Several species of simultaneously hermaphroditic land snails show a genital dimorphism: aphallic individuals differ from euphallic ones by a lack of male copulatory organs (penis plus genital retractor muscle). Aphallic individuals can self-fertilize or outcross as females but not as males. Thus, the mating system of a population may be significantly influenced by the proportion of aphallic individuals.
We present data on the frequency of aphally in 23 natural populations of the rock-dwelling land snail Chondrina clienta on the Baltic island of Öland, Sweden. The populations varied greatly in percentage of aphallic individuals, ranging from 52.2 to 99.1%, (grand mean 77.7%)). This variation did not follow any geographical pattern.
In a laboratory experiment, we examined whether food supply (high or low) and/or population density (high or low) experienced during ontogeny affected the expression of genital dimorphism. Snails derived from a population with 99.1% aphallic individuals and raised under different food and density conditions did not differ from the original population in frequency of aphally. By contrast, when snails from a population with 66.7% aphallic individuals were raised on a low food supply, more individuals became euphallic than expected under complete genetic determination. These results suggest that, in addition to a genetic component. the expression of the genital dimorphism can be influenced by environmental conditions.