Résumé
Understanding the mechanisms responsible for phenotypic diversification
within and among species ultimately rests with linking naturally occurring
mutations to functionally and ecologically significant traits. Colour
polymorphisms are of great interest in this context because discrete
colour patterns within a population are often controlled by just a few
genes in a common environment. We investigated how and why phenotypic
diversity arose and persists in the Zosterops borbonicus white-eye of
Reunion (Mascarene archipelago), a colour polymorphic songbird in which
all highland populations contain individuals belonging to either a brown
or a grey plumage morph. Using extensive phenotypic and genomic data, we
demonstrate that this melanin-based colour polymorphism is controlled by a
single locus on chromosome 1 with two large-effect alleles, which was not
previously described as affecting hair or feather colour. Differences
between colour morphs appear to rely upon complex cis-regulatory variation
that either prevents the synthesis of pheomelanin in grey feathers, or
increases its production in brown ones. We used coalescent analyses to
show that, from a ‘brown’ ancestral population, the dominant ‘grey’ allele
spread quickly once it arose from a new mutation. Since colour morphs are
always found in mixture, this implies that the selected allele does not go
to fixation, but instead reaches an intermediate frequency, as would be
expected under balancing selection.