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 bristles
present on the tarsal segments of the prothoracic leg (T1) in Sophophoran
species of Drosophila (fig 1.10a). It is absent from the rest of the species of
Drosophilidae. 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 from
the chaetotaxy of the prothoracic leg of males of the non-Sophophoran group
(Kopp and True, 2002; Barmina and Kopp, 2007). As previously mentioned
there exist a dramatic variation in the number, position and morphology of sex
comb in different species of Sophophora enabling sex comb to enjoy a diverse
evolutionary 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 been
driven by sexual selection. The sex comb develops from a group of bristles
called the transverse bristle row (TBR) which is present on the T1 of both the
males and females of Drosophilidea (Hannah-Alavah, 1958; Tokunaga, 1962).
The TBR which are recruited to develop into sex comb undergo two important
events of modifications i) rotation ii) chitinization (Held et al., 2004; Atallah et
al., 2009; Tanaka et al., 2009). The transformation of the primitive bristle to the
newly evolved sex comb should have been resulted from a genetic shift in the
developmental circuit regulating the formation of the primitive bristles on the
tarsal segments of the prothoracic leg.