Evolutionary Dynamics of Unreduced Gametes

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.

Interspecific transfer of genetic information through polyploid bridges

Hybridization blurs species boundaries and leads to intertwined lineages resulting in reticulate evolution. Polyploidy, the outcome of whole genome duplication (WGD), has more recently been implicated in promoting and facilitating hybridization between polyploid species, potentially leading to adaptive introgression. However, because polyploid lineages are usually ephemeral states in the evolutionary history of life it is unclear whether WGD-potentiated hybridization has any appreciable effect on their diploid counterparts. Here, we develop a model of cytotype dynamics within mixed-ploidy populations to demonstrate that polyploidy can in fact serve as a bridge for gene flow between diploid lineages, where introgression is fully or partially hampered by the species barrier. Polyploid bridges emerge in the presence of triploid organisms, which despite critically low levels of fitness, can still allow the transfer of alleles between diploid states of independently evolving mixed-ploidy species. Notably, while marked genetic divergence prevents polyploid-mediated interspecific gene flow, we show that increased recombination rates can offset these evolutionary constraints, allowing a more efficient sorting of alleles at higher-ploidy levels before introgression into diploid gene pools. Additionally, we derive an analytical approximation for the rate of gene flow at the tetraploid level necessary to supersede introgression between diploids with nonzero introgression rates, which is especially relevant for plant species complexes, where interspecific gene flow is ubiquitous. Altogether, our results illustrate the potential impact of polyploid bridges on the (re)distribution of genetic material across ecological communities during evolution, representing a potential force behind reticulation.

The global distribution of angiosperm genome size is shaped by climate

Angiosperms, which inhabit diverse environments across all continents, exhibit significant variation in genome sizes, making them an excellent model system for examining hypotheses about the global distribution of genome size. These include the previously proposed large genome constraint, mutational hazard, polyploidy-mediated, and climate-mediated hypotheses. We compiled the largest genome size dataset to date, encompassing 16 017 (> 5% of known) angiosperm species, and analyzed genome size distribution using a comprehensive geographic distribution dataset for all angiosperms. We observed that angiosperms with large range sizes generally had small genomes, supporting the large genome constraint hypothesis. Climate was shown to exert a strong influence on genome size distribution along the global latitudinal gradient, while the frequency of polyploidy and the type of growth form had negligible effects. In contrast to the unimodal patterns along the global latitudinal gradient shown by plant size traits and polyploid proportions, the increase in angiosperm genome size from the equator to 40-50°N/S is probably mediated by different (mostly climatic) mechanisms than the decrease in genome sizes observed from 40 to 50°N northward. Our analysis suggests that the global distribution of genome sizes in angiosperms is mainly shaped by climatically mediated purifying selection, genetic drift, relaxed selection, and environmental filtering.

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.

Identification of two mutant JASON-RELATED genes associated with unreduced pollen production in potato

KEY MESSAGE

Multiple QTLs control unreduced pollen production in potato. Two major-effect QTLs co-locate with mutant alleles of genes with homology to AtJAS, a known regulator of meiotic spindle orientation. In diploid potato the production of unreduced gametes with a diploid (2n) rather than a haploid (n) number of chromosomes has been widely reported. Besides their evolutionary important role in sexual polyploidisation, unreduced gametes also have a practical value for potato breeding as a bridge between diploid and tetraploid germplasm. Although early articles argued for a monogenic recessive inheritance, the genetic basis of unreduced pollen production in potato has remained elusive. Here, three diploid full-sib populations were genotyped with an amplicon sequencing approach and phenotyped for unreduced pollen production across two growing seasons. We identified two minor-effect and three major-effect QTLs regulating this trait. The two QTLs with the largest effect displayed a recessive inheritance and an additive interaction. Both QTLs co-localised with genes encoding for putative AtJAS homologs, a key regulator of meiosis II spindle orientation in Arabidopsis thaliana. The function of these candidate genes is consistent with the cytological phenotype of mis-oriented metaphase II plates observed in the parental clones. The alleles associated with elevated levels of unreduced pollen showed deleterious mutation events: an exonic transposon insert causing a premature stop, and an amino acid change within a highly conserved domain. Taken together, our findings shed light on the natural variation underlying unreduced pollen production in potato and will facilitate interploidy breeding by enabling marker-assisted selection for this trait.

Gamete sex and elevation affect genetically based variation for unreduced gamete production in a mixed-ploidy plant

PREMISE

Unreduced gametes are the primary mechanism of neopolyploid formation. Their production in diploid populations is arguably maladaptive, but the magnitude and patterns of genetically based variation maintained in natural populations are poorly understood.

METHODS

We examined variation in male and female unreduced gamete production among plants from different elevations in fireweed, Chamerion angustifolium, grown in a common environment. Using seeds from three high-elevation and three low-elevation diploid populations in one study, and a single diploid population in another, we estimated realized rates of unreduced male (sperm) and female (egg) gamete production by reciprocally pollinating diploid and tetraploid plants and estimating the incidence of tetraploid seeds using flow cytometry.

RESULTS

Unreduced gamete frequencies per plant were similar in the two studies (0.12% vs. 0.08%). High-elevation populations had a greater percentage of fruit with seeds from unreduced gametes, but a lower percentage of seeds per fruit than low-elevation populations. Female unreduced gamete frequencies differed among elevations, but male frequencies did not, and the gamete sexes were not correlated at the plant level.

CONCLUSIONS

We conclude that genetically based variation for unreduced gametes is maintained within and among natural populations, despite their fitness disadvantages, suggesting that local selection may be ineffective at purging them under some conditions.

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.

