Spatial and temporal patterns of parthenogenesis and parasitism in the freshwater snail Melanoides tuberculata.

The Red Queen hypothesis predicts that sex should be more common in populations heavily infested with parasites, than in those without. This hypothesis was investigated in the aquatic snail Melanoides tuberculata, in which both sexual and parthenogenetic individuals exist in natural populations, and some populations are heavily infested by trematodes. The presence of fertile males and the higher genetic diversity of bisexual populations are indicative of sexual reproduction. We compared sites in 1990, 1999, and 2001, and we looked for a positive correlation between male and parasite frequencies. Male frequency was not correlated with the frequency of individuals infected by trematodes. This lack of correlation was reconfirmed in a retrospective power analysis. In a period of 9 years, male frequencies decreased but infection levels increased. These results do not support the Red Queen hypothesis. In samples with high male frequency the number of embryos was low, perhaps indicating that males may have a negative effect on embryo numbers. This effect of males on fitness could perhaps suggest that the cost of sex is fewer embryos. The reduction in embryo numbers may also represent a trade-off between mating and egg production costs.