Evolution of mirror images by sexually asymmetric mating behavior in hermaphroditic snails

Directionally asymmetric animals generally exhibit no variation in handedness of whole-body architecture. In contrast, reversed chirality in both coil and entire anatomy has frequently evolved in snails. We demonstrate a nonrandom pattern and deterministic process of chiral evolution, as predicted by the following hypothesis. Bimodal shell shapes are associated with discrete mating behaviors in hermaphroditic pulmonates. Flat-shelled species mate reciprocally, face-to-face. This sexual symmetry prevents interchiral mating because genitalia exposed by a sinistral on its left side cannot be joined with those exposed by a dextral on its right. Thus, selection against the chiral minority, resulting from mating disadvantage, stabilizes chiral monomorphism. Tall-shelled species mate nonreciprocally the "male" copulates by mounting the "female's" shell, mutually aligned in the same direction. This sexual asymmetry permits interchiral copulation with small behavioral adjustments. Therefore, the positive frequency-dependent selection is relaxed, and reversal alleles persist longer in populations of tall-shelled species. We verified both the assumption and the prediction of this hypothesis: significantly lower interchiral mating success in a low-spired species and higher chiral evolution rate ill high-spired taxa. Sexual asymmetry is the key to understanding the accelerated chiral evolution in high-spired pulmonates.