Rate of crop-weed hybridization in Sorghum bicolor × Sorghum halepense is influenced by genetic background, pollen load, and the environment

The potential for gene flow between cultivated species and their weedy relatives poses agronomic and environmental concerns, particularly when there are opportunities for the transfer of adaptive or agronomic traits such as herbicide resistance into the weedy forms. Grain sorghum (Sorghum bicolor) is an important crop capable of interspecific hybridization with its weedy relative johnsongrass (Sorghum halepense). Previous findings have shown that triploid progenies resulting from S. bicolor × S. halepense crosses typically collapse with only a few developing into mature seeds, whereas tetraploids often fully develop. The objective of this experiment was to determine the impact of S. bicolor genotype and pollen competition on the frequency of hybridization between S. bicolor and S. halepense. A total of 12 different cytoplasmic male sterile S. bicolor genotypes were compared with their respective male fertile lines across 2 years, to assess the frequency of hybridization and seed set when S. halepense served as the pollinator parent. Results indicate significant differences in the frequency of interspecific hybridization among the S. bicolor genotypes, and pollen fertility in S. bicolor reduced the rate of this interspecific hybridization by up to two orders of magnitude. Further, hybridization rates greatly varied across the two study environments. Results are helpful for developing appropriate gene flow mitigation strategies and indicate that gene flow could be reduced by the selection of appropriate seed parents for sorghum hybrids.

Ancestral self-compatibility facilitates the establishment of allopolyploids in Brassicaceae

Self-incompatibility systems based on self-recognition evolved in hermaphroditic plants to maintain genetic variation of offspring and mitigate inbreeding depression. Despite these benefits in diploid plants, for polyploids who often face a scarcity of mating partners, self-incompatibility can thwart reproduction. In contrast, self-compatibility provides an immediate advantage: a route to reproductive viability. Thus, diploid selfing lineages may facilitate the formation of new allopolyploid species. Here, we describe the mechanism of establishment of at least four allopolyploid species in Brassicaceae (Arabidopsis suecica, Arabidopsis kamchatica, Capsella bursa-pastoris, and Brassica napus), in a manner dependent on the prior loss of the self-incompatibility mechanism in one of the ancestors. In each case, the degraded S-locus from one parental lineage was dominant over the functional S-locus of the outcrossing parental lineage. Such dominant loss-of-function mutations promote an immediate transition to selfing in allopolyploids and may facilitate their establishment.

Reproductive isolation between diploid and tetraploid individuals in mixed-cytotype populations of Lycium australe

PREMISE

Whole-genome duplication is considered a major mechanism of sympatric speciation due to the creation of strong and instantaneous reproductive barriers. Although postzygotic reproductive isolation between diploids and polyploids is often expected, the extent of reproductive incompatibility must be empirically determined and compared to patterns of genetic isolation to fully characterize the reproductive dynamics between cytotypes.

METHODS

We investigated reproductive compatibility between diploid and tetraploid Lycium australe in two mixed-cytotype populations using (1) controlled crossing experiments to evaluate fruit and seed production and (2) germination trials to test seed viability following homoploid and heteroploid crosses. We contrast these experiments with a single-nucleotide polymorphism (SNP) data set to measure genetic isolation between cytotypes and explore whether cytotype or population origin better explains patterns of genetic variation. Finally, we explore mating patterns using the observed germination rates of naturally produced seeds in each population.

RESULTS

Although homoploid and heteroploid crosses resulted in similar fruit and seed production, reproductive isolation between co-occurring diploids and tetraploids was nearly complete, due to low seed viability following heteroploid crosses. Of 191,182 total SNPs, 21,679 were present in ≥90% of individuals and replicate runs using unlinked SNPs revealed strong clustering by cytotype and differentiation of tetraploids based on population origin.

CONCLUSIONS

As often reported, diploid and tetraploid L. australe experience strong postzygotic isolation via hybrid seed inviability. Consistent with this result, cytotype explained a greater amount of variation in the SNP data set than population origin, despite some evidence of historical introgression.

Two pathways of 2n gamete formation and differences in the frequencies of 2n gametes between wild species and interspecific hybrids

Epidendrum produces 2n gametes with high frequency. This paper is the first to report on multiple pathways for forming 2n gametes, meiotic defeats, and pre-meiotic chromosome doubling. Unreduced 2n reproductive cells are predominantly involved in pathways that lead to polyploid plants. Although one of the most common pathways for inducing 2n gametes is through meiotic defects, a small set of isolated species alternatively generates 2n gametes from tetraploid pollen mother cells in the pre-meiotic phase. Hence, determining the mechanisms underlying 2n gamete formation is critical to improving breeding programmes and understanding plant evolution. We investigated sporads to reveal the pathway(s) accounting for the formation and frequencies of 2n gametes in wild species and interspecific hybrids in the genus Epidendrum. We investigated different types of sporads with varying frequencies, sizes, and viability in the wild species and hybrids of the genus Epidendrum. Large tetrad-estimated pre-meiotic chromosome doubling was observed in wild species. The Epidendrum is unique in that it forms 2n pollens via two pathways, namely, meiotic defects and pre-meiotic chromosome doubling. These two pathways of 2n pollen formation could influence the high diversity generation of polyploidy with different degrees of heterozygosity and genetic backgrounds in the genus Epidendrum. Therefore, these findings are proposed to influence polyploid breeding of Epidendrum via 2n pollen, helping us understand evolution and speciation via unreduced 2n gamete formation in Orchidaceae.

