dsxM and fruM isoforms. Females express the RNA splicing factor transformer (tra), which drives female-specific splicing of dsx and fru-P1 into a coding dsxF isoform and a non-coding fruF

INTRODUCTION Behavioral differences between males and females often arise from sexually dimorphic neurons and circuits (Cooke et al., 1998; Wade and Arnold, 2004; Villella and Hall, 2008; Paus, 2010). Stereotyped male behaviors in Drosophila melanogaster provide the basis for our current understanding of the genetic mechanisms and neural substrates that generate sexually dimorphic behaviors (Yamamoto, 2007; Villella and Hall, 2008). The sex determination cascade generates dimorphic neuronal populations largely through the sex-specific RNA splicing of the transcription factor genes doublesex (dsx) and fruitless (fru) (Salz and Erickson, 2010; Dauwalder, 2011). In males, dsx and a fru transcript driven from its P1 promoter (fru-P1) undergo default RNA splicing into coding dsxM and fruM isoforms. Females express the RNA splicing factor transformer (tra), which drives female-specific splicing of dsx and fru-P1 into a coding dsxF isoform and a non-coding fruF isoform. Both Fru and Dsx are expressed in a largely overlapping set of ~2000 neurons that play crucial roles in sexually dimorphic behaviors (Cachero et al., 2010; Rideout et al., 2010; Robinett et al., 2010; Yu et al., 2010), in which Fru and Dsx direct sexual dimorphic neuronal gene expression and functional properties, as well as differences in branching and connectivity (Yamamoto, 2007; Villella and Hall, 2008; Dauwalder, 2011). Curiously, only males are reported to have numerically expanded neuronal populations or unique populations not found in females (Yamamoto, 2007; Cachero et al., 2010; Rideout et al., 2010; Yu et al., 2010; Kimura, 2011). Much of our understanding of the genetic and neural substrates of sexually dimorphic behavior comes from analysis of males, with comparatively less work having been performed on female behavior (Ferveur, 2010). Egg laying in females is under tight neuronal control and its regulatory circuitry is one of the best understood female behaviors (Middleton et al., 2006; Rodríguez-Valentín et al., 2006; Yapici et al., 2008; Yang et al., 2009; Rezával et al., 2012). After eggs exit the ovary, they are propelled through the oviduct by somatic-like muscles that ring the oviduct (Hudson et al., 2008). Peristaltic contraction/relaxation activity of these muscles is directed by unidentified excitatory glutamatergic motoneurons and inhibitory octopaminergic neurons (Middleton et al., 2006; Rodríguez-Valentín et al., 2006; Kapelnikov et al., 2008). Insulinlike peptide 7 (Ilp7)-expressing neurons are also reported to innervate the oviduct, and their electrical silencing blocks egg laying (Yang et al., 2008); yet, as Ilp7 mutants have no egg-laying phenotype (Grönke et al., 2010), the function of these neurons is uncertain. Here, we identify a post-embryonic population of Ilp7-expressing neurons in the posterior adult ventral nerve cord that innervates the female oviduct and the male seminal vesicles. This population exhibits a functionally biased role in females as well as a rare phenomenon in Drosophila: a female-specific subset of CNS neurons. Examination of the role of the sex determination cascade in the dimorphisms displayed by these neurons indicates that a postmitotic traand fruM-dependent mechanism accounts for the dimorphisms of the shared population of Ilp7 neurons, but that a postmitotic tra-dependent and fruand dsx-independent mechanism is responsible for generating the female-specific neuronal subset in females. 1Department of Cellular and Physiological Sciences, 2401 Life Sciences Centre, 2350 Health Sciences Mall, University of British Columbia, Vancouver, BC, V6T 1Z3, Canada. 2Department of Genetics, Room 360, New Research Building, Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115, USA.