Sexual Species Research Articles

Recombination: the good, the bad and the variable.

Recombination, the process by which DNA strands are broken and repaired, producing new combinations of alleles, occurs in nearly all multicellular organisms and has important implications for many evolutionary processes. The effects of recombination can be good, as it can facilitate adaptation, but also bad when it breaks apart beneficial combinations of alleles, and recombination is highly variable between taxa, species, individuals and across the genome. Understanding how and why recombination rate varies is a major challenge in biology. Most theoretical and empirical work has been devoted to understanding the role of recombination in the evolution of sex—comparing between sexual and asexual species or populations. How recombination rate evolves and what impact this has on evolutionary processes within sexually reproducing organisms has received much less attention. This Theme Issue focusses on how and why recombination rate varies in sexual species, and aims to coalesce knowledge of the molecular mechanisms governing recombination with our understanding of the evolutionary processes driving variation in recombination within and between species. By integrating these fields, we can identify important knowledge gaps and areas for future research, and pave the way for a more comprehensive understanding of how and why recombination rate varies.

Low recombination rates in sexual species and sex-asex transitions.

In most sexual, diploid eukaryotes, at least one crossover occurs between each pair of homologous chromosomes during meiosis, presumably in order to ensure proper segregation. Well-known exceptions to this rule are species in which one sex does not recombine and specific chromosomes lacking crossover. We review other possible exceptions, including species with chromosome maps of less than 50 cM in one or both sexes. We discuss the idea that low recombination rates may favour sex-asex transitions, or, alternatively may be a consequence of it. We then show that a yet undescribed species of brine shrimp Artemia from Kazakhstan (A sp. Kazakhstan), the closest known relative of the asexual Artemia parthenogenetica, has one of the shortest genetic linkage maps known. Based on a family of 42 individuals and 589 RAD markers, we find that many linkage groups are considerably shorter than 50 cM, suggesting either no obligate crossover or crossovers concentrated at terminal positions with little effect on recombination. We contrast these findings with the published map of the more distantly related sexual congener, A. franciscana, and conclude that the study of recombination in non-model systems is important to understand the evolutionary causes and consequences of recombination.This article is part of the themed issue 'Evolutionary causes and consequences of recombination rate variation in sexual organisms'.

Phenotypic influences on the reproductive strategy of the facultative sexual rotifer Brachionus rubens (Monogononta)

Facultative sexual species employ a dual reproductive strategy (heterogony) comprising primarily asexual reproduction with intermittent sexual reproduction. Given the higher relative costs of sexual reproduction, elucidating the triggers underlying these transitions might help our understanding of the evolution of (obligate) sex in general. Existing hypotheses into how and when facultative sexuals invest into sex focus largely either on environmental (habitat-deterioration and resource-demanding hypotheses) or genetic factors (condition-dependent hypothesis), but tend to lack experimental evidence, especially with respect to within-population variation. To address this deficit, we examined the influence of several variables that potentially affect fitness (food quality, water temperature, physiological acclimation, and all combinations thereof) on both the lifetime reproduction (total number of offspring) and investment into sexual offspring per female in a clonal population of the monogonont rotifer Brachionus rubens. Investment into sex, both absolutely and relative to lifetime reproduction, was tied most closely to and positively correlated with individual fitness (i.e., lifetime reproduction): individuals with higher fitness invested more into sexual reproduction. These results run contra to the condition-dependent hypothesis and indicate an energy-budget analogue of the resource-demanding hypothesis. Furthermore, investment into sex increased after a period of physiological acclimation to the new conditions, probably because of the amelioration of short-term stress effects or clonal selection. Our results underscore that life history and general phenotypic considerations—here, energetic provisioning of offspring, the presence of a sexual resting stage, and the relative timing of sexual versus asexual reproduction—can modify existing hypotheses based either on environmental or genetic factors alone.

Do male fish prefer them big and colourful? Non-random male courtship effort in a viviparous fish with negligible paternal investment

