Population genetic structure in a self-compatible hermaphroditic snail is driven by drift independently of its contemporary mating system.

Genetic drift, gene flow, and natural selection commonly influence population genetic diversity. In populations of self-compatible hermaphrodites, the mating system (e.g., self-fertilization) further reduces individual heterozygosity. Furthermore, selfing, as a form of inbreeding, significantly impacts genetic drift by reducing effective population size (N e). This can potentially accelerate genetic drift, particularly in small populations where self-fertilization is likely during founder events. To investigate the roles of genetic drift and contemporary mating system in populations of the freshwater snail Lymnaea stagnalis, we examined their effective population sizes (N e) and Tajima's D values, which reflect genetic drift over extended time periods, as well as estimates of within-population selfing rates and pairwise relatedness reflecting contemporary mating system. We used 4054 SNP markers obtained using restriction site associated DNA (RAD) sequencing from individuals in five snail populations originating from geographically closely located ponds. We found strong population genetic structure and differences in genetic diversity among populations. Covariation between genetic diversity and N e estimates and Tajima's D values suggested drift being an important determinant of genetic diversity and structure in these populations. However, this effect was independent of the contemporary mating system, as indicated by the similarity of selfing rates and relatedness estimates among populations. Thus, founder events (possibly including historical inbreeding) and/or drift due to small sizes of L. stagnalis populations are likely to explain their genetic structure and limit within-population genetic diversity.

Parasites driving host diversity: Incidence of disease correlated with Daphnia clonal turnover.

According to the Red Queen hypothesis, clonal diversity in asexual populations could be maintained by negative frequency-dependant selection by coevolving parasites. If common clones are selected against and rare clones gain a concomitant advantage, we expect that clonal turnover should be faster during parasite epidemics than between them. We tested this hypothesis exploring field data of the Daphnia-Caullerya host-parasite system. The clonal make-up and turnover of the Daphnia host population was tracked with high temporal resolution from 1998 until 2013, using first allozyme and later microsatellite markers. Significant differences in the clonal composition between random and infected subsamples of Daphnia populations were detected on six of seven tested occasions, confirming genetic specificity of the host-parasite interaction in this system. We used time series analysis to compare the rates of host clonal turnover to the incidence of parasitism, and found that Caullerya prevalence was significantly associated with microsatellite-based clonal turnover. As alternate hypotheses, we further tested whether turnover was related to a variety of biotic, abiotic, and host demographic parameters. Other significant correlates of turnover were cyanobacterial biomass and (weakly) temperature. Overall, parasitism seems to be a strong driver of host clonal turnover, in support of the Red Queen hypothesis.