The genetic basis of sexual dimorphism in birds.

The genetic basis of sexual dimorphisms is an intriguing problem of evolutionary genetics because dimorphic traits are limited to one sex. Such traits can arise genetically in two ways. First, the alleles that cause dimorphisms could be limited in expression to only one sex at their first appearance. Alternatively, dimorphism alleles could initially be expressed in both sexes, but subsequently be repressed or promoted in only one sex by the evolution of modifier genes or regulatory elements. We investigated these alternatives by looking for the expression of sexually dimorphic traits in female hybrids between bird species whose males show different types of ornaments. If modifier alleles or regulatory elements involved in sex-limited traits are not completely dominant, the modification should break down in female hybrids, which might then show dimorphic traits resembling those seen in males. Of 13 interspecific hybridizations examined, we found not a single instance of the expression of male-limited ornaments in female hybrids. This suggests that male ornaments were sex limited from the outset or that those traits became sex limited through the evolution of dominant modifiers -- possibly cis-dominant regulatory elements. Observing hybrid phenotypes is a useful approach to studying the genetics and evolution of dimorphic traits.