Population structure, parasitism, and survivorship of sexual and autodiploid parthenogenetic Campeloma limum

Two theories for the maintenance of sexual reproduction, the Red Queen hypo thesis and mutation accumulation, suggest that the dispersal rates of sexua ls and asexuals may determine the elimination or persistence of asexuals. U nder higher dispersal rates of asexuals, asexuals may temporarily escape vi rulent parasites and reduce the effects of deleterious mutations. In the pr esent study, I examine the population structure, parasite loads, and juveni le survivorship of Campeloma limum sexuals and autodiploid parthenogens fro m the southeastern U.S. Atlantic coastal plain. Using mtDNA sequence variat ion, it is shown that parthenogenetic haplotypes with limited sequence dive rgence are geographically widespread throughout this region and there is no significant population differentiation over a broad geographical scale. Se xual C. limum populations show significant mtDNA differentiation among and within river drainages and there is significant isolation by distance. Thes e patterns are consistent with a recent origin and range expansion of parth enogens. Prevalence of infection by digenetic trematodes is significantly h igher in autodiploid parthenogens, and the variance of prevalence is also h igher in autodiploid parthenogens. I argue that the latter pattern indicate s that unparasitized parthenogens have temporarily escaped these virulent p arasites, but recolonization of these populations by trematodes results in high infection levels (> 40%), possibly due to reduced variation in resista nce genes. I also examined whether the survivorship of juvenile sexuals and parthenogens varied under different stress levels. Sexual juveniles had tw ofold higher survivorship in all environments. Compared to polyploid parthe nogens, autodiploid parthenogens may be less buffered against the effects o f deleterious recessive alleles. I propose that the combined effects of hig her parasitism and reduced juvenile survivorship of these autodiploid parth enogens accounts for the spatial distribution of sexual and parthenogenetic C. limum in the Atlantic coastal plain. Parthenogens may persist by higher dispersal rates into marginal habitats where there is a temporary escape f rom digenetic trematodes and competition with sexuals.