Formation of the male-specific muscle in female Drosophila by ectopic fruitless expression

The Drosophila fruitless (fru) gene product Fru has been postulated to be a neural sex-determination factor that directs the development of at least t wo male-specific characteristics, namely courtship behaviour and formation of the muscle of Lawrence (MOL). The fru gene encodes a putative transcript ion factor with a BTB domain and two zinc-finger motifs, and with consensus Tra-binding sequences. The binding of Tra to these sequences results in se x-specific alternative splicing of the fru mRNA, leading to production of t he 'male-type' or 'female-type' Fru protein. We show here that the Fru prot ein is not detected in the female central nervous system (CNS), despite the similar level of expression of fru mRNA in both male and female CNS. As ec topic expression of both the 'male-type' (with the sequence for the aminote rminal extension) and 'female-type' (without the sequence for the amino-ter minal extension) fru cDNA can induce formation of the MOL in females, the p resence or absence of the Fru protein, and not its sex-specific structure, seems to be responsible for the sexually dimorphic actions of the fru gene.