Evolution of sex comb from the primitive bristle pattern in Drosophila is associated with modification in the...

Sex comb is a recently evolved male specific character confined to the Sophophoran group of Drosophila. Such innovations in phenotypes as Waddington proposed, are, outcome of "canalization" in developmental pathways that occur due to mutations creating "choice points" in genetic regulatory pathways. Our interest in the present study is to understand the shifts in genetic network, which has lead to the origin of sex comb from the basic bristle pattern that is seen in rest of the members of Drosophilidae. Here we have made a comparative analysis of expression of some of the key regulators of sex comb morphogenesis, between D. melanogaster and a group of selected species, which primitively lack sex comb. Sex combs reduced (Scr), dachshund (dac), and bric-a-brac (bab) gene expression were studied. We show that, primitive bristle pattern is marked by a strikingly down regulated expression of Sex combs reduced in the first tarsal segment of the prothoracic leg discs of male flies. Further a remarkable change with respect to Dachshund, an activator of sex combs reduced gene in the sex comb regulatory pathway, is seen. This is attributed to changes in DAC protein that might have taken place between the two groups of species. bric-a-brac does not reveal any significant expression modulation between the sex comb bearing and the primitive patterned species. Earlier works had shown that within the Sophophoran group, dynamic changes in SCR expression is responsible for the diversity seen in sex comb morphology, where as no such variation is witnessed with respect to DAC expression. Our findings have demonstrated that the scenario is different between the group primitively lacking sex comb and D. melanogaster wherein an obvious change in the protein has taken place. genesis 51:97-109, 2013, 2013. © 2012 Wiley Periodicals, Inc.