Penetrance interactions of colour pattern loci in the African Monarch and their implications for the evolution of dominance.

Scoring the penetrance of heterozygotes in complex phenotypes, like colour pattern, is difficult and complicates the analysis of systems in which dominance is incomplete or evolving. The African Monarch () represents an example where colour pattern heterozygotes, formed in the contact zone between the different subspecies, show such intermediate dominance. Colour pattern in this aposematic butterfly is controlled by three loci , and . The and loci are closely linked in a / supergene and significant interaction of and phenotypes is therefore expected via linkage alone. The locus, however, is not linked to / and is found on a different chromosome. To study interactions between these loci we generated colour pattern heterozygotes by crossing males and females bearing different and / genotypes, collected from different parts of Africa. We derived a novel scoring system for the expressivity of the heterozygotes and, as predicted, we found significant interactions between the genotypes of the closely linked and loci. Surprisingly, however, we also found highly significant interactions between and the unlinked locus, modifications that generally increased the resemblance of heterozygotes to homozygous ancestors. In contrast, we found no difference in the penetrance of any of the corresponding heterozygotes from crosses conducted either in allopatry or sympatry, in reciprocal crosses of males and females, or in the presence or absence of endosymbiont mediated male-killing or its associated mediated sex-linkage of colour pattern. Together, this data supports the idea that the different colour morphs of the African Monarch meet transiently in the East African contact zone and that genetic modifiers act to mask inappropriate expression of colour patterns in the incorrect environments.