Coil and shape in Partula suturalis: the rules of form revisited

Early research on the direction of coiling (chirality) in snails had a prominent role in the history of biology, and continues to mold current thinking about developmental evolution. Here, we investigated the pleiotropic effects of chirality alleles on shell shape and length, using a large set of data from the Polynesian land snail Partula suturalis. By examining the shells of individuals with different combinations of their own and their mother's chiral genotypes, we find that the effects of chiral genes on length and shape involve both the maternal genotype and the genotype of the individual itself. Thus, differences in shape are symptoms of developmental shifts caused by the chirality alleles, or by alleles at other loci in close linkage disequilibrium with them. Although changes in shell shape are not necessary concomitants of chiral evolution, the results illustrate the wider pleiotropic effects of the genes, or haplotypes, that influence chirality. As the data are a rare record of the association in nature between the phenotype and genotype of a locus showing maternal effects, the conclusions are likely to be relevant to the general understanding of the dynamics of maternal-effect genes, including how selection acts on them.