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.