Sexual differentiation in Drosophila is controlled by a short cascade of regulatory genes, the expression pattern of which determines all aspects of maleness and femaleness, including complex behaviours displayed by males and females [1-3]. One sex-determining gene is transformer (tra), the activity of which is needed for female development. Flies with a female karyotype (XX) but which are mutant for tra develop and behave as males. In such flies, a female phenotype can be restored by a transgene that carries the female-specific cDNA of tra under the control of a heat-shock promoter. This transgene, called hs[trafem], also transforms XY animals into sterile females [4]. When we raised these XX and XY 'females' at 25 degreesC, however, they displayed vigorous male courtship while at the same time, as a result of their female pheromone pattern, they were attractive to males. Intriguingly, their male courtship behaviour was indiscriminately directed towards both females and males. When we forced expression of tra by heat shock, applied during a limited period around puparium formation, male behaviour was abolished and replaced by female behaviour. We conclude that sexual behaviour is irreversibly programmed during a critical period as a result of the activity or inactivity of a single control gene.

A male Drosophila melanogaster deposits many more sperm in a female's bursa copulatrix than are stored in her ventral receptacle or paired spermathecae soon after copula has ended. The remaining sperm are expelled by the female. These observations suggest a sexual con£ict over the processes involved in sperm storage. We used genetically manipulated £ies to study the role of the central nervous system in sperm storage. Flies with female bodies but masculinized nervous systems, or isolated female abdomens, stored signi¢cantly fewer sperm than did control females. Furthermore, compared with control £ies, there were relatively more sperm in the ventral receptacle and relatively fewer in the spermathecae. These results suggest that the female nervous input counteracts the male's attempts to force sperm into the ventral receptacle during copula and promotes active transport of sperm to the spermathecae during and after copula. The female is clearly a very active partner in in£uencing processes involved in sperm competition, especially as only stored sperm can be used later to fertilize eggs. To our knowledge, this