Autopolyploid establishment depends on life-history strategy and the mating outcomes of clonal architecture

Polyploidy is a significant component in the evolution of many taxa, particularly plant groups. However, new polyploids face substantial fitness disadvantages due to a lack of same-cytotype mates, and the factors promoting or preventing polyploid establishment in natural populations are often unclear. We develop spatially explicit agent-based simulation models to test the hypothesis that a perennial life history and clonal propagation facilitate the early stages of polyploid establishment and persistence. Our models show that polyploids are more likely to establish when they have longer life spans than diploids, especially when self-fertilization rates are high. Polyploids that combine sexual and clonal reproduction can establish across a wide range of life histories, but their success is moderated by clonal strategy. By tracking individuals and mating events, we reveal that clonal architecture has a substantial impact on the spatial structure of the mixed diploid-polyploid population during polyploid establishment: altering patterns of mating within or between cytotypes via geitonogamous self-fertilization, the mechanisms through which polyploid establishment proceeds, and the final composition of the polyploid population. Overall, our findings provide novel insight into the role of clonal structure in modulating the complex relationship between polyploidy, perenniality, and clonality and offer testable predictions for future empirical work.

Polyploidization as an opportunistic mutation: The role of unreduced gametes formation and genetic drift in polyploid establishment

It is broadly assumed that polyploidy success reflects an increase in fitness associated with whole-genome duplication (WGD), due to higher tolerance to stressful conditions. Nevertheless, WGD also arises with several costs in neo-polyploid lineages, like genomic instability, or cellular mis-management. In addition to these costs, neo-polyploid individuals also face frequency dependent selection because of frequent low-fitness triploids formed by cross-ploidy pollinations when tetraploids are primarily rare in the population. Interestingly, the idea that polyploidy can be fixed by genetic drift as a neutral or deleterious mutation is currently underexplored in the literature. To test how and when polyploidy can fix in a population by chance, we built a theoretical model in which autopolyploidization occurs through the production of unreduced gametes, a trait modelled as a quantitative trait that is allowed to vary through time. We found that when tetraploid individuals are less or as fit as their diploid progenitors, fixation of polyploidy is only possible when genetic drift is stronger than natural selection. The necessity of drift for tetraploid fixation holds even when polyploidy confers a selective advantage, except for scenarios where tetraploids are much fitter than diploids. Finally, we found that self-fertilization is less beneficial for tetraploid establishment than previously thought, notably when polyploids harbour an initial decrease in fitness. Our results bring a novel, non-exclusive explanation for the unequal temporal and spatial distribution of polyploid species.

Pervasive genome duplications across the plant tree of life and their links to major evolutionary innovations and transitions

Whole-genome duplication (WGD) has occurred repeatedly during plant evolution and diversification, providing genetic layers for evolving new functions and phenotypes. Advances in long-read sequencing technologies have enabled sequencing and assembly of over 1000 plant genomes spanning nearly 800 species, in which a large set of ancient WGDs has been uncovered. Here, we review the recently reported WGDs that occurred in major plant lineages and key evolutionary positions, and highlight their contributions to morphological innovation and adaptive evolution. Current gaps and challenges in integrating enormous volumes of sequenced plant genomes, accurately inferring WGDs, and developing web-based analysis tools are emphasized. Looking to the future, ambitious genome sequencing projects and global efforts may substantially recapitulate the plant tree of life based on broader sampling of phylogenetic diversity, reveal much of the timetable of ancient WGDs, and address the biological significance of WGDs in plant adaptation and radiation.

Exploiting Unreduced Gametes for Improving Ornamental Plants

The formation of gametes with somatic chromosome number or unreduced gametes (2n gametes) is an important process involved in the origin of polyploid plants in nature. Unreduced gametes are the result of meiotic mutations occurring during micro- and mega-sporogenesis. 2n gametes have been identified or artificially induced in a large number of plant species. Breeding of plants through 2n gametes can be advantageous because it combines genetic effects of polyploidy with meiotic recombination and sexual hybridization to produce tremendous genetic variation and heterosis. 2n gametes also occur in ornamental plants, but the potential of using 2n gametes in ornamental plant breeding has not been extensively exploited. Ornamental plants are primarily produced for their esthetic appearance and novelty, not for food and yield, and they can be readily propagated through vegetative means. Triploids, tetraploids, and plants with even higher ploidy levels produced through 2n gametes can be propagated through tissue culture to fix their phenotypes, thus leading to the development of new cultivars. In this review article, we intend to discuss the mechanisms underlying the formation of 2n gametes, techniques for 2n gamete identification, methods for enhancing 2n gamete formation, and the current status in the use of 2n gametes for development of novel ornamental plants. We believe that polyploidy breeding through 2n gametes represents a viable way of developing new cultivars, new species, and even new genera of ornamental plants.

Polyploidy in urban environments

Polyploidy is a major driver of evolutionary change in plants, but many aspects of polyploidy in natural systems remain enigmatic. We argue that urban landscapes present an unprecedented opportunity to observe polyploidy in action. Integrating polyploid biology and urban evolutionary ecology, we discuss multiple factors expected to promote polyploid formation, establishment, and persistence in urban systems. We develop a predictive framework for the contemporary ecology and evolution of polyploid plants in cities, and through this novel perspective propose that studying polyploidy in an urban context could lead to breakthroughs in understanding fundamental processes in polyploid evolution. We conclude by highlighting the potential consequences of polyploidy in urban environments, and outline a roadmap for research into this currently unexplored field.