In the majority of sexual species, there are asymmetries in reproductive effort, with males typically investing more in securing matings and females investing more in producing offspring. This causes males to mate less discriminately than females. Yet males may also become choosy if the following conditions are met: (a) that females vary in their reproductive value, (b) that males can perceive this variation, and (c) that mating with one female reduces the possibility of mating with another. These conditions may be met in the livebearing Goodeidae, a clade of Mexican fish whose females are often brightly coloured and whose males display costly ornaments and courtship as the only means to obtain matings. Males of the black-finned goodeid (Girardinichthys viviparus) have relatively simple, yet costly courtship behaviour, with mating probability depending on the duration of one-to-one courtship episodes, thus by courting one female they must ignore others. We evaluated whether the decision to court a female depends on her phenotype. Three variables of female phenotype were positively linked to the duration of male visits and to the frequency of displays performed by males: belly area, hue (“orangeness”) and size. Since fecundity and offspring survival were also a positive function of female size, we conclude that male G. viviparus evaluate the potential female reproductive value and allocate their courtship effort accordingly. Since male courtship effort is also influenced by female colouration, we suggest that our findings may help explaining the recurrent evolution of sexually dimorphic female colouration in this clade. Amongst sexually reproducing species, females often invest heavily on offspring and mate only after selecting partners carefully, while males invest little on offspring but mate indiscriminately. In other cases, males carry the burden of raising offspring and are choosy. Thus, we see female mate choice in species with a bias towards maternal investment, and male mate choice in species with a bias towards paternal investment. Here we report male mate choice in a species with predominantly maternal investment; a viviparous fish whose females are choosy and whose males invest heavily on courtship. Males made longer visits to the wider bellied, and more orange-looking females, and larger (but not more orange) females produced more offspring which survived better, thus some attributes of females linked to their reproductive value influence how much time and effort males devote to court them.

Is evolution of apomicts driven by the phylogeography of the sexual ancestor? Insights from European and Caucasian brambles (Rubus, Rosaceae)

AbstractAimApomixis, i.e. asexual reproduction via seeds, occurs in many plant taxa of diverse phylogenetic origins and has resulted in a high abundance and wide distribution of some groups. When and where apomicts arose and how their evolution is linked to their sexual ancestors is poorly understood. We aimed at detecting phylogeographical patterns in Rubus ulmifolius–R. sanctus agg., a diploid sexual species aggregate from Rubus series Discolores (Rosaceae), and asked where and when its polyploid apomictic descendants originated.LocationEurope and adjacent regions (Caucasus, Macaronesia, Morocco).MethodsNext‐generation sequencing of 10 nuclear microsatellite loci, Sanger sequencing of two plastid loci and ecological niche modelling.ResultsThe data reveal strong, continental‐scale genetic structuring within Rubus ulmifolius–R. sanctus agg. Geographical patterns of genetic diversity and ecological niche models indicate its survival mainly on the Iberian Peninsula and in Morocco during the Last Glacial Maximum, as well as population bottlenecks in the eastern Mediterranean and the Caucasus, whereas low allelic diversity in north‐western Europe stems from post‐glacial re‐colonization from southern refugia. The distribution of alleles among diploids and polyploids indicates that the first allopolyploidization events occurred prior to the last glaciation, but also reflects post‐glacial gene flow from diploids to polyploids.Main conclusionsPolyploid apomicts both preserve ancestral alleles lost in their diploid ancestors because of ice‐age bottlenecks and harbour also derived, i.e. younger, alleles obtained from diploid taxa via recent gene flow. Diversification of apomicts as a result of the diploid's genetic deterioration is hypothesized. Then, geographical parthenogenesis in Rubus could also be explained by genetic diversity patterns in the diploid, sexual ancestor.

Analysis of simple sequence repeat (SSR) structure and sequence within Epichloë endophyte genomes reveals impacts on gene structure and insights into ancestral hybridization events.

Epichloë grass endophytes comprise a group of filamentous fungi of both sexual and asexual species. Known for the beneficial characteristics they endow upon their grass hosts, the identification of these endophyte species has been of great interest agronomically and scientifically. The use of simple sequence repeat loci and the variation in repeat elements has been used to rapidly identify endophyte species and strains, however, little is known of how the structure of repeat elements changes between species and strains, and where these repeat elements are located in the fungal genome. We report on an in-depth analysis of the structure and genomic location of the simple sequence repeat locus B10, commonly used for Epichloë endophyte species identification. The B10 repeat was found to be located within an exon of a putative bZIP transcription factor, suggesting possible impacts on polypeptide sequence and thus protein function. Analysis of this repeat in the asexual endophyte hybrid Epichloë uncinata revealed that the structure of B10 alleles reflects the ancestral species that hybridized to give rise to this species. Understanding the structure and sequence of these simple sequence repeats provides a useful set of tools for readily distinguishing strains and for gaining insights into the ancestral species that have undergone hybridization events.

