A Genetic Tale Tracing the Origin and Evolution of Drosophila's Sex Comb

Sex comb is a sexually dimorphic trait-a group of male specific bristlespresent on the tarsal segments of the prothoracic leg (T1) in Sophophoranspecies of Drosophila (fig 1.10a). It is absent from the rest of the species ofDrosophilidae. Thus the sex comb is an evolutionary innovative trait(Lakovaara and Soura, 1982; Lemeunier et al., 1986; Kopp and True, 2002).The chaetotaxy of the prothoracic leg of females do not show variation fromthe chaetotaxy of the prothoracic leg of males of the non-Sophophoran group(Kopp and True, 2002; Barmina and Kopp, 2007). As previously mentionedthere exist a dramatic variation in the number, position and morphology of sexcomb in different species of Sophophora enabling sex comb to enjoy a diverseevolutionary phylogenetic position (Kopp and True, 2002; Kopp and Barmina,2005). Sex comb has got diverse roles in courtship and mating (Spieth, 1952;Cook, 1977; Ng and Kopp, 2008), indicating that their evolution has beendriven by sexual selection. The sex comb develops from a group of bristlescalled the transverse bristle row (TBR) which is present on the T1 of both themales and females of Drosophilidea (Hannah-Alavah, 1958; Tokunaga, 1962).The TBR which are recruited to develop into sex comb undergo two importantevents of modifications i) rotation ii) chitinization (Held et al., 2004; Atallah etal., 2009; Tanaka et al., 2009). The transformation of the primitive bristle to thenewly evolved sex comb should have been resulted from a genetic shift in thedevelopmental circuit regulating the formation of the primitive bristles on thetarsal segments of the prothoracic leg.