Mapping of dextral : sinistral proportions in the chirally dimorphic land snail Amphidromus inversus

One of the very few snail taxa that display genetic antisymmetry (that is, roughly equal mixes of genetically determined clockwise [D] and anticlockwise [S] coiled individuals within a single population) are the circa 35 species of the tropical tree snail subgenus Amphidromus . Previous work has shown that this may be due to a particular type of sexual selection, in which sperm transfer is improved in copulations between the two mirror-image morphs. However, it is not yet clear why so often significant deviations from 50:50 proportions are found. Modelling studies show that population structure will affect the degree by which the dimorphism is skewed towards the morph associated with the recessive allele. In this study, we mapped the proportions of sinistrals (PropS) in 56 demes in A. inversus on the Malaysian island of Kapas. We also mapped population density, predation rates, and several measures of vegetation structure. Our results show that PropS amounts on average to 0.65, but across the island it varies from 0.30 to 0.85. Density and overall predation are inversely correlated. A general linear model selection procedure results in the proportion of sinistrals to be positively correlated with density and predation on dextrals. We find no overwhelming evidence for a role for drift in explaining the deviations from equal S:D proportions, but we do argue that further study of crab-snail interactions may be warranted.