1. Sexual organisms should have half the reproductive rate of their parthenogenetic counterparts (i.e. twofold cost of sex), so the plethora of sexual species relative to parthenogenetic species remains an evolutionary paradox. The rarity of parthenogenesis may in part be due to the accumulation of deleterious mutations. Indeed, parthenogenetic populations of the freshwater snail Campeloma limum have a greater mutation load relative to sexual populations of C. limum, although this does not directly affect their reproductive fitness. We hypothesise that although parthenogenesis has no direct effect on fitness in C. limum, mutation accumulation and environmental stress act synergistically to limit the distribution of parthenogenetic populations.
2. We evaluated this hypothesis, predicting that parthenogenetic populations of C. limum would inhabit sites with fewer environmental stressors than their sexual counterparts. We collected water quality, population density and individual size data at multiple time points from eight parthenogenetic and five sexual populations in the south-eastern United States (Georgia and South Carolina).
3. Consistent with our hypothesis, sexual populations of C. limum inhabited poorer-quality areas (sites with significantly lower dissolved oxygen and significantly more faecal coliform bacteria) than parthenogenetic populations. Despite these stressors, sexual populations still exhibited significantly higher population density than parthenogenetic populations.
4. Our findings support the hypothesis that mutation-laden parthenogenetic C. limum populations occupy habitats with fewer environmental stressors relative to their sexual counterparts. Moreover, sexual C. limum populations inhabit lower-quality habitats where they can presumably evade the twofold cost of sex in the absence of competition from their parthenogenetic counterparts.