Among- and within-population variation in outcrossing rate of a mixed-mating freshwater snail

Mixed-mating animals self-fertilize a proportion of their offspring. Outcrossing rate may covary with the ecological and historical factors affecting the population. Theory predicts that outcrossing is favored when inbreeding depression is high and when individual heterozygosity is important. Self-fertilization is predicted to be favored when costs of male function, or mate finding are high, for example, when empty patches are colonized by few individuals. In this study, we assessed primary (after hatching) and secondary (after juvenile mortality) outcrossing rates of two mixed-mating snail populations. Our purpose was to assess the variation in mating-system parameters and estimate significance of inbreeding depression for secondary outcrossing rate (the realized outcrossing rate of parents that produce the next generation). Secondary outcrossing rate was higher than the primary outcrossing rate in one of the two populations, suggesting considerable inbreeding depression. In the other study population, secondary outcrossing rates were found to increase when initially low, or decrease when initially high, depending on the family. Moderate outcrossing rates were found to be more stable. Parental inbreeding coefficients were close to zero in both populations. Outcrossing rate was much more variable among families in the population with the lower average outcrossing rate, suggesting that individuals differed considerably in their mating system. Our results add to recent studies suggesting that populations of mixed-mating animals may differ in their mating system parameters and expression of inbreeding depression.