EVOLUTION OF MIRROR IMAGES BY SEXUALLY ASYMMETRIC MATING-BEHAVIOR IN HERMAPHRODITIC SNAILS

Directionally asymmetric animals generally exhibit no variation in han dedness of whole-body architecture. In contrast, reversed chirality in both coil and entire anatomy has frequently evolved in snails. We dem onstrate a nonrandom pattern and deterministic process of chiral evolu tion, as predicted by the following hypothesis. Bimodal shell shapes a re associated with discrete mating behaviors in hermaphroditic pulmona tes. Flat-shelled species mate reciprocally, face-to-face. This sexual symmetry prevents interchiral mating because genitalia exposed by a s inistral on its left side cannot be joined with those exposed by a dex tral on its right. Thus, selection against the chiral minority, result ing from mating disadvantage, stabilizes chiral monomorphism. Tall-she lled species mate nonreciprocally the ''male'' copulates by mounting t he ''female's'' shell, mutually aligned in the same direction. This se xual asymmetry permits interchiral copulation with small behavioral ad justments. Therefore, the positive frequency-dependent selection is re laxed, and reversal alleles persist longer in populations of tall-shel led species. We verified both the assumption and the prediction of thi s 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 ev olution in high-spired pulmonates.