Molecular genetics of satori, a Drosophila mutant with altered sexual orientation

satori (sat) is a Drosophila mutant in which male sexual orientation changes from heterosexual to homosexual. Another phenotype of the satori mutant is the absence of the male-specific muscle of Lawrence (MOL). The formation of MOL is repressed in females by the action of a female- determinant protein, Transformer (Tra), while a Tra-target doublesex (dsx) has no role in the MOL formation. Molecular cloning of the fruitless (fru) gene responsible for the sat mutation revealed that it encodes a putative transcription factor with a BTB domain and Zn finger motifs, and that the second exon of this gene contains three repeats of the Tra-binding consensus sequence. These observations suggest that fru is a novel target of Tra in the sex determination cascade of Drosophila. MOL formation is known to occur when the sex of the innervating motoneuron is male regardless of the sex of the MOL itself. Taking into account the fact that Fru is preferentially expressed in the nervous system, Fru is likely involved in neuronal sex determination. We hypothesize that the sat mutation feminizes a class of neurons which otherwise promote heterosexual courtship and the formation of the MOL in males.