Ease of fixation of a change in coiling: computer experiments on chirality in snails

Abstract

This paper deals with chirality of snails. It explores the ease of fixation of a change in coiling direction, caused by an invasion of snails carrying a mutant chirality allele into a normal, homogeneous population, by using Monte-Carlo simulation. Additionally, single-gene speciation on the basis of chirality is discussed. Six factors are studied in particular, namely: (i) the absolute and relative population size; (ii) the mating success, which is known to be related to shell shape, especially the height/width ratio; (iii) the maternal effect, underlying chirality; (iv) the (low) mobility of the snails; (v) fitness differences (heterosis); and (vi) the invading mutant allele being either dominant or recessive. The impact of these factors is quantified. Small populations with not too few invaders and dominance of the mutant chirality allele are of paramount relevance for its occasional fixation. In comparison to this, the maternal effect turns out to be considerably less significant, whereas the mobility of the snails plays only a minor role. The simulations confirm the expectation that heterosis can be very effective. Logically, the mating success is also a very important factor. This reflects the observation in nature that among snails with slender shells, when mating between mirror-image individuals is still possible to a certain extent, reverse populations have originated far more frequently than among snails with globular shells, where such matings are impossible. Only the latter cases, which are very rare in nature, may concern single-gene speciation.

Population structure and coil dimorphism in a tropical land snail

“…This is due to the fact that during mating between individuals with opposite coil, successful contact between the genital openings (located on the right-hand side of the body in a dextral, on the left-hand side of the body in a sinistral snail) is impeded: hindrance is particularly severe in snails with globular shells, but also, to a lesser extent, in high-spired ones (Asami et al, 1998). Computer simulations and field observations confirm that only in rare cases does a sinistral population become established within a dextral species or vice versa (Johnson et al, 1990;Orr, 1991;van Batenburg and Gittenberger, 1996;Stone and Bjö rklund, 2002;Ueshima and Asami, 2003).…”

“…Studies on phylogenetically unrelated species (Degner, 1952;Murray and Clarke, 1976;Freeman and Lundelius, 1982) suggest that coiling direction is normally determined by the maternal genotype at a single locus, with genetic dominance of either the dextral or the sinistral allele (van Batenburg and Gittenberger, 1996). Most pulmonate species, however, are uniform in coiling, with reverse-coiled individuals only appearing as rare mutants (Gittenberger, 1988).…”

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

“…7). Because of the role of the maternal effect, however, the dominant allele for LR polarity could be fixed more easily than the recessive allele in small populations isolated from each other (Asami, 1993;van Batenburg and Gittenberger, 1996). The recessive allele is also unlikely to prevail in populations in which migration and interchiral mating continue between sinistral and dextral demes (Johnson et al, 1990;Davison et al, 2005).…”

“…Because the maternal genotype determines the offspring phenotype, each mother should only produce either dextral or sinistral offspring, if alleles determine dextrality and sinistrality unlike LRdepolarizing genes known in vertebrates . Simulation studies with mathematical models have shown that the maternal effect enhances the probability of chiral fixation in small, isolated populations (Orr, 1991;van Batenburg and Gittenberger, 1996;Stone and Björk-lund, 2002), which may lead to chiral speciation by the establishment of premating isolation from other populations. As long as the dextral and sinistral forms coexist, however, they keep sharing the same gene pool even if they cannot mate with each other at all, because of chirality inheritance by the maternal effect (Johnson et al, 1990;Davison et al, 2005).…”

Coil and shape in Partula suturalis: the rules of form revisited

“…In snails, there have been some models and simulations of the chirality-determining locus (Orr, 1991;van Batenburg and Gittenberger, 1996;Stone and Bjö rklund, 2002;Davison et al, 2005), but there is a scarcity of empirical data. This is probably because dimorphic populations of snails are rare, but also because it is laborious to infer genotype from juvenile phenotypes when all the eggs that hatch from a female produce only a single data point.…”