Animals perform or terminate particular behaviors by integrating external cues and internal states through neural circuits. Identifying neural substrates and their molecular modulators promoting or inhibiting animal behaviors are key steps to understand how neural circuits control behaviors. Here, we identify the Cholecystokinin-like peptide Drosulfakinin (DSK) that functions at single-neuron resolution to suppress male sexual behavior in Drosophila. We found that Dsk neurons physiologically interact with male-specific P1 neurons, part of a command center for male sexual behaviors, and function oppositely to regulate multiple arousal-related behaviors including sex, sleep and spontaneous walking. We further found that the DSK-2 peptide functions through its receptor CCKLR-17D3 to suppress sexual behaviors in flies. Such a neuropeptide circuit largely overlaps with the fruitless-expressing neural circuit that governs most aspects of male sexual behaviors. Thus DSK/CCKLR signaling in the sex circuitry functions antagonistically with P1 neurons to balance arousal levels and modulate sexual behaviors.

SUMMARYUncovering the direct regulatory targets of doublesex (dsx) and fruitless (fru) is crucial for an understanding of how they regulate sexual development, morphogenesis, differentiation and adult functions (including behavior) in Drosophila melanogaster. Using a modified DamID approach, we identified 650 DSX-binding regions in the genome from which we then extracted an optimal palindromic 13 bp DSX-binding sequence. This sequence is functional in vivo, and the base identity at each position is important for DSX binding in vitro. In addition, this sequence is enriched in the genomes of D. melanogaster (58 copies versus approximately the three expected from random) and in the 11 other sequenced Drosophila species, as well as in some other Dipterans. Twenty-three genes are associated with both an in vivo peak in DSX binding and an optimal DSX-binding sequence, and thus are almost certainly direct DSX targets. The association of these 23 genes with optimum DSX binding sites was used to examine the evolutionary changes occurring in DSX and its targets in insects.

The development of sexually dimorphic morphology and the potential for sexually dimorphic behavior in Drosophila are regulated by the Fruitless (Fru) and Doublesex (Dsx) transcription factors. Several direct targets of Dsx have been identified, but direct Fru targets have not been definitively identified. We show that Drosophila leucine-rich repeat G protein-coupled receptor 3 (Lgr3) is regulated by Fru and Dsx in separate populations of neurons. Lgr3 is a member of the relaxin-receptor family and a receptor for Dilp8, necessary for control of organ growth. Lgr3 expression in the anterior central brain of males is inhibited by the B isoform of Fru, whose DNA binding domain interacts with a short region of an Lgr3 intron. Fru A and C isoform mutants had no observed effect on Lgr3 expression. The female form of Dsx (Dsx F ) separately up-and down-regulates Lgr3 expression in distinct neurons in the abdominal ganglion through female-and male-specific Lgr3 enhancers. Excitation of neural activity in the Dsx F -up-regulated abdominal ganglion neurons inhibits female receptivity, indicating the importance of these neurons for sexual behavior. Coordinated regulation of Lgr3 by Fru and Dsx marks a point of convergence of the two branches of the sex-determination hierarchy.M ost animal species are comprised of female and male individuals, in which sex differences in form and behavior are specified by their genetic makeup. The developmental processes by which genes build sex-specific differences into the nervous system, and hence encode the potential for sex-specific behavior, have long been of interest (1).In Drosophila melanogaster the assessment of the number of X chromosomes leads to sex-differential splicing of transcripts from genes making up the sex-determination hierarchy, in particular the terminal genes of that hierarchy, fruitless (fru) and doublesex [dsx (2), reviewed in ref. 3]. fru and dsx encode sexspecific Zn-finger transcription factors that alter, either directly or indirectly, the expression of downstream genes to produce the sexually dimorphic elements of flies. The male-specific forms of Fru (Fru M ) act in a subset of the neurons within the male's nervous system to establish the potential for social interactions such as courtship behavior and aggression (reviewed in ref.3). In contrast, Dsx acts in subsets of both neural and nonneural tissues of males and females to regulate behavioral and nonbehavioral aspects of sexual development (reviewed in ref.3).Although the mechanisms regulating the production of the sexspecific isoforms of the Fru and Dsx proteins are well-established (4), how these proteins in turn function is only beginning to be elucidated. Several direct Dsx targets and a well-conserved 13-bp Dsx binding site have been identified (5-13). Many Dsx target genes encode well-known transcription factors and cell-cell signaling molecules that function sex-nonspecifically in most tissues in which they are expressed. However, in other tissues, Dsx directs the sexspecific expression of these gene...