Diversity of lichens and bryophytes in hybrid aspen plantations in Estonia depends on landscape structure

The importance of single-species forest plantations in sustaining biodiversity could be bigger than expected. We described the diversity of lichens and bryophytes in 15 midterm (16- to 17-year-old) hybrid aspen (Populus tremula × Populus tremuloides) plantations in Estonia. Species richness and composition data were linked with environmental and landscape-scale variables. Altogether, 44 lichen and 37 bryophyte species were recorded from plantations; richnesses of bryophytes and lichens were positively correlated. Lichen species composition was significantly affected by landscape-scale parameters (distance to the nearest present forest, distance to the nearest forest continuously occurring in the same place since the 1930s, and forest area in the study plot vicinity in the 1930s). Bryophyte species composition was affected mainly by light conditions and forest area in the 1930s. Among lichens, the sexual crustose species functional group dominated in midterm plantations; appearance of asexual lichens of di...

Signals and Speciation:AnolisDewlap Color as a Reproductive Barrier

Abstract The divergence of signals used in sexual selection and species recognition is thought to play an important role in speciation. As such, the striking diversity of dewlap color in Anolis lizards may have contributed to the diversification of this species-rich group. Whether the dewlap acts as a reproductive isolating barrier however remains unclear. We tested the prediction that the degree of dewlap divergence between the two closely related and highly polymorphic Hispaniolan trunk anoles Anolis distichus and Anolis brevirostris is correlated with genetic divergence and the frequency of hybridization in nature. We use integrative analyses of dewlap color variation and molecular genetics to investigate two pairs of localities where the species co-occur, including one pair of localities where the two species exhibit dissimilar dewlaps and a second pair of localities where they have similar dewlaps. At one site where species share similar dewlap coloration, we found evidence of hybridization and lower...

The metaphysics of evolution.

This paper briefly describes process metaphysics, and argues that it is better suited for describing life than the more standard thing, or substance, metaphysics. It then explores the implications of process metaphysics for conceptualizing evolution. After explaining what it is for an organism to be a process, the paper takes up the Hull/Ghiselin thesis of species as individuals and explores the conditions under which a species or lineage could constitute an individual process. It is argued that only sexual species satisfy these conditions, and that within sexual species the degree of organization varies. This, in turn, has important implications for species' evolvability. One important moral is that evolution will work differently in different biological domains.

Neural Circuitry That Mediates Behavior Governing the Tradeoffs Between Survival and Reproduction in Caenorhabditis elegans.

In all outcrossing sexual species there is a mechanism that brings two parents together. For animals, this reproductive requirement may at times conflict with other needs, such as foraging for food. This tension has been studied using the tiny (1 mm) nematode worm, Caenorhabditis elegans. In a trade off between certainty of survival and possibility of reproduction, the C. elegans male will abandon a food patch lacking mates and explore its environment to find one where mates are present. A quantitative behavioral assay has been used to study the behavioral mechanism of mate searching and nutritional, sexual, and neurohormonal pathways that influence the underlying drive state. Taking advantage of the known connectivity of the C. elegans nervous system, neural pathways have been identified that influence the male's behavior in the presence of food with and without mates.

Evolving mutation rate advances the invasion speed of a sexual species

BackgroundMany species are shifting their ranges in response to global climate change. Range expansions are known to have profound effects on the genetic composition of populations. The evolution of dispersal during range expansion increases invasion speed, provided that a species can adapt sufficiently fast to novel local conditions. Genetic diversity at the expanding range border is however depleted due to iterated founder effects. The surprising ability of colonizing species to adapt to novel conditions while being subjected to genetic bottlenecks is termed ‘the genetic paradox of invasive species’. Mutational processes have been argued to provide an explanation for this paradox. Mutation rates can evolve, under conditions that favor an increased rate of adaptation, by hitchhiking on beneficial mutations through induced linkage disequilibrium. Here we argue that spatial sorting, iterated founder events, and population structure benefit the build-up and maintenance of such linkage disequilibrium. We investigate if the evolution of mutation rates could play a role in explaining the ‘genetic paradox of invasive species’ for a sexually reproducing species colonizing a landscape of gradually changing conditions.ResultsWe use an individual-based model to show the evolutionary increase of mutation rates in sexual populations during range expansion, in coevolution with the dispersal probability. The observed evolution of mutation rate is adaptive and clearly advances invasion speed both through its effect on the evolution of dispersal probability, and the evolution of local adaptation. This also occurs under a variable temperature gradient, and under the assumption of 90% lethal mutations.ConclusionsIn this study we show novel consequences of the particular genetic properties of populations under spatial disequilibrium, i.e. the coevolution of dispersal probability and mutation rate, even in a sexual species and under realistic spatial gradients, resulting in faster invasions. The evolution of mutation rates can therefore be added to the list of possible explanations for the ‘genetic paradox of invasive species’. We conclude that range expansions and the evolution of mutation rates are in a positive feedback loop, with possibly far-reaching ecological consequences concerning invasiveness and the adaptability of species to novel environmental conditions.

