication compensates for reduced fertility in natural autopolyploids

Exploring the fitness consequences of whole-genome multiplication (WGM) is essential for understanding the establishment of autopolyploids in diploid parental populations, but suitable model systems are rare. We examined the impact of WGM on reproductive traits in three major cytotypes (2x, 3x, 4x) of Pilosella rhodopea, a species with recurrent formation of neo-autopolyploids in mixed-ploidy populations. We found that diploids had normal female sporogenesis and gametogenesis, high fertility, and produced predominantly euploid seed progeny. By contrast, autopolyploids had highly disturbed developmental programs that resulted in significantly lower seed set and a high frequency of aneuploid progeny. All cytotypes, but particularly triploids, produced gametes of varying ploidy, including unreduced ones, that participated in frequent intercytotype mating. Noteworthy, the reduced investment in sexual reproduction in autopolyploids was compensated by increased production of axillary rosettes and the novel expression of two clonal traits: adventitious rosettes on roots (root-sprouting), and aposporous initial cells in ovules which, however, do not result in functional apomixis. The combination of increased vegetative clonal growth in autopolyploids and frequent intercytotype mating are key mechanisms involved in the formation and maintenance of the largest diploid-autopolyploid primary contact zone ever recorded in angiosperms.

Genome of tetraploid sour cherry (Prunus cerasus L.) 'Montmorency' identifies three distinct ancestral Prunus genomes

Sour cherry (Prunus cerasus L.) is a valuable fruit crop in the Rosaceae family and a hybrid between progenitors closely related to extant Prunus fruticosa (ground cherry) and Prunus avium (sweet cherry). Here we report a chromosome-scale genome assembly for sour cherry cultivar Montmorency, the predominant cultivar grown in the USA. We also generated a draft assembly of P. fruticosa to use alongside a published P. avium sequence for syntelog-based subgenome assignments for 'Montmorency' and provide compelling evidence P. fruticosa is also an allotetraploid. Using hierarchal k-mer clustering and phylogenomics, we show 'Montmorency' is trigenomic, containing two distinct subgenomes inherited from a P. fruticosa-like ancestor (A and A') and two copies of the same subgenome inherited from a P. avium-like ancestor (BB). The genome composition of 'Montmorency' is AA'BB and little-to-no recombination has occurred between progenitor subgenomes (A/A' and B). In Prunus, two known classes of genes are important to breeding strategies: the self-incompatibility loci (S-alleles), which determine compatible crosses, successful fertilization, and fruit set, and the Dormancy Associated MADS-box genes (DAMs), which strongly affect dormancy transitions and flowering time. The S-alleles and DAMs in 'Montmorency' and P. fruticosa were manually annotated and support subgenome assignments. Lastly, the hybridization event 'Montmorency' is descended from was estimated to have occurred less than 1.61 million years ago, making sour cherry a relatively recent allotetraploid. The 'Montmorency' genome highlights the evolutionary complexity of the genus Prunus and will inform future breeding strategies for sour cherry, comparative genomics in the Rosaceae, and questions regarding neopolyploidy.

Polyploidy impacts population growth and competition with diploids: multigenerational experiments reveal key life-history trade-offs

Ecological theory predicts that early generation polyploids ('neopolyploids') should quickly go extinct owing to the disadvantages of rarity and competition with their diploid progenitors. However, polyploids persist in natural habitats globally. This paradox has been addressed theoretically by recognizing that reproductive assurance of neopolyploids and niche differentiation can promote establishment. Despite this, the direct effects of polyploidy at the population level remain largely untested despite establishment being an intrinsically population-level process. We conducted population-level experiments where life-history investment in current and future growth was tracked in four lineage pairs of diploids and synthetic autotetraploids of the aquatic plant Spirodela polyrhiza. Population growth was evaluated with and without competition between diploids and neopolyploids across a range of nutrient treatments. Although neopolyploid populations produce more biomass, they reach lower population sizes and have reduced carrying capacities when growing alone or in competition across all nutrient treatments. Thus, contrary to individual-level studies, our population-level data suggest that neopolyploids are competitively inferior to diploids. Conversely, neopolyploid populations have greater investment in dormant propagule production than diploids. Our results show that neopolyploid populations should not persist based on current growth dynamics, but high potential future growth may allow polyploids to establish in subsequent seasons.

Dispersal and establishment traits provide a colonization advantage for a polyploid apomictic plant

PREMISE

Apomictic plants (reproducing asexually through seed) often have larger ranges and occur at higher latitudes than closely related sexuals, a pattern known as geographical parthenogenesis (GP). Explanations for GP include differences in colonizing ability due to reproductive assurance and direct/indirect effects of polyploidy (most apomicts are polyploid) on ecological tolerances. While life history traits associated with dispersal and establishment also contribute to the potential for range expansion, few studies compare these traits in related apomicts and sexuals.

METHODS

We investigated differences in early life history traits between diploid-sexual and polyploid-apomictic Townsendia hookeri (Asteraceae), which displays a classic pattern of GP. Using lab and greenhouse experiments, we measured seed dispersal traits, germination success, and seedling size and survival in sexual and apomictic populations from across the range of the species.

