Serotonergic neuronal death and concomitant serotonin deficiency curb copulation ability of Drosophila platonic mutants

Drosophila platonic (plt) males court females, but fail to copulate. Here we show that plt is an allele of scribbler (sbb), a BMP signalling component. sbb knockdown in larvae leads to the loss of approximately eight serotonergic neurons, which express the sex-determinant protein Doublesex (Dsx). Genetic deprivation of serotonin (5-HT) from dsx-expressing neurons results in copulation defects. Thus, sbb⁺ is developmentally required for the survival of a specific subset of dsx-expressing neurons, which support the normal execution of copulation in adults by providing 5-HT. Our study highlights the conserved involvement of serotonergic neurons in the control of copulatory mechanisms and the key role of BMP signalling in the formation of a sex-specific circuitry.