Genomic map of the fruitless gene. Top: Coordinates of fru exons derived from cDNA sequences mapped onto the genomic sequence. Black rectangles represent exons and inverted triangles represent the positions of p-element insertion mutations. Middle: Diagram of cDNA classes with introns denoted by lines connecting the exons (black rectangles). Alternative splicing at the sex-specific sites is shown by the open rectangle, which is used only in females. Alternative internal 32 nt, 15 nt and 148 nt micro-exons are shown without connections to other exons. Question marks indicate where the complete array of exons used in a particular transcript class is unknown. Bottom: Proteins encoded by the fru P1-derived mRNAs.That fru acts during development to establish the potential for male sexual behavior is suggested by the patterns of expression of male-specific P1 products. The FRUM proteins are expressed almost exclusively in the CNS, where they are first detected in a small number of cells at the end of the larval period. Expression is maximal about two days into the pupal period when ca. 1700 cells (2% of the CNS) express FRUM proteins; this time coincides with the period of major morphogenetic events that shape the adult CNS (Figure 1). Cells expressing the FRUM proteins are not localized to one part of the CNS. They are scattered throughout the brain and VNC, most frequently found in small groups of cells, and less frequently as single cells. There are approximately 20 groups of FRUM neurons (Fig. 2). Some of these groups are in specific regions of the CNS previously shown to be involved in particular steps of courtship behavior. In addition, a number of regions containing FRUM neurons had not been previously implicated in courtship. Classification of the types of neurons in which fru is expressed-based on the locations, size and morphologies of their cell bodies-provides insight into fru's function. Most cells expressing these products are in higher order sensory or motor neuropils and have morphologies suggesting that they are either local circuit neurons or interganglionic interneurons. Few of the cells expressing the male-specific fru products appear to be either sensory or motor neurons. One exception is a group of serotonergic FRUM cells in the abdominal ganglion that provide the sole innervation of much of the internal male genitalia. These findings suggest that male sexual behavior is the result of sensory information entering the CNS via largely sex-nonspecific sensory systems, and being processed by sex-specific (fru-specified) circuitry in higher order neuropils. This information is then used to execute the particular steps of courtship behavior by modulating largely sex-nonspecific motor systems.
Figure 2. Anti FRUM staining of wild-type male CNS.The findings that FRUM proteins are expressed in only a small portion of the CNS and are necessary for all, or nearly all, aspects of male sexual behavior were provocative and pleasing. They suggested fru provided a handle for understanding the developmental, genetic, molecular, and neuronal bases of male courtship behavior. Yet there is a formidable gap between the two aspects of fru about which we have substantial information: (1) the properties and expression patterns of its products and (2) fru's phenotypic effects on male sexual behavior. It is on bridging this gap that our current research is focused. We believe the key to gaining an understanding of how the potential for a complex behavior is built into the CNS will be to go from the gene (fru) to the properties of the cells in which it functions and from those cells to behavior. Thus it is on the developmental origins of these neurons, the characteristics that distinguish them, and the roles they play in adult male sexual behavior that we are focused. We are addressing the following topics.
Taylor, B. J., Villella, A., Ryner, L. C., Baker, B. S., and Hall, J. C. (1994). Behavioral and Neurobiological implications of sex-determining factors in Drosophila. Devel. Genetics 15: 275-296.
Ryner, L. C., Goodwin, S. F., Castrillon, D. H., Anand, A., Villella, A., Baker, B., Hall, J. C., Taylor, B. J., and Wasserman, S. A. (1996). Control of male sexual behavior and sexual orientation in Drosophila by the fruitless gene. Cell, 87:1079-1089.
Villella, A., Gailey, D. A., Berwald, B., Ohshima, S., Barnes, P. T., and Hall, J. C. (1997). Extended reproductive roles of the fruitless gene in Drosophila melanogaster revealed by behavioral analysis of new fru mutants. Genetics 147: 1107-1130.
Heinrichs, V., Ryner, L.C. and Baker, B.S. (1998). Regulation of sex-specific selection of fruitless 5' splice sites by transformer and transformer-2. Mol. Cell. Biol. 18: 450-458.
Goodwin S.F., Taylor, B.J., Villella, A., Foss, M., Ryner, L. C., Baker, B. S., and Hall, J. C., (2000) Aberrant splicing and altered spatial expression patterns in fruitless mutants of Drosophila melanogaster. Genetics 154: 725-45.
Lee, G., Foss, M., Goodwin, S., Carlo, T., Taylor, B. J., and Hall, J. C. (2000). Spatial, temporal and sexually dimorphic expression patterns of the fruitless gene in the Drosophila CNS. J. Neurobiol. 43: 404-426.
Lee, G., and Hall, J. C., (2000). A newly uncovered phenotype associated with the fruitless gene of Drosophila melanogaster: aggression-like head interactions between mutant males. Behav. Genet. 30: 263-275.
Lee, G., Villella, A., Taylor, B. J., Hall, J. C. (2001). New reproductive anomalies in fruitless-mutant Drosophila males: Extreme lengthening of mating durations and infertility associated with defective serotonergic innervation of reproductive organs. J. Neurobiol. 47: 121-149.
Lee, G. and Hall, J.C. (2001). Abnormalities of male-specific FRU protein and serotonin expression in the CNS of fruitless mutants in Drosophila. J. Neurosci. 21: 513-526.
Anand, A., Villella, A., Ryner, L. C., Carlo, T., Goodwin, S. F., Song, H.-J., Gailey, D. A., Hall, J. C., Baker, B. S., and Taylor, B. J., (2001). The sex determination gene fruitless , in addition to controlling male sexual behaviors, has sex-nonspecific vital functions. Genetics, 158: 1569-1595.
Song, H.-J.,. Goodwin , S. F., Reynaud, E., Carlo, T., Billeter, J.-C., Eric P. Spana, E. P., Perrimon, N., Baker, B. S., and Taylor, B. J., (2002). The fruitless gene is required for the proper formation of axonal tracts in the embryonic CNS of Drosophila. Genetics, 162: 1703-1724.
Song, H.-J. and Taylor, B. J. (2003). The fruitless gene is required to maintain neuronal identity in evenskipped-expressing neurons in the embryonic CNS of Drosophila. J. Neurobiol., in press.