Whether it is the brightly colored feathers of the male peacock or the horns of male dung beetles, scientists have been intrigued by sexually dimorphic traits. However, the molecular mechanisms responsible for the origin of these sex-specific traits are not well known. Comparative studies investigating the genetic basis of convergent, male-limited phenotypes, will lead to a better understanding of the constraints in the genes and pathways used in the generation of new traits.
We have studied two novel but phenotypically similar traits that arose independently: the sex combs bristles found in forelegs of D. melanogaster, and the brush bristles found in the forelegs of the D. immigrans species group. The sex combs and foreleg brush are both modifications of the transverse bristle rows of the first tarsal segment in the forelegs and are male-specific . In Drosophila, the transcription factor doublesex establishes sex-specific transcription in males and females, but is expressed in only a subset of cells. Therefore, for a new cell-type to become sexually dimorphic or for a trait to expand beyond the current set of doublesex expressing cells, expression of doublesex must be modified.
We have previously shown that changes in the expression of doublesex explains a large portion of phenotypic divergence in sex combs. In parallel, we have found that the gene doublesex shows a similar pattern in the origin and diversification of the foreleg brush. Species that have secondarily lost the foreleg brush have lost doublesex expression, while species that have reduced brush sizes also have corresponding reduced doublesex expression. These results indicate that changes to the regulation of doublesex may be vital in the origin and secondary modification of sexually dimorphic traits.