RESULTS

While theory predicts that trade-offs between dispersal and establishment traits should be common, this was largely not the case in T. hookeri. Apomictic seeds had both lower terminal velocity (staying aloft longer when dropped) and higher germination success than sexual seeds. While there were no differences in seedling size between reproductive types, apomicts did, however, have slightly lower seedling survival than sexuals.

CONCLUSIONS

These differences in early life history traits, combined with reproductive assurance conferred by apomixis, suggest that apomicts achieve a greater range through advantages in their ability to both spread and establish.

Learning to tango with four (or more): the molecular basis of adaptation to polyploid meiosis

Polyploidy, which arises from genome duplication, has occurred throughout the history of eukaryotes, though it is especially common in plants. The resulting increased size, heterozygosity, and complexity of the genome can be an evolutionary opportunity, facilitating diversification, adaptation and the evolution of functional novelty. On the other hand, when they first arise, polyploids face a number of challenges, one of the biggest being the meiotic pairing, recombination and segregation of the suddenly more than two copies of each chromosome, which can limit their fertility. Both for developing polyploidy as a crop improvement tool (which holds great promise due to the high and lasting multi-stress resilience of polyploids), as well as for our basic understanding of meiosis and plant evolution, we need to know both the specific nature of the challenges polyploids face, as well as how they can be overcome in evolution. In recent years there has been a dramatic uptick in our understanding of the molecular basis of polyploid adaptations to meiotic challenges, and that is the focus of this review.

Complex patterns of ploidy in a holocentric plant clade (Schoenus, Cyperaceae) in the Cape biodiversity hotspot

BACKGROUND AND AIMS

It is unclear how widespread polyploidy is throughout the largest holocentric plant family - the Cyperaceae. Because of the prevalence of chromosomal fusions and fissions, which affect chromosome number but not genome size, it can be impossible to distinguish if individual plants are polyploids in holocentric lineages based on chromosome count data alone. Furthermore, it is unclear how differences in genome size and ploidy levels relate to environmental correlates within holocentric lineages, such as the Cyperaceae.

METHODS

We focus our analyses on tribe Schoeneae, and more specifically the southern African clade of Schoenus. We examine broad-scale patterns of genome size evolution in tribe Schoeneae and focus more intensely on determining the prevalence of polyploidy across the southern African Schoenus by inferring ploidy level with the program ChromEvol, as well as interpreting chromosome number and genome size data. We further investigate whether there are relationships between genome size/ploidy level and environmental variables across the nutrient-poor and summer-arid Cape biodiversity hotspot.

KEY RESULTS

Our results show a large increase in genome size, but not chromosome number, within Schoenus compared to other species in tribe Schoeneae. Across Schoenus, there is a positive relationship between chromosome number and genome size, and our results suggest that polyploidy is a relatively common process throughout the southern African Schoenus. At the regional scale of the Cape, we show that polyploids are more often associated with drier locations that have more variation in precipitation between dry and wet months, but these results are sensitive to the classification of ploidy level.

CONCLUSIONS

Polyploidy is relatively common in the southern African Schoenus, where a positive relationship is observed between chromosome number and genome size. Thus, there may be a high incidence of polyploidy in holocentric plants, whose cell division properties differ from monocentrics.

Morphological and environmental differentiation as prezygotic reproductive barriers between parapatric and allopatric Campanula rotundifolia agg. cytotypes

BACKGROUND AND AIMS

Reproductive isolation and local establishment are necessary for plant speciation. Polyploidy, the possession of more than two complete chromosome sets, creates a strong postzygotic reproductive barrier between diploid and tetraploid cytotypes. However, this barrier weakens between polyploids (e.g. tetraploids and hexaploids). Reproductive isolation may be enhanced by cytotype morphological and environmental differentiation. Moreover, morphological adaptations to local conditions contribute to plant establishment. However, the relative contributions of ploidy level and the environment to morphology have generally been neglected. Thus, the extent of morphological variation driven by ploidy level and the environment was modelled for diploid, tetraploid and hexaploid cytotypes of Campanula rotundifolia agg. Cytotype distribution was updated, and morphological and environmental differentiation was tested in the presence and absence of natural contact zones.

METHODS

Cytotype distribution was assessed from 231 localities in Central Europe, including 48 localities with known chromosome counts, using flow cytometry. Differentiation in environmental niche and morphology was tested for cytotype pairs using discriminant analyses. A structural equation model was used to explore the synergies between cytotype, environment and morphology.

KEY RESULTS

Tremendous discrepancies were revealed between the reported and detected cytotype distribution. Neither mixed-ploidy populations nor interploidy hybrids were detected in the contact zones. Diploids had the broadest environmental niche, while hexaploids had the smallest and specialized niche. Hexaploids and spatially isolated cytotype pairs differed morphologically, including allopatric tetraploids. While leaf and shoot morphology were influenced by environmental conditions and polyploidy, flower morphology depended exclusively on the cytotype.

