Female-biased sexual dimorphism of neurons in Drosophila

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Our program of research will examine the genetic and cellular determinants of female-specific neuronal circuits in Drosophila melanogaster. Behavioral differences between males and females arise from ~1500 dimorphic neurons. The field has focused on male behavior, thus our understanding has a strong male bias, leaving female behavior poorly understood. Study of males has identified a sex determination cascade that directs Drosophila dimorphism. This cascade centers on sex-specific RNA splicing of the transcription factors doublesex (dsx) and fruitless (fru). In males, dsx and fru-P1 transcripts are spliced into coding dsxM and fruM isoforms. Females express the RNA splicing factor transformer (tra) that directs splicing of the distinct dsxF and the non-coding fruF isoforms. Prevailing models state that the sexual identity of sex-specific neurons is determined by a double switch system mediated by differences in fruitless and doublesex isoform function.PRELIMINARY DATA. We identified the first CNS neurons that are female-specific, the Ilp7-motoneurons (see Research Contribution 1). We found that the genetic regulation of their generation defies conventional models. In particular, Ilp7-neurons are generated independently of the function of fruitless and doublesex.HYPOTHESIS: We hypothesize that the generation of female-specific neurons and circuits requires novel genetic mechanisms that are distinct from those defined in males.AIM 1) Determine the cellular mechanisms underlying the generation of female-specific neuronal populations. Aim 1A) Determine the fate of female-specific neurons in males. We will use a battery of genetic approaches to test whether the male counterparts die or transfate. Aim 1B) We will identify other female-specific neurons to expand our analysis and uncover common principals. Aim 1Bi takes a targeted approach to identify female reproductive tract neurons and their male counterparts. Aim 1Bii takes advantage of the fact that many neurons become male specific due to their death in female. We will identify dying dimorphic neurons in males using a 'positive identification' protocol. AIM 2) Determine the genetic mechanisms directing female-specific neuron dimorphism. We focus on the roles of tra, fru, dsx, as well as the poorly studied, but described, regulators of female dimorphism, dissatisfaction, intersex and hermaphrodite. In Aim 2A, we examine their roles in known female-specific neurons and in neuronal subset that we expect to be female-specific. In Aim 2B, we take a broader view to examine the roles of these regulators in generating the unique female-type morphologies and gene expression profiles of all dimorphic neuronal populations.AIM 3) Determine the genetic regulation of the formation of female-specific circuits. Aim 3A examines the connectivity between neurons (identified in Aim 1B) that coordinately regulate the female reproductive tract. Aim 3B examines the female-specific connectivity of circuits that are known to have different function in male and females. These studies will be performed by a combination of intersectional reporter genetics and split GFP methods that positively identify synaptic connectivity between specific neurons.IMPACT AND TRAINING. These studies will provide an in depth understanding of how female-specific neuronal circuits are constructed and function, providing an important counterpoint to the decades of analysis of male circuits that generate male-specific behaviors. Students working on this project will obtain valuable training in advanced Drosophila molecular genetics, 3D neuronal imaging and behavioral analysis.