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)

Abstract

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).

Habitat Shape Affects Polyploid Establishment in a Spatial, Stochastic Model

“…The theoretical impacts of reproductive assurance and population size on polyploid establishment have been examined in previously published deterministic and stochastic models (Rodriguez, 1996;Baack, 2005;Rausch and Morgan, 2005;Oswald and Nuismer, 2011;Fowler and Levin, 2016;Chrtek et al, 2017), and the results presented here are qualitatively similar to the results of those studies. However, these factors have not been studied extensively within a spatial context (aside from Baack, 2005).…”

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

“…In recent decades, several mechanisms facilitating successful polyploid establishment and/or cytotype coexistence have been proposed (reviewed by Kolář et al, 2017). For example, the minority status of one of the coexisting cytotypes may be overcome by its recurrent origin (Ramsey, 2007), efficient vegetative spread (Chrtek et al, 2017), autogamy (Petit et al, 1997), non-random mating (Husband et al, 2008) or a substantial competitive advantage (Felber, 1991). Prominent among these mechanisms is the (fine-scale) spatial segregation of cytotypes, which can increase the rate of compatible, within-ploidy mating (Baack, 2005;Kolář et al, 2017).…”

Genetically divergent cytotypes of Vaccinium uliginosum co-occurring in the central Eastern Alps can be distinguished based on the morphology of their flowers

“…Longevity along with the ability to reproduce vegetatively and by selfing and, consequently, the lack of a need for regular sexual reproduction probably enabled V. uliginosum s. lat. to escape minority cytotype exclusion (see also Chrtek et al 2017). In other polyploid complexes spatial segregation is identified as an important driver of successful cytotype co-occurrence, for example in Chamerion angustifolium (Sabara et al 2013), Gymnadenia conopsea (Trávníček et al 2011b), Knautia arvensis (Kolář et al 2009), Pilosella echioides (Trávníček et al 2011a) and Senecio carniolicus s. lat.…”