One of the very few snail taxa that display genetic antisymmetry (that is, roughly even mixes of genetically determined clockwise and anticlockwise 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 varies from 0.30 to 0.85. Density and overall predation are inversely correlated, but neither predicts PropS. Vegetation parameters also do not correlate with the proportion of sinistrals. We do, however, find a negative correlation between the predation rate on sinistrals and PropS, a finding which may warrant further study.
Self-Fertilization and Genetic Population Structure in a Colonizing Land Snail