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
(Danaus chrysippus) represents an example where colour pattern
heterozygotes, formed in the contact zone between the different ...
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
(Danaus chrysippus) 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 A, B and C. The B and C
loci are closely linked in a B/C supergene and significant interaction of B
and C phenotypes is therefore expected via linkage alone. The A locus,
however, is not linked to B/C and is found on a different chromosome.
To study interactions between the loci we generated colour pattern
heterozygotes by crossing males and females bearing different A and
B/C 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 B and C loci. Surprisingly, however, we also found
highly significant interactions between C and the unlinked A 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 neoW 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.