Genetic Variability in Balkan Paleoendemic Resurrection Plants Ramonda serbica and R. nathaliae Across Their Range and in the Zone of Sympatry

The genus Ramonda includes three Paleoendemic and Tertiary relict species that survived in refugial habitats of the Balkan Peninsula (R. nathaliae and R. serbica) and the Iberian Peninsula (R. myconi). They are all "resurrection plants," a rare phenomenon among flowering plants in Europe. Ramonda myconi and R. nathaliae are diploids (2n = 2x = 48), while R. serbica is a hexaploid (2n = 6x = 144). The two Balkan species occur in sympatry in only two localities in eastern Serbia, where tetraploid potential hybrids (2n = 4x = 96) were found. This observation raised questions about the existence of gene flow between the two species and, more generally, about the evolutionary processes shaping their genetic diversity. To address this question, genetic markers (AFLP) and an estimate of genome size variation were used in a much larger sample and at a larger geographic scale than previously. The combination of AFLP markers and genome size results suggested ongoing processes of interspecific and interploidy hybridization in the two sites of sympatry. The data also showed that interspecific gene flow was strictly confined to sympatry. Elsewhere, both Ramonda species were characterized by low genetic diversity within populations and high population differentiation. This is consistent with the fact that the two species are highly fragmented into small and isolated populations, likely a consequence of their postglacial history. Within sympatry, enormous variability in cytotypes was observed, exceeding most reported cases of mixed ploidy in complex plant species (from 2x to >8x). The AFLP profiles of non-canonical ploidy levels indicated a diversity of origin pathways and that backcrosses probably occur between tetraploid interspecific hybrids and parental species. The question arises whether this diversity of cytotypes corresponds to a transient situation. If not, the question arises as to the genetic and ecological mechanisms that allow this diversity to be maintained over time.

Application-based guidelines for best practices in plant flow cytometry

Flow cytometry (FCM) is currently the most widely-used method to establish nuclear DNA content in plants. Since simple, 1-3-parameter, flow cytometers, which are sufficient for most plant applications, are commercially available at a reasonable price, the number of laboratories equipped with these instruments, and consequently new FCM users, has greatly increased over the last decade. This paper meets an urgent need for comprehensive recommendations for best practices in FCM for different plant science applications. We discuss advantages and limitations of establishing plant ploidy, genome size, DNA base composition, cell cycle activity, and level of endoreduplication. Applications of such measurements in plant systematics, ecology, molecular biology research, reproduction biology, tissue cultures, plant breeding, and seed sciences are described. Advice is included on how to obtain accurate and reliable results, as well as how to manage troubleshooting that may occur during sample preparation, cytometric measurements, and data handling. Each section is followed by best practice recommendations; tips as to what specific information should be provided in FCM papers are also provided.

On the origin of the widespread self-compatible allotetraploid Capsella bursa-pastoris (Brassicaceae)

Polyploidy, or whole-genome duplication, is a common speciation mechanism in plants. An important barrier to polyploid establishment is a lack of compatible mates. Because self-compatibility alleviates this problem, it has long been hypothesized that there should be an association between polyploidy and self-compatibility (SC), but empirical support for this prediction is mixed. Here, we investigate whether the molecular makeup of the Brassicaceae self-incompatibility (SI) system, and specifically dominance relationships among S-haplotypes mediated by small RNAs, could facilitate loss of SI in allopolyploid crucifers. We focus on the allotetraploid species Capsella bursa-pastoris, which formed ~300 kya by hybridization and whole-genome duplication involving progenitors from the lineages of Capsella orientalis and Capsella grandiflora. We conduct targeted long-read sequencing to assemble and analyze eight full-length S-locus haplotypes, representing both homeologous subgenomes of C. bursa-pastoris. We further analyze small RNA (sRNA) sequencing data from flower buds to identify candidate dominance modifiers. We find that C. orientalis-derived S-haplotypes of C. bursa-pastoris harbor truncated versions of the male SI specificity gene SCR and express a conserved sRNA-based candidate dominance modifier with a target in the C. grandiflora-derived S-haplotype. These results suggest that pollen-level dominance may have facilitated loss of SI in C. bursa-pastoris. Finally, we demonstrate that spontaneous somatic tetraploidization after a wide cross between C. orientalis and C. grandiflora can result in production of self-compatible tetraploid offspring. We discuss the implications of this finding on the mode of formation of this widespread weed.

A Hypomorphic Mutant of PHD Domain Protein Male Meiocytes Death 1

Meiosis drives reciprocal genetic exchanges and produces gametes with halved chromosome number, which is important for the genetic diversity, plant viability, and ploidy consistency of flowering plants. Alterations in chromosome dynamics and/or cytokinesis during meiosis may lead to meiotic restitution and the formation of unreduced microspores. In this study, we isolated an Arabidopsis mutant male meiotic restitution 1 (mmr1), which produces a small subpopulation of diploid or polyploid pollen grains. Cytological analysis revealed that mmr1 produces dyads, triads, and monads indicative of male meiotic restitution. Both homologous chromosomes and sister chromatids in mmr1 are separated normally, but chromosome condensation at metaphase I is slightly affected. The mmr1 mutant displayed incomplete meiotic cytokinesis. Supportively, immunostaining of the microtubular cytoskeleton showed that the spindle organization at anaphase II and mini-phragmoplast formation at telophase II are aberrant. The causative mutation in mmr1 was mapped to chromosome 1 at the chromatin regulator Male Meiocyte Death 1 (MMD1/DUET) locus. mmr1 contains a C-to-T transition at the third exon of MMD1/DUET at the genomic position 2168 bp from the start codon, which causes an amino acid change G618D that locates in the conserved PHD-finger domain of histone binding proteins. The F1 progenies of mmr1 crossing with knockout mmd1/duet mutant exhibited same meiotic defects and similar meiotic restitution rate as mmr1. Taken together, we here report a hypomorphic mmd1/duet allele that typically shows defects in microtubule organization and cytokinesis.

The Application of Flow Cytometry for Estimating Genome Size, Ploidy Level Endopolyploidy, and Reproductive Modes in Plants

Over the years, the amount of DNA in a nucleus (genome size) has been estimated using a variety of methods, but increasingly, flow cytometry (FCM) has become the method of choice. The popularity of this technique lies in the ease of sample preparation and in the large number of particles (i.e., nuclei) that can be analyzed in a very short period of time. This chapter presents a step-by-step guide to estimating the nuclear DNA content of plant nuclei using FCM. Attempting to serve as a tool for daily laboratory practice, we list, in detail, the equipment required, specific reagents and buffers needed, as well as the most frequently used protocols to carry out nuclei isolation. In addition, solutions to the most common problems that users may encounter when working with plant material and troubleshooting advice are provided. Finally, information about the correct terminology to use and the importance of obtaining chromosome counts to avoid cytological misinterpretations of the FCM data are discussed.