Meiotic Genes in Colpodean Ciliates Support Secretive Sexuality.

The putatively asexual Colpodean ciliates potentially pose a problem to macro-organismic theories of evolution. They are extremely ancient (although asexuality is thought to hasten extinction), and yet there is one apparently derived sexual species (implying an unlikely regain of a complex trait). If macro-organismic theories of evolution also broadly apply to microbial eukaryotes, though, then most or all of the colpodean ciliates should merely be secretively sexual. Here we show using de novo genome sequencing, that colpodean ciliates have the meiotic genes required for sex and these genes are under functional constraint. Along with these genomic data, we argue that these ciliates are sexual given the cytological observations of both micronuclei and macronuclei within their cells, and the behavioral observations of brief fusions as if the cells were mating. The challenge that colpodean ciliates pose is therefore not to evolutionary theory, but to our ability to induce microbial eukaryotic sex in the laboratory.

The power of evolutionary rescue is constrained by genetic load.

The risk of extinction faced by small isolated populations in changing environments can be reduced by rapid adaptation and subsequent growth to larger, less vulnerable sizes. Whether this process, called evolutionary rescue, is able to reduce extinction risk and sustain population growth over multiple generations is largely unknown. To understand the consequences of adaptive evolution as well as maladaptive processes in small isolated populations, we subjected experimental Tribolium castaneum populations founded with 10 or 40 individuals to novel environments, one more favorable, and one resource poor, and either allowed evolution, or constrained it by replacing individuals one‐for‐one each generation with those from a large population maintained in the natal environment. Replacement individuals spent one generation in the target novel environment before use to standardize effects due to the parental environment. After eight generations we mixed a subset of surviving populations to facilitate admixture, allowing us to estimate drift load by comparing performance of mixed to unmixed groups. Evolving populations had reduced extinction rates, and increased population sizes in the first four to five generations compared to populations where evolution was constrained. Performance of evolving populations subsequently declined. Admixture restored their performance, indicating high drift load that may have overwhelmed the beneficial effects of adaptation in evolving populations. Our results indicate that evolution may quickly reduce extinction risk and increase population sizes, but suggest that relying solely on adaptation from standing genetic variation may not provide long‐term benefits to small isolated populations of diploid sexual species, and that active management facilitating gene flow may be necessary for longer term persistence.

Aging asexual lineages and the evolutionary maintenance of sex.

Finite populations of asexual and highly selfing species suffer from a reduced efficacy of selection. Such populations are thought to decline in fitness over time due to accumulating slightly deleterious mutations or failing to adapt to changing conditions. These within-population processes that lead nonrecombining species to extinction may help maintain sex and outcrossing through species level selection. Although inefficient selection is proposed to elevate extinction rates over time, previous models of species selection for sex assumed constant diversification rates. For sex to persist, classic models require that asexual species diversify at rates lower than sexual species; the validity of this requirement is questionable, both conceptually and empirically. We extend past models by allowing asexual lineages to decline in diversification rates as they age, that is nonrecombining lineages "senesce" in diversification rates. At equilibrium, senescing diversification rates maintain sex even when asexual lineages, at young ages, diversify faster than their sexual progenitors. In such cases, the age distribution of asexual lineages contains a peak at intermediate values rather than showing the exponential decline predicted by the classic model. Coexistence requires only that the average rate of diversification in asexuals be lower than that of sexuals.

Driving factors and temporal fluctuation of Collembola communities and reproductive mode across forest types and regions.

