Population structure and coil dimorphism in a tropical land snail

Abstract

Tree snails of the subgenus Amphidromus s. str. are unusual because of the chiral dimorphism that exists in many species, with clockwise (dextrally) and counter-clockwise (sinistrally) coiled individuals co-occurring in the same population. Given that mating in snails is normally impeded when the two partners have opposite coil, positive frequencydependent selection should prevent such dimorphism from persisting. We test the hypothesis that a strong population structure with little movement between tree-based demes may result in the fixation of coiling morphs at a very small spatial scale, but apparent dimorphism at all larger scales. To do so, we describe the spatial structure in a Malaysian population of A. inversus (Mü ller, 1774) with 36% dextrals.We marked almost 700 juvenile and adult snails in a piece of forest consisting of 92 separate trees, and recorded dispersal and the proportions of dextrals and sinistrals in all trees over a 7-day period. We observed frequent movement between trees (155 events), and found that no trees had snail populations with proportions of dextrals and sinistrals that were significantly different from random. Upon recapture 1 year later, almost two-thirds of the snails had moved away from their original tree. We conclude that population structure alone cannot stabilise the coil dimorphism in Amphidromus. Heredity (2005) 95, 216-220.

The fate of balanced, phenotypic polymorphisms in fragmented metapopulations

“…Frequency‐dependent sexual selection can maintain coil dimorphism (Schilthuizen et al. , 2007) but in the field populations often show large, consistent bias in favour of one morph (Schilthuizen et al. , 2005; Sutcharit et al.…”

Genetic support for random mating between left and right‐mouth morphs in the dimorphic scale‐eating cichlid fish Perissodus microlepis from Lake Tanganyika

“…phenotypically plastic), except for the one case of direction of style bend in enantiostylous flowers of one Heteranthera species (Jesson & Barrett, 2002). Another exception to this has since been reported for the direction of shell coiling in the south‐east Asian camaenid tree snail subgenus Amphidromus sensu stricto (Schilthuizen et al , 2005). By contrast, in every case of directional asymmetry, the direction of asymmetry is heritable, mostly via Mendelian inheritance (Palmer, 2004).…”

Speciation and Gene Flow between Snails of Opposite Chirality

“…One approach has been to study the exceptions, species that are chirally dimorphic, but even in these most populations are fixed for a particular type [11–14] (with the exception of Amphidromus [15]). This is because positive frequency-dependent selection tends to drive the chiral majority to fixation; rarer snails of opposite chirality are less likely to find a mate and successfully copulate [13].…”