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.

High heteroplasmy is associated with low mitochondrial copy number and selection against non-synonymous mutations in the snail Cepaea nemoralis.

Molluscan mitochondrial genomes are unusual because they show wide variation in size, radical genome rearrangements and frequently show high variation (> 10%) within species. As progress in understanding this variation has been limited, we used whole genome sequencing of a six-generation matriline of the terrestrial snail Cepaea nemoralis, as well as whole genome sequences from wild-collected C. nemoralis, the sister species C. hortensis, and multiple other snail species to explore the origins of mitochondrial DNA (mtDNA) variation. The main finding is that a high rate of SNP heteroplasmy in somatic tissue was negatively correlated with mtDNA copy number in both Cepaea species. In individuals with under ten mtDNA copies per nuclear genome, more than 10% of all positions were heteroplasmic, with evidence for transmission of this heteroplasmy through the germline. Further analyses showed evidence for purifying selection acting on non-synonymous mutations, even at low frequency of the rare allele, especially in cytochrome oxidase subunit 1 and cytochrome b. The mtDNA of some individuals of Cepaea nemoralis contained a length heteroplasmy, including up to 12 direct repeat copies of tRNA-Val, with 24 copies in another snail, Candidula rugosiuscula, and repeats of tRNA-Thr in C. hortensis. These repeats likely arise due to error prone replication but are not correlated with mitochondrial copy number in C. nemoralis. Overall, the findings provide key insights into mechanisms of replication, mutation and evolution in molluscan mtDNA, and so will inform wider studies on the biology and evolution of mtDNA across animal phyla.