Propagule pressure and the establishment of emergent polyploid populations

BACKGROUND

Whereas the incidence or rate of polyploid speciation in flowering plants is modest, the production of polyploid individuals within local populations is widespread. Explanations for this disparity primarily have focused on properties or interactions of polyploids that limit their persistence.

HYPOTHESIS

The emergence of local polyploid populations within diploid populations is similar to the arrival of invasive species at new, suitable sites, with the exception that polyploids suffer interference from their progenitor(s). The most consistent predictor of successful colonization by invasive plants is propagule pressure, i.e. the number of seeds introduced. Therefore, insufficient propagule pressure, i.e. the formation of polyploid seeds within diploid populations, ostensibly is a prime factor limiting the establishment of newly emergent polyploids within local populations. Increasing propagule number reduces the effects of genetic, environmental and demographic stochasticity, which thwart population survival. As with invasive species, insufficient seed production within polyploid populations limits seed export, and thus reduces the chance of polyploid expansion.

CONCLUSION

The extent to which propagule pressure limits the establishment of local polyploid populations remains to be determined, because we know so little. The numbers of auto- or allopolyploid seed in diploid populations rarely have been ascertained, as have the numbers of newly emergent polyploid plants within diploid populations. Moreover, seed production by these polyploids has yet to be assessed.

Targeted generation of polyploids in Hydrangea macrophylla through cross-based breeding

Background

Up to now, diploid and triploid cultivars were reported for the ornamental crop Hydrangea macrophylla. Especially, the origin of triploids and their crossing behaviors are unknown, but the underlying mechanisms are highly relevant for breeding polyploids.

Results

By screening a cultivar collection, we identified diploid, triploid, tetraploid and even aneuploid H. macrophylla varieties. The pollen viability of triploids and tetraploids was comparable to that of diploids. Systematic crosses with these cultivars resulted in viable diploid, triploid, tetraploid and aneuploid offspring. Interestingly, crosses between diploids produced diploid and 0 or 1–94% triploid offspring, depending on the cultivars used as pollen parent. This finding suggests that specific diploids form unreduced pollen, either at low or high frequencies. In contrast, crosses of triploids with diploids or tetraploids produced many viable aneuploids, whose 2C DNA contents ranged between the parental 2C values. As expected, crosses between diploid and tetraploid individuals generated triploid offspring. Putative tetraploid plants were obtained at low frequencies in crosses between diploids and in interploid crosses of triploids with either diploid or tetraploid plants. The analysis of offspring populations indicated the production of 1n = 2x gametes for tetraploid plants, whereas triploids produced obviously reduced, aneuploid gametes with chromosome numbers ranging between haploid and diploid level. While euploid offspring grew normally, aneuploid plants showed mostly an abnormal development and a huge phenotypic variation within offspring populations, most likely due to the variation in chromosome numbers. Subsequent crosses with putative diploid, triploid and aneuploid offspring plants from interploid crosses resulted in viable offspring and germination rates ranging from 21 to 100%.

Conclusions

The existence of diploids that form unreduced pollen and of tetraploids allows the targeted breeding of polyploid H. macrophylla. Different ploidy levels can be addressed by combining the appropriate crossing partners. In contrast to artificial polyploidization, cross-based polyploidization is easy, cheap and results in genetically variable offspring that allows the direct selection of more robust and stress tolerant polyploid varieties. Furthermore, the generation of polyploid H. macrophylla plants will favor interspecific breeding programs within the genus Hydrangea.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00954-z.

Chromosome Pairing in Polyploid Grasses

Polyploids are species in which three or more sets of chromosomes coexist. Polyploidy frequently occurs in plants and plays a major role in their evolution. Based on their origin, polyploid species can be divided into two groups: autopolyploids and allopolyploids. The autopolyploids arise by multiplication of the chromosome sets from a single species, whereas allopolyploids emerge from the hybridization between distinct species followed or preceded by whole genome duplication, leading to the combination of divergent genomes. Having a polyploid constitution offers some fitness advantages, which could become evolutionarily successful. Nevertheless, polyploid species must develop mechanism(s) that control proper segregation of genetic material during meiosis, and hence, genome stability. Otherwise, the coexistence of more than two copies of the same or similar chromosome sets may lead to multivalent formation during the first meiotic division and subsequent production of aneuploid gametes. In this review, we aim to discuss the pathways leading to the formation of polyploids, the occurrence of polyploidy in the grass family (Poaceae), and mechanisms controlling chromosome associations during meiosis, with special emphasis on wheat.

Unreduced Male Gamete Formation in Cymbidium and Its Use for Developing Sexual Polyploid Cultivars

Polyploidy plays an important role in crop improvement. Polyploid plants, particularly those produced through unreduced gametes (2n gametes), show increased organ size, improved buffering capacity for deleterious mutations, and enhanced heterozygosity and heterosis. Induced polyploidy has been widely used for improving floriculture crops, however, there are few reported sexual polyploid plants in the floriculture industry. This study evaluated nine cultivars of Cymbidium Swartz and discovered that 2n male gametes occurred in this important orchid. Depending on cultivars, 2n male gamete formation frequencies varied from 0.15 to 4.03%. Interspecific hybrids generally produced more 2n male gametes than traditional cultivars. To generate sexual polyploid plants, seven pairs of crosses were made, which produced five triploid and two tetraploid hybrids. Two triploid hybrids were evaluated for in vitro regeneration and growth characteristics. Compared to the diploid parents, the triploids were more easily regenerated through rhizomes or protocorms, and regenerated plants had improved survival rates after transplanting to the greenhouse. Furthermore, the sexual polyploid plants had more compact growth style, produced fragrant flowers, and demonstrated heterosis in plant growth. Through this study, a reliable protocol for selection of appropriate parents for 2n gamete production, ploidy level evaluation, in vitro culture of polyploid progenies, and development of new polyploid cultivars was established. Our study with Cymbidium suggests that the use of 2n gametes is a viable approach for improving floriculture crops.

