Sexual selection maintains whole‐body chiral dimorphism in snails

Abstract

Although the vast majority of higher animals are fixed for one chiral morph or another, the cause for this directionality is known in only a few cases. In snails, for example, rare individuals of the opposite coil are unable to mate with individuals of normal coil, so directionality is maintained by frequency-dependent selection. The snail subgenus Amphidromus presents an unexplained exception, because dextral (D) and sinistral (S) individuals occur sympatrically in roughly equal proportions (so-called ‘antisymmetry’) in most species. Here we show that in Amphidromus there is sexual selection for dimorphism, rather than selection for monomorphism. We found that matings between D and S individuals occur more frequently than expected by chance. Anatomical investigations showed that the chirality of the spermatophore and the female reproductive tract probably allow a greater fecundity in such inter-chiral matings. Computer simulation confirms that under these circumstances, sustained dimorphism is the expected outcome.

The development and evolution of left‐right asymmetry in invertebrates: Lessons from Drosophila and snails

“…7; Gittenberger, 1988;Orr, 1991;Ueshima and Asami, 2003). Asami et al (1998) showed that the physical difficulty of interchiral mating differs significantly between the two discrete mating behaviors associated with the bimodal distribution of shell shape in shell-bearing terrestrial pulmonates, although exceptional cases have raised new questions (Sutcharit et al, 2007;Schilthuizen et al, 2007;Davison and Mordan, 2007). Flat-shelled to globular species mate reciprocally face to face (Fig.…”

“…The LR polarity of Amphidromus, in which populations of many species are enantiomorphic, is also probably determined by the dextral and sinistral alleles as well (Sutcharit et al, 2007;Schilthuizen et al, 2007). In other animals, however, most cases in which a handedness gene is mutated result in randomized LR polarity , although one exception is the mouse inv mutant (Yokoyama et al, 1993).…”

“…Fluctuating asymmetry (van Valen, 1962;Polak, 2003) refers to random individual variation in the amount of difference between the right and left sides of a trait that is symmetric in the population mean. Enantiomorphy (Palmer, 2005;Sutcharit et al, 2007;Schilthuizen et al, 2007;Schilthuizen, 2007) is chiral dimorphism in which the dextral and sinistral morphs coexist in a population. Directional asymmetry is chiral monomorphism in which a population is fixed for either enantiomorph.…”

Evidence of disassortative mating in a Tanganyikan cichlid fish and its role in the maintenance of intrapopulation dimorphism

“…Experimental evidence of disassortative mating has been reported in mice (Isles et al 2001;Carroll et al 2002) and humans (Wedekind & Fü ri 1997;Santos et al 2004), and in both cases, females preferred males with odours that differed from their own. However, only a few empirical examples have been reported in the wild (Roulin 2004;Schilthuizen et al 2007).…”

The effect of different chiral morphs on visitation rates and fruit set in the orchid Spiranthes spiralis

“…For instance, in snails, rare individuals with anti-clockwise shell coiling are unable to mate with individuals with clockwise shell coiling, so directionality is maintained by frequency-dependent sexual selection (Schilthuizen et al 2007). In our case study, chirality does not represent an isolating barrier as pollinators move pollen (as estimated by pollen-analogue fluorescent dyes) between the two morphs, and hand pollinations confirmed that the two morphs are fully compatible.…”