Many animals exhibit innate sexually dimorphic sexual behaviours. Male flies, for example, follow elaborate courtship rituals to secure a female mate. Nervous system sexual dimorphisms probably underlie many sex-specific behaviours, but how are these neuronal dimorphisms generated? On p. , David Mellert and colleagues report that one such dimorphism – midline crossing by gustatory (taste) receptor neuron (GRN) axons in male, but not female, Drosophila – is regulated by the sex determination genes fruitless (fru) and doublesex (dsx) and the Roundabout (Robo) axon guidance receptors. fru and dsx are regulated during pre-RNA splicing to produce male- and female-specific isoforms, with female-specific fru mRNA not being translated. The researchers show that, in male flies, male-specific Fru and Dsx promote midline crossing by foreleg GRN axons, whereas in female flies, female-specific Dsx represses midline crossing. They also show that male-specific Fru exerts its effect on midline crossing by directly or indirectly regulating Robo signalling. Together, these results begin to reveal how the neuronal connections that underpin Drosophila sex-specific behaviour are established.
