A relay model for the neuronal control of sexual size dimorphism (SSD) in Drosophila.

Sex-lethal (Sxl) expression in insulin-producing cells (IPCs) and other neurons of the central nervous system (CNS) acts, largely independently of the female-specific tranformer splice variant (TraF), to relay a signal(s) to peripheral larval tissues. This signal specifies the female-specific growth trajectory of larval tissues and does not appear to involve IPC-derived insulin-like peptides (Ilps) or insulin signalling. Imaginal tissue growth during larval stages is insensitive to the neuronal Sxl signal. Sxl also acts cell-autonomously via TraF in both larval and imaginal tissues to increase female growth. Red arrows indicate female-specific regulation; black arrows indicate non-sex-specific growth regulatory pathways. Additional SSD mechanisms not depicted in this model are proposed in other studies [25,43].