Forma, Vol. 8 (No. 2), pp. 263-276, 1993

Genetic Variation and Evolution of Coiling Chirality in Snails

Takahiro Asami

Division of Biology, Tachikawa College of Tokyo, Azuma-cho, Akishima-shi, Tokyo 196, Japan

(Received July 10, 1993; Accepted July 15, 1993)

Keywords: asymmetry, helix, Body Shape, behavior, Gastropoda

Abstract. This paper reviews recent studies on the genetics, ecology and evolution of the direction of coiling in gastropods and discusses five questions on 1) genetic dominance in chirality, 2) bias for dextrality, 3) ineffectiveness of chirality for genetic subdivision of species, 4) biased distribution of chiral polymorphism to high-spired snails, and 5) association of high-spired shell shape and mating by shell mounting. A hypothesis is proposed to explain why the chiral polymorphism is more frequently found in high-spired snails than in discoidal or globular ones. High-spired and discoidal pulmonates perform contrasting patterns of mating behavior. Because of their behavioral patterns, discoidal snails can hardly accomplish inter-chiral mating, whereas high-spired snails succeed occasionally. Frequency-dependent selection against the minor morph resulting from its mating disadvantage is, therefore, relatively relaxed in high-spired snails, compared with discoidal ones. Thus, chiral polymorphism persists longer in high-spired snails, providing more chances for evolution of chiral differences than in discoidal or globose snails.