Population genetic structure of parthenogenetic flatworm populations with occasional sex

Abstract

Summary 1. Sexual populations are expected to perform better in fluctuating environments than asexuals because recombination provides the potential to adapt to changing environments due to increased genetic variation. Nevertheless, some asexual species show comparably high levels of genotypic diversity. Such diversity might be achieved through gene flow between coexisting sexual and asexual populations or through sexual events within asexual populations. 2. Evidence for occasional sex in the flatworm Schmidtea polychroa was previously found at one specific site that is inhabited by parthenogenetic forms. There, varying rates of sex between subpopulations, reaching up to 12%, were observed. Past recurrent sexual processes left a significant genetic signature in the population genetic structure of this population. In the present study, we examined the population genetic structure of six independent metapopulations (lakes) of the freshwater planarian flatworm S. polychroa, to confirm the presence of occasional sex and that its population genetic consequences can be generalised. 3. Using microsatellites, we found varying rates of occasional sex among subpopulations. Metapopulations showed medium to high levels of genotypic diversity that correlated with the rate of sex. 4. We conclude that occasional sex has considerable consequences for population genetic structure of parthenogenetic species and promotes diversity that might allow response to the particular type of selection that is usually predicted to favour sexual reproduction. This reproductive strategy provides genetic characteristics required for selection to act on, and might, therefore, explain the success of this parthenogenetic species.

Faster clonal turnover in high‐infection habitats provides evidence for parasite‐mediated selection

“…Earlier studies in other sexual/asexual systems have shown that the frequency of sexual reproduction varies among natural populations and that the genetic signature of sexual reproduction can be seen in many predominately asexual populations (Michiels et al ., ; Bruvo et al ., ; Allen & Lynch, ; Sánchez Navarro et al ., ). However, direct comparisons of temporal variation in sexual frequency in mixed populations remain rare.…”

Recent progress and challenges in population genetics of polyploid organisms: an overview of current state‐of‐the‐art molecular and statistical tools

“…In some cases, such as found in the planarian flatworm, Schmidtea polychroa , polyploid individuals can produce viable sperm that may lead to rare sexual processes (Sánchez‐Navarro et al . ). As asexually reproducing plants and animals often have uneven ploidy levels (e.g.…”

“…In the animal kingdom, the majority of polyploid invertebrates and reptiles reproduce asexually, and it has been estimated that 99% of apomictic plant species are polyploids (Suomalainen et al 1987). In some cases, such as found in the planarian flatworm, Schmidtea polychroa, polyploid individuals can produce viable sperm that may lead to rare sexual processes (S anchez-Navarro et al 2013). As asexually reproducing plants and animals often have uneven ploidy levels (e.g.…”

Exploring the sexual diversity of flatworms: Ecology, evolution, and the molecular biology of reproduction

“…Second, parthenogenetic forms also harbor more infections by a presumed parasitic amoeboid protozoan, which is in some respects consistent with the Red Queen hypothesis that predicts sexuality protects against parasites, though in this case the costs of infection (required for the hypothesis to work) are unclear (Michiels et al, ). Subsequent efforts have focused on explaining the maintenance of both sexual and parthenogenetic forms, with phylogeographic and experimental evidence indicating that hybridization between the two occurs, as does genetic exchange between parthenogenetic individuals via various mechanisms that lead to the occasional retention of paternal chromosomes and exclusion of maternal chromosomes, and frequently involve transitions in ploidy (Pongratz et al, ; D'Souza et al, ; Sánchez Navarro and Jokela, ). This may help to maintain genetic diversity in parthenogens and explain their persistence.…”