1. Laboratory of Biology, Iwamizawa Campus, Hokkaido University of Education, Iwamizawa 068-8642, Japan
2. Waseda University, Advanced Institute for Science and Engineering, Tokyo 169-8555, Japan
3. Graduate School of Life Sciences, Tohoku University, Sendai 980-8578, Japan

Correspondence to: Ken-Ichi Kimura¹ Correspondence and requests for materials should be addressed to K-I.K. (Email: kimura@iwa.hokkyodai.ac.jp).

Abstract

The Drosophila fruitless (fru) gene product Fru has been postulated to be a neural sex determination factor that directs development of the central nervous system (CNS), thereby producing male-typical courtship behaviour and inducing male-specific muscle^{1, 2, 3, 4, 5, 6}. Male-specific Fru protein is expressed in small groups of neurons scattered throughout the CNS of male, but not female, Drosophila^{4, 7}. Collectively, these observations suggest that Fru 'masculinizes' certain neurons, thereby establishing neural substrates for male-typical behaviour. However, specific differences between neurons resulting from the presence or absence of Fru are unknown. Previous studies have suggested that Fru might result in sexual differences in the CNS at the functional level, as no overt sexual dimorphism in CNS structure was discernible^{8, 9, 10}. Here we identify a subset of fru-expressing interneurons in the brain that show marked sexual dimorphism in their number and projection pattern. We also demonstrate that Fru supports the development of neurons with male-specific dendritic fields, which are programmed to die during female development as a result of the absence of Fru. Thus, Fru expression can produce a male-specific neural circuit, probably used during heterosexual courtship, by preventing cell death in identifiable neurons.

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