Abstract
Facultative clonality is associated with complex life cycles where sexual and asexual forms can be exposed to contrasting selection pressures. Facultatively clonal animals often have distinct developmental capabilities that depend on reproductive mode (e.g., negligible senescence and exceptional regeneration ability in asexual individuals, which are lacking in sexual individuals). Understanding how these differences in life history strategies evolved is hampered by limited knowledge of the population structure underlying sexual and asexual forms in nature. Here we studied genetic differentiation of coexisting sexual and asexual Hydra oligactis polyps, a freshwater cnidarian where reproductive mode-dependent life history patterns are observed. We collected asexual and sexual polyps from 13 Central European water bodies and used restriction-site associated DNA sequencing to infer population structure. We detected high relatedness among populations and signs that hydras might spread with resting eggs through zoochory. We found no genetic structure with respect to mode of reproduction (asexual vs. sexual). On the other hand, clear evidence was found for phenotypic plasticity in mode of reproduction, as polyps inferred to be clones differed in reproductive mode. Moreover, we detected two cases of apparent sex change (males and females found within the same clonal lineages) in this species with supposedly stable sexes. Our study describes population genetic structure in Hydra for the first time, highlights the role of phenotypic plasticity in generating patterns of life history variation, and contributes to understanding the evolution of reproductive mode-dependent life history variation in coexisting asexual and sexual forms.
Highlights
Sexual reproduction is the most widespread mode of reproduction in multicellular eukaryotic species as more than 99% of these organisms reproduce sexually (Bell, 1982)
We found that: (i) there is clear evidence for phenotypic plasticity in mode of reproduction: polyps inferred to be clones had contrasting modes of reproduction; and (ii) there was no genetic structuring with respect to the mode of reproduction: sexual polyps showed no difference from the asexual polyps
We found high genetic relatedness between populations and we observed two cases of apparent sex change
Summary
Sexual reproduction is the most widespread mode of reproduction in multicellular eukaryotic species as more than 99% of these organisms reproduce sexually (Bell, 1982). In some cases sexual and asexual lineages occurring in sympatry do not compete with each other for the same habitats or resources but rather specialize on distinct ecological niches (Barraclough et al, 2003; Maynard-Smith, 1978; Vrijenhoek, 1984), resulting in the stable coexistence of the two forms Such specialization can lead to speciation and generation of cryptic species— morphologically similar but genetically more or less distinct entities (Birky & Barraclough, 2009; Mayr, 1948; Peccoud et al, 2009)—with reproductive isolation further reinforced by differences in reproductive strategies among sexual and asexual lineages. Facultative clonality usually occurs in highly variable environments, where asexual and sexual modes of reproduction are observed depending on environmental conditions In such facultative clonal organisms, the presence of both asexual and sexual strategies within the same population is the result of phenotypic plasticity in mode of reproduction. This could work in the following possible ways. (i) If there is notable genetic differentiation among reproductive mode categories (asexual and sexual), we expect that sexually reproducing individuals would cluster into different groups from asexual ones. (ii) If phenotypic plasticity plays a major role in shaping the mode of reproduction, individuals of the same genotype may follow different reproduction modes. (iii) Alternatively, both genetic differentiation and phenotypic plasticity might be involved; in this case we expect variable proportions of sexual/asexual individuals within distinct genotypes
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