Despite the major role of Collembola in forest soil animal food webs, ecological and evolutionary determinants of their community composition are not well understood. We investigated abundance, community structure, life forms, and reproductive mode of Collembola in four different forest types (coniferous, young managed beech, old managed beech, and unmanaged beech forests) representing different management intensities. Forest types were replicated within three regions across Germany: the Schorfheide‐Chorin, the Hainich, and the Swabian Alb, differing in geology, altitude, and climate. To account for temporal variation, samples were taken twice with an interval of 3 years. To identify driving factors of Collembola community structure, we applied structural equation modeling, including an index of forest management intensity, abiotic and biotic factors such as pH, C‐to‐N ratio of leaf litter, microbial biomass, and fungal‐to‐bacterial ratio. Collembola abundance, biomass, and community composition differed markedly between years, with most pronounced differences in the Schorfheide, the region with the harshest climatic conditions. There, temporal fluctuations of parthenogenetic Collembola were significantly higher than in the other regions. In the year with the more favorable conditions, parthenogenetic species flourished, with their abundance depending mainly on abiotic, density‐independent factors. This is in line with the “Structured Resource Theory of Sexual Reproduction,” stating that parthenogenetic species are favored if density‐independent factors, such as desiccation, frost or flooding, prevail. In contrast, sexual species in the same year were mainly influenced by resource quality‐related factors such as the fungal‐to‐bacterial ratio and the C‐to‐N ratio of leaf litter. The influence of forest management intensity on abundances was low, indicating that disturbance through forest management plays a minor role. Accordingly, differences in community composition were more pronounced between regions than between different forest types, pointing to the importance of regional factors.

Within-population variation in the chemistry of life: the stoichiometry of sexual dimorphism in multiple dimensions

In sexual species, phenotypic divergence between males and females, or sexual dimorphism, is often the source of the most staggering examples of phenotypic variation in nature. Theory suggests that exaggerated sexual traits should drive sex-specific nutritional demands. Advances in spectrometry enable rapid quantification of the elements that make up individuals and traits, which can be used to assess patterns of intraspecific variation and the contribution of nutritionally-demanding sexual traits to these patterns. We measured dimorphism in the whole body stoichiometry of Hyalella amphipods and examined whether nutritional demands of exaggerated sexual traits differ from those of similar traits not under sexual selection. We found striking sexual dimorphism in multivariate whole body elemental composition (i.e., the ionome), including elements important for organismal growth and performance. In males, the exaggerated, sexually-selected claw-like appendage (posterior gnathopod) differed significantly in mass-specific stoichiometry from a similarly sized and serially homologous non-sexual trait (fifth pereopod), indicating that there are fundamental differences in the construction of sexual traits in relation to similar traits that are not under sexual selection. While sexually selected traits do differ from non-sexual traits in their ionomes, we found that possessing an exaggerated trait does not change organismal stoichiometry, indicating that trait exaggeration may not be directly driving ionomic sexual dimorphism. Finally, we found that larger traits are not comparatively larger resource sinks for any element, suggesting that the possession of larger traits is not a function of greater allocation of resources. Together, we discovered substantial sexual dimorphism at the lowest level of organization, chemical elements. Such information illuminates predictions about dimorphisms in foraging behavior, nutritional physiology, and sex-specific selection on the underlying loci. High throughput, multidimensional data on sexual divergence in stoichiometric composition is a powerful tool in understanding the evolutionary ecology of sexual dimorphisms.

Effect of methylparaben in Artemia franciscana

In this study, the toxicity of methylparaben (MeP) an emerging contaminant, was analysed in the sexual species Artemia franciscana, due to its presence in coastal areas and marine saltworks in the Mediterranean region. The acute toxicity (24h-LC50) of MeP in nauplii was tested and its chronic effect (9days) evaluated by measuring survival and growth under two sublethal concentrations (0.0085 and 0.017mg/L). Also, the effect on several key enzymes involved in: antioxidant defences (catalase (CAT) and gluthathion-S-transferase (GST)), neural activity (cholinesterase (ChE)) and xenobiotic biotransformation (carboxylesterase (CbE), was assessed after 48h under sublethal exposure. The results of acute exposure indicate that MeP is harmful to A. franciscana (24h-LC50=36.7mg/L). MeP causes a decrease in CAT activity after 48h exposure to both concentration tested, that points out at the oxidative stress effect of MeP in A. franciscana. However, no significant effect on ChE, CbE and GST activities was found.In addition, MeP does not affect survival and growth in chronic exposure at the sublethal concentrations tested. The results of this study indicate that MeP is not a threat for A. franciscana under the experimental conditions used. Additional studies should be done considering long-term exposure and reproduction studies to analyse the potential risk of MeP as emerging contaminant in marine and hypersaline environments.

Hybridization drives evolution of apomicts in Rubus subgenus Rubus: evidence from microsatellite markers.

