Continuous variation in the shell colour of the snail Cepaea nemoralis is associated with the colour locus of the supergene.

While the shell of the land snail Cepaea nemoralis is typically classed as yellow, pink, or brown, the reality is that colour variation is continuously distributed. To further understand the origin of the continuous variation, we used crosses of C. nemoralis to compare quantitative measures of the colour with the inferred genotype of the underlying supergene locus. We also used a recently developed linkage map to find quantitative trait loci that may influence colour. The results show that the colour locus of the supergene-at around 31.385 cM on linkage group 11-is involved in determining the quantitative chromatic differences that are perceptible to human vision. We also found some evidence that variation within colour classes may be due to allelic variation at or around the supergene. There are likely other unlinked loci involved in determining colour within classes, but confirmation will require greater statistical power. Although not investigated here, environmental factors, including diet, may also impact upon variation within colour types.