female-specific Ilp7 motoneurons are generated by Fruitless-dependent cell death in males and by a double-assurance survival role for Transformer in females.

Female-specific Ilp7 neuropeptide-expressing motoneurons (FS-Ilp7 motoneurons) are required in for oviduct function in egg laying. Here， we uncover cellular and genetic mechanisms underlying their female-specific generation. We demonstrate that programmed cell death (PCD) eliminates FS-Ilp7 motoneurons in males， and that this requires male-specific splicing of the sex-determination gene () into the Fru isoform. However， in females， alleles that only generate Fru isoforms failed to kill FS-Ilp7 motoneurons. This blockade of Fru-dependent PCD was not attributable to gene function but to a non-canonical role for ()， a gene encoding the RNA splicing activator that regulates female-specific splicing of and transcripts. In both sexes， we show that Tra prevents PCD even when the Fru isoform is expressed. In addition， we found that Fru eliminated FS-Ilp7 motoneurons in both sexes， but only when Tra was absent. Thus， Fru-dependent PCD eliminates female-specific neurons in males， and Tra plays a double-assurance function in females to establish and reinforce the decision to generate female-specific neurons.