Rubus subgenus Rubus is a group of mostly apomictic and polyploid species with a complicated taxonomy and history of ongoing hybridization. The only polyploid series with prevailing sexuality is the series Glandulosi , although the apomictic series Discolores and Radula also retain a high degree of sexuality, which is influenced by environmental conditions and/or pollen donors. The aim of this study is to detect sources of genetic variability, determine the origin of apomictic taxa and validate microsatellite markers by cloning and sequencing. A total of 206 individuals from two central European regions were genotyped for 11 nuclear microsatellite loci and the chloroplast trn L- trn F region. Microsatellite alleles were further sequenced in order to determine the exact repeat number and to detect size homoplasy due to insertions/deletions in flanking regions. The results confirm that apomictic microspecies of ser. Radula are derived from crosses between sexual series Glandulosi and apomictic series Discolores , whereby the apomict acts as pollen donor. Each apomictic microspecies is derived from a single distinct genotype differing from the parental taxa, suggesting stabilized clonal reproduction. Intraspecific variation within apomicts is considerably low compared with sexual series Glandulosi , and reflects somatic mutation accumulation. While facultative apomicts produce clonal offspring, sexual species are the conduits of origin for new genetically different apomictic lineages. One of the main driving forces of evolution and speciation in the highly apomictic subgenus Rubus in central Europe is sexuality in the series Glandulosi . Palaeovegetation data suggest that initial hybridizations took place over different time periods in the two studied regions, and that the successful origin and spread of apomictic microspecies of the series Radula took place over several millennia. Additionally, the cloning and sequencing show that standard evaluations of microsatellite repeat numbers underestimate genetic variability considering homoplasy in allele size.

Genome Expression Balance in a Triploid Trihybrid Vertebrate.

Polyploidy is increasingly recognized as a driver of biological diversity. How and why polyploidization affects gene expression is critical to understanding the link between ploidy elevation and diversification. In polyploid plants, multiple studies have demonstrated that ploidy elevation can confer major but variable consequences for gene expression, ranging from gene-by-gene alterations to entirely silenced genomes. By contrast, animal polyploids remain largely uncharacterized. Accordingly, how animals respond to and manage polyploidy events is not understood. Here, we address this important knowledge gap by analyzing transcriptomes from a triploid hybrid animal, a unisexual Ambystoma salamander, and three sexual Ambystoma species that represent all three parental genomes in the unisexual. We used a novel bioinformatics pipeline that includes competitively mapping triploid sequences to a reference set of orthologous genes in the sexual species to evaluate subgenome expression. Our comparisons of gene expression levels across the three parental genomes revealed that the unisexual triploid displays a pattern of genome balance, where 72% of the genes analyzed were expressed equally among the subgenomes. This result is strikingly different from the genome imbalance typically observed in hybrid polyploid plants. Our analyses represent the first to address gene expression in a triploid hybrid animal and introduce a novel bioinformatic framework for analyzing transcriptomic data.

Cytotype coexistence in the field cannot be explained by inter-cytotype hybridization alone: linking experiments and computer simulations in the sexual species Pilosella echioides (Asteraceae)

BackgroundProcesses driving ploidal diversity at the population level are virtually unknown. Their identification should use a combination of large-scale screening of ploidy levels in the field, pairwise crossing experiments and mathematical modelling linking these two types of data. We applied this approach to determine the drivers of frequencies of coexisting cytotypes in mixed-ploidy field populations of the fully sexual plant species Pilosella echioides. We examined fecundity and ploidal diversity in seeds from all possible pairwise crosses among 2x, 3x and 4x plants. Using these data, we simulated the dynamics of theoretical panmictic populations of individuals whose progeny structure is identical to that determined by the hybridization experiment.ResultsThe seed set differed significantly between the crossing treatments, being highest in crosses between diploids and tetraploids and lowest in triploid-triploid crosses. The number of progeny classes (with respect to embryo and endosperm ploidy) ranged from three in the 2x-2x cross to eleven in the 3x-3x cross. Our simulations demonstrate that, provided there is no difference in clonal growth and/or survival between cytotypes, it is a clear case of minority cytotype exclusion depending on the initial conditions with two stable states, neither of which corresponds to the ploidal structure in the field: (i) with prevalent diploids and lower proportions of other ploidies, and (ii) with prevalent tetraploids and 9% of hexaploids. By contrast, if clonal growth differs between cytotypes, minority cytotype exclusion occurs only if the role of sexual reproduction is high; otherwise differences in clonal growth are sufficient to maintain triploid prevalence (as observed in the field) independently of initial conditions.ConclusionsThe projections of our model suggest that the ploidal structure observed in the field can only be reached via a relatively high capacity for clonal growth (and proportionally lower sexual reproduction) in all cytotypes combined with higher clonal growth in the prevailing cytotype (3x).