Maintenance mechanism of a supergene for shell colour polymorphism in the terrestrial pulmonate Bradybaena similaris

The land snail Bradybaena similaris exhibits polymorphism for the ground colour of periostracum and the banding pattern of ostracum and periostracum. The ground colour is either dark reddish brown or pale light brown. In addition, a single chestnut-color band is either present or absent along the ridge of the whorl. Thus, there are four morphs in combination: dark and banded, dark and unbanded, light and banded, and light and unbanded. Earlier breeding experiments have shown that the phenotypes of ground colour and banding are controlled by two linked loci, where the dark (C+) and banded (B+) alleles are dominant to the light (C-) and unbanded (B-) alleles. Thus, four different cis configurations of alleles (C+B+, C+B-, C-B+ and CB-) could be expected. However, the C+B+ chromosome has not been found, i.e. all the examined specimens of the dark/banded morph were trans heterozygotes (C+B-/C-B+). This linkage disequilibrium may result from tight linkage, lethality of the dark/banded haplotype or recombination suppression. We found no recombinant phenotype in 3287 progeny obtained by the test cross of C+B-/C-B+ with C-B-/C-B- in 16 pairs. Our results suggest that recombination between the colour and banding loci is suppressed in effect.