CONCLUSIONS

Reproductive isolation mechanisms vary between cytotypes. While diploids and polyploids are isolated postzygotically, the environmental niche shift is essential between higher polyploids. The impact of polyploidy and the environment on plant morphology implies the adaptive potential of polyploids, while the exclusive relationship between flower morphology and cytotype highlights the role of polyploidy in reproductive isolation.

New cup out of old coffee: contribution of parental gene expression legacy to phenotypic novelty in coffee beans of the allopolyploid Coffea arabica L

BACKGROUND AND AIMS

Allopolyploidization is a widespread phenomenon known to generate novel phenotypes by merging evolutionarily distinct parental genomes and regulatory networks in a single nucleus. The objective of this study was to investigate the transcriptional regulation associated with phenotypic novelty in coffee beans of the allotetraploid Coffea arabica.

METHODS

A genome-wide comparative transcriptomic analysis was performed in C. arabica and its two diploid progenitors, C. canephora and C. eugenioides. Gene expression patterns and homeologue expression were studied on seeds at five different maturation stages. The involvement of homeologue expression bias (HEB) in specific traits was addressed both by functional enrichment analyses and by the study of gene expression in the caffeine and chlorogenic acid biosynthesis pathways.

KEY RESULTS

Expression-level dominance in C. arabica seed was observed for most of the genes differentially expressed between the species. Approximately a third of the genes analysed showed HEB. This proportion increased during seed maturation but the biases remained equally distributed between the sub-genomes. The relative expression levels of homeologues remained relatively constant during maturation and were correlated with those estimated in leaves of C. arabica and interspecific hybrids between C. canephora and C. eugenioides. Functional enrichment analyses performed on genes exhibiting HEB enabled the identification of processes potentially associated with physiological traits. The expression profiles of the genes involved in caffeine biosynthesis mirror the differences observed in the caffeine content of mature seeds of C. arabica and its parental species.

CONCLUSIONS

Neither of the two sub-genomes is globally preferentially expressed in C. arabica seeds, and homeologues appear to be co-regulated by shared trans-regulatory mechanisms. The observed HEBs are thought to be a legacy of gene expression differences inherited from diploid progenitor species. Pre-existing functional divergences between parental species appear to play an important role in controlling the phenotype of C. arabica seeds.

Inferring Chromosome Number Changes Along a Phylogeny Using chromEvol

Chromosome numbers have long been used for the identification of key genomic events such as polyploidy and dysploidy. These inferences are often challenging, particularly when applied to large phylogenies, or clades in which more than a few chromosome number transitions had occurred. Here we describe the chromEvol computational framework that infers shifts in chromosome numbers along a phylogeny using probabilistic models of chromosome number change. Given chromosome count data and an associated phylogeny, chromEvol identifies such patterns by fitting probabilistic models of chromosome number evolution to the data. We describe the chromEvol workflow using available online tools, including the specification of the desired models, the examination of model fit to the data, and the inference of ploidy levels. The pipeline can be used by the wide scientific community and requires no previous computational or programming skills.

Inference of Polyploid Origin and Inheritance Mode from Population Genomic Data

Whole-genome duplications yield varied chromosomal pairing patterns, ranging from strictly bivalent to multivalent, resulting in disomic and polysomic inheritance modes. In the bivalent case, homeologous chromosomes form pairs, where in a multivalent pattern all copies are homologous and are therefore free to pair and recombine. As sufficient sequencing data is more readily available than high-quality cytological assessments of meiotic behavior or population genetic assessment of allelic segregation, especially for non-model organisms, bioinformatics approaches to infer origins and inheritance modes of polyploids using short-read sequencing data are attractive. Here we describe two such approaches, where the first is based on distributions of allelic read depth at heterozygous sites within an individual, as the expectations of such distributions are different for disomic and polysomic inheritance modes. The second approach is more laborious and based on a phylogenetic assessment of partially phased haplotypes of a polyploid in comparison to the closest diploid relatives. We discuss the sources of deviations from expected inheritance patterns, advantages and pitfalls of both methods, effects of mating types on the performance of the methods, and possible future developments.

The immediate effects of polyploidization of Spirodela polyrhiza change in a strain-specific way along environmental gradients

The immediate effects of plant polyploidization are well characterized and it is generally accepted that these morphological, physiological, developmental, and phenological changes contribute to polyploid establishment. Studies on the environmental dependence of the immediate effects of whole-genome duplication (WGD) are, however, scarce but suggest that these immediate effects are altered by stressful conditions. As polyploid establishment seems to be associated with environmental disturbance, the relationship between ploidy-induced phenotypical changes and environmental conditions is highly relevant. Here, we use a common garden experiment on the greater duckweed Spirodela polyrhiza to test whether the immediate effects of WGD can facilitate the establishment of tetraploid duckweed along gradients of two environmental stressors. Because successful polyploid establishment often depends on recurrent polyploidization events, we include four genetically diverse strains and assess whether these immediate effects are strain-specific. We find evidence that WGD can indeed confer a fitness advantage under stressful conditions and that the environment affects ploidy-induced changes in fitness and trait reaction norms in a strain-specific way.