Selective egg cell polyspermy bypasses the triploid block

Polyploidization, the increase in genome copies, is considered a major driving force for speciation. We have recently provided the first direct in planta evidence for polyspermy induced polyploidization. Capitalizing on a novel sco1-based polyspermy assay, we here show that polyspermy can selectively polyploidize the egg cell, while rendering the genome size of the ploidy-sensitive central cell unaffected. This unprecedented result indicates that polyspermy can bypass the triploid block, which is an established postzygotic polyploidization barrier. In fact, we here show that most polyspermy-derived seeds are insensitive to the triploid block suppressor admetos. The robustness of polyspermy-derived plants is evidenced by the first transcript profiling of triparental plants and our observation that these idiosyncratic organisms segregate tetraploid offspring within a single generation. Polyspermy-derived triparental plants are thus comparable to triploids recovered from interploidy crosses. Our results expand current polyploidization concepts and have important implications for plant breeding.

Clonal Reproduction through Seeds in Sight for Crops

Apomixis or asexual reproduction through seeds, enables the preservation of hybrid vigor. Hybrids are heterozygous and segregate for genotype and phenotype upon sexual reproduction. While apomixis, that is, clonal reproduction, is intuitively antithetical to diversity, it is rarely obligate and actually provides a mechanism to recover and maintain superior hybrid gene combinations for which sexual reproduction would reveal deleterious alleles in less fit genotypes. Apomixis, widespread across flowering plant orders, does not occur in major crop species, yet its introduction could add a valuable tool to the breeder's toolbox. In the past decade, discovery of genetic mechanisms regulating meiosis, embryo and endosperm development have facilitated proof-of-concept for the synthesis of apomixis, bringing apomictic crops closer to reality.

Pairing and Exchanging between Daypyrum villosum Chromosomes 6V#2 and 6V#4 in the Hybrids of Two Different Wheat Alien Substitution Lines

Normal pairing and exchanging is an important basis to evaluate the genetic relationship between homologous chromosomes in a wheat background. The pairing behavior between 6V#2 and 6V#4, two chromosomes from different Dasypyrum villosum accessions, is still not clear. In this study, two wheat alien substitution lines, 6V#2 (6A) and 6V#4 (6D), were crossed to obtain the F₁ hybrids and F₂ segregating populations, and the testcross populations were obtained by using the F₁ as a parent crossed with wheat variety Wan7107. The chromosomal behavior at meiosis in pollen mother cells (PMCs) of the F₁ hybrids was observed using a genomic in situ hybridization (GISH) technique. Exchange events of two alien chromosomes were investigated in the F₂ populations using nine polymerase chain reaction (PCR) markers located on the 6V short arm. The results showed that the two alien chromosomes could pair with each other to form ring- or rod-shaped bivalent chromosomes in 79.76% of the total PMCs, and most were pulled to two poles evenly at anaphase I. Investigation of the F₂ populations showed that the segregation ratios of seven markers were consistent with the theoretical values 3:1 or 1:2:1, and recombinants among markers were detected. A genetic linkage map of nine PCR markers for 6VS was accordingly constructed based on the exchange frequencies and compared with the physical maps of wheat and barley based on homologous sequences of the markers, which showed that conservation of sequence order compared to 6V was 6H and 6B > 6A > 6D. In the testcross populations with 482 plants, seven showed susceptibility to powdery mildew (PM) and lacked amplification of alien chromosomal bands. Six other plants had amplification of specific bands of both the alien chromosomes at multiple sites, which suggested that the alien chromosomes had abnormal separation behavior in about 1.5% of the PMCs in F₁, which resulted in some gametes containing two alien chromosomes. In addition, three new types of chromosome substitution were developed. This study lays a foundation for alien allelism tests and further assessment of the genetic relationship among 6V#2, 6V#4, and their wheat homoeologous chromosomes.

Sorghum bicolor x S. halepense interspecific hybridization is influenced by the frequency of 2n gametes in S. bicolor

Tetraploid johnsongrass [Sorghum halepense (L.) Pers.] is a sexually-compatible weedy relative of diploid sorghum [Sorghum bicolor (L.) Moench]. To determine the extent of interspecific hybridization between male sterile grain sorghum and johnsongrass and the ploidy of their progeny, cytoplasmic (CMS), genetic (GMS) and chemically induced male sterile lines of Tx623 and Tx631 were pollinated with johnsongrass pollen. At maturity 1% and 0.07% of the developing seeds of Tx623 and Tx631 respectively were recovered. Ninety-one percent of recovered hybrids were tetraploid and two percent were triploid, the tetraploids resulting from 2n gametes present in the sorghum female parent. Their formation appears to be genotype dependent as more tetraploids were recovered from Tx623 than Tx631. Because a tetraploid sorghum x johnsongrass hybrid has a balanced genome, they are male and female fertile providing opportunities for gene flow between the two species. Given the differences in 2n gamete formation among Tx623 and Tx631, seed parent selection may be one way of reducing the likelihood of gene flow. These studies were conducted in controlled and optimum conditions; the actual outcrossing rate in natural conditions is expected to be much lower. More studies are needed to assess the rates of hybridization, fitness, and fertility of the progeny under field conditions.

Plant speciation in the age of climate change

BACKGROUND

Species diversity is likely to undergo a sharp decline in the next century. Perhaps as many as 33 % of all plant species may expire as a result of climate change. All parts of the globe will be impacted, and all groups of organisms will be affected. Hundreds of species throughout the world have already experienced local extinction.

PERSPECTIVES

While thousands of species may become extinct in the next century and beyond, species formation will still occur. I consider which modes of plant species formation are likely to prevail in the next 500 years. I argue that speciation primarily will involve mechanisms that produce reproductively isolated lineages within less (often much less) than 100 generations. I will not especially consider the human element in promoting species formation, because it will continue and because the conclusions presented here are unaffected by it. The impact of climate change may be much more severe and widespread.

CONCLUSIONS

The most common modes of speciation likely to be operative in the next 500 years ostensibly will be auto- and allopolyploidy. Polyploid species or the antecedents thereof can arise within two generations. Moreover, polyploids often have broader ecological tolerances, and are likely to be more invasive than are their diploid relatives. Polyploid species may themselves spawn additional higher level polyploids either through crosses with diploid species or between pre-existing polyploids. The percentage of polyploid species is likely to exceed 50 % within the next 500 years vs. 35 % today. The stabilized hybrid derivatives (homoploid hybrid speciation) could emerge within a hundred generations after species contact, as could speciation involving chromosomal rearrangements (and perhaps number), but the number of such events is likely to be low. Speciation involving lineage splitting will be infrequent because the formation of substantive pre- and post-zygotic barriers typically takes many thousands of years.

Transcriptome Analysis of Young Ovaries Reveals Candidate Genes Involved in Gamete Formation in Lantana camara

Lantana (Lantana camara L., Verbenaceae) is an important ornamental crop, yet can be a highly invasive species. The formation of unreduced female gametes (UFGs) is a major factor contributing to its invasiveness and has severely hindered the development of sterile cultivars. To enrich the genomic resources and gain insight into the genetic mechanisms of UFG formation in lantana, we investigated the transcriptomes of young ovaries of two lantana genotypes, GDGHOP-36 (GGO), producing 100% UFGs, and a cultivar Landmark White Lantana (LWL), not producing UFGs. The de novo transcriptome assembly resulted in a total of 90,641 unique transcript sequences with an N50 of 1692 bp, among which, 29,383 sequences contained full-length coding sequences (CDS). There were 214 transcripts associated with the biological processes of gamete production and 10 gene families orthologous to genes known to control unreduced gamete production in Arabidopsis. We identified 925 transcription factor (TF)-encoding sequences, 91 nucleotide-binding site (NBS)-containing genes, and gene families related to drought/salt tolerance and allelopathy. These genomic resources and candidate genes involved in gamete formation will be valuable for developing new tools to control the invasiveness in L. camara, protect native lantana species, and understand the formation of unreduced gametes in plants.

Widespread co-occurrence of multiple ploidy levels in fragile ferns (Cystopteris fragilis complex; Cystopteridaceae) probably stems from similar ecology of cytotypes, their efficient dispersal and inter-ploidy hybridization

BACKGROUND AND AIMS

Polyploidy has played an important role in the evolution of ferns. However, the dearth of data on cytotype diversity, cytotype distribution patterns and ecology in ferns is striking in comparison with angiosperms and prevents an assessment of whether cytotype coexistence and its mechanisms show similar patterns in both plant groups. Here, an attempt to fill this gap was made using the ploidy-variable and widely distributed Cystopteris fragilis complex.

METHODS

Flow cytometry was used to assess DNA ploidy level and monoploid genome size (Cx value) of 5518 C. fragilis individuals from 449 populations collected over most of the species' global distributional range, supplemented with data from 405 individuals representing other related species from the complex. Ecological preferences of C. fragilis tetraploids and hexaploids were compared using field-recorded parameters and database-extracted climate data.

KEY RESULTS

Altogether, five different ploidy levels (2x, 4x, 5x, 6x, 8x) were detected and three species exhibited intraspecific ploidy-level variation: C. fragilis, C. alpina and C. diaphana. Two predominant C. fragilis cytotypes, tetraploids and hexaploids, co-occur over most of Europe in a diffuse, mosaic-like pattern. Within this contact zone, 40 % of populations were mixed-ploidy and most also contained pentaploid hybrids. Environmental conditions had only a limited effect on the distribution of cytotypes. Differences were found in the Cx value of tetraploids and hexaploids: between-cytotype divergence was higher in uniform-ploidy than in mixed-ploidy populations.

CONCLUSIONS

High ploidy-level diversity and widespread cytotype coexistence in the C. fragilis complex match the well-documented patterns in some angiosperms. While ploidy coexistence in C. fragilis is not driven by environmental factors, it could be facilitated by the perennial life-form of the species, its reproductive modes and efficient wind dispersal of spores. Independent origins of hexaploids and/or inter-ploidy gene flow may be expected in mixed-ploidy populations according to Cx value comparisons.

Partitioning Apomixis Components to Understand and Utilize Gametophytic Apomixis

Apomixis is a method of reproduction to generate clonal seeds and offers tremendous potential to fix heterozygosity and hybrid vigor. The process of apomictic seed development is complex and comprises three distinct components, viz., apomeiosis (leading to formation of unreduced egg cell), parthenogenesis (development of embryo without fertilization) and functional endosperm development. Recently, in many crops, these three components are reported to be uncoupled leading to their partitioning. This review provides insight into the recent status of our understanding surrounding partitioning apomixis components in gametophytic apomictic plants and research avenues that it offers to help understand the biology of apomixis. Possible consequences leading to diversity in seed developmental pathways, resources to understand apomixis, inheritance and identification of candidate gene(s) for partitioned components, as well as contribution towards creation of variability are all discussed. The potential of Panicum maximum, an aposporous crop, is also discussed as a model crop to study partitioning principle and effects. Modifications in cytogenetic status, as well as endosperm imprinting effects arising due to partitioning effects, opens up new opportunities to understand and utilize apomixis components, especially towards synthesizing apomixis in crops.

Polyploidy in Fruit Tree Crops of the Genus Annona (Annonaceae)

Genome duplication or polyploidy is one of the main factors of speciation in plants. It is especially frequent in hybrids and very valuable in many crops. The genus Annona belongs to the Annonaceae, a family that includes several fruit tree crops, such as cherimoya (Annona cherimola), sugar apple (Annona squamosa), their hybrid atemoya (A. cherimola × A. squamosa) or pawpaw (Asimina triloba). In this work, genome content was evaluated in several Annona species, A. triloba and atemoya. Surprisingly, while the hybrid atemoya has been reported as diploid, flow cytometry analysis of a progeny obtained from an interspecific cross between A. cherimola and A. squamosa showed an unusual ploidy variability that was also confirmed karyotype analysis. While the progeny from intraspecific crosses of A. cherimola showed polyploid genotypes that ranged from 2.5 to 33%, the hybrid atemoyas from the interspecific cross showed 35% of triploids from a total of 186 genotypes analyzed. With the aim of understanding the possible implications of the production of non-reduced gametes, pollen performance, pollen size and frequency distribution of pollen grains was quantified in the progeny of this cross and the parents. A large polymorphism in pollen grain size was found within the interspecific progeny with higher production of unreduced pollen in triploids (38%) than in diploids (29%). Moreover, using PCR amplification of selected microsatellite loci, while 13.7% of the pollen grains from the diploids showed two alleles, 41.28% of the grains from the triploids amplified two alleles and 5.63% showed up to three alleles. This suggests that the larger pollen grains could correspond to diploid and, in a lower frequency, to triploid pollen. Pollen performance was also affected with lower pollen germination in the hybrid triploids than in both diploid parents. The results confirm a higher percentage of polyploids in the interspecific cross, affecting pollen grain size and pollen performance. The occurrence of unreduced gametes in A. cherimola, A. squamosa and their interspecific progeny that may result in abnormalities of ploidy such as the triploids and tetraploids observed in this study, opens an interesting opportunity to study polyploidy in Annonaceae.

Silurana Chromosomal Evolution: A New Piece to the Puzzle

The African clawed frogs of the subgenus Silurana comprise both diploid and tetraploid species. The root of the polyploidization event leading to the extant Xenopus calcaratus, X. mellotropicalis, and X. epitropicalis is not fully understood so far. In X. mellotropicalis, we previously proposed 2 evolutionary scenarios encompassing complete (scenario A) or incomplete (scenario B) translocation of a heterochromatic block from chromosome 9 to 2 in a diploid ancestor. To resolve this puzzle, we performed FISH coupled with tyramide signal amplification (FISH-TSA) using 5 X. tropicalis and X. mellotropicalis single copy gene probes (gyg2, cept1, fn1, ndufs1, and sf3b1) reflecting borders of the heterochromatic blocks in X. tropicalis chromosome 9 (XTR 9) and X. mellotropicalis chromosome 9b (XME 9b) and XME 2a. cDNA sequencing recognized both homoeologous genes in X. mellotropicalis. Comparison of gene physical mapping between X. tropicalis and X. mellotropicalis clearly confirmed complete rather than incomplete translocation t(9;2) of the heterochromatic block in the diploid predecessor and thus favored scenario A regarding the formation of an ancestral allotetraploid karyotype.

Speciation Success of Polyploid Plants Closely Relates to the Regulation of Meiotic Recombination

Polyploidization is a widespread phenomenon, especially in flowering plants that have all undergone at least one event of whole genome duplication during their evolutionary history. Consequently, a large range of plants, including many of the world's crops, combines more than two sets of chromosomes originating from the same (autopolyploids) or related species (allopolyploids). Depending on the polyploid formation pathway, different patterns of recombination will be promoted, conditioning the level of heterozygosity. A polyploid population harboring a high level of heterozygosity will produce more genetically diverse progenies. Some of these individuals may show a better adaptability to different ecological niches, increasing their chance for successful establishment through natural selection. Another condition for young polyploids to survive corresponds to the formation of well-balanced gametes, assuring a sufficient level of fertility. In this review, we discuss the consequences of polyploid formation pathways, meiotic behavior and recombination regulation on the speciation success and maintenance of polyploid species.

Plastid genomes reveal recurrent formation of allopolyploid Fragaria

PREMISE OF THE STUDY

Recurrent formation of polyploid taxa is a common observation in many plant groups. Haploid, cytoplasmic genomes like the plastid genome can be used to overcome the problem of homeologous genes and recombination in polyploid taxa. Fragaria (Rosaceae) contains several octo- and decaploid species. We use plastome sequences to infer the plastid ancestry of these taxa with special focus on the decaploid Fragaria cascadensis.

METHODS

We used genome skimming of 96 polyploid Fragaria samples on a single Illumina HiSeq 3000 lane to obtain whole plastome sequences. These sequences were used for phylogenetic reconstructions and dating analyses. Ploidy of all samples was inferred with flow cytometry, and plastid inheritance was examined in a controlled cross of F. cascadensis.

KEY RESULTS

The plastid genome phylogeny shows that only the octoploid F. chiloensis is monophyletic, all other polyploid taxa were supported to be para- or polyphyletic. The decaploid Fragaria cascadensis has biparental plastid inheritance and four different plastid donors. Diversification of the F. cascadensis clades occurred in the last 230,000 years. The southern part of its distribution range harbors considerably higher genetic diversity, suggestive of a potential refugium.

CONCLUSIONS

Fragaria cascadensis had at least four independent origins from parents with different plastomes. In contrast, para- and polyphyletic taxa of the octoploid Fragaria species are best explained by incomplete lineage sorting and/or hybridization. Biogeographic patterns in F. cascadensis are probably a result of range shift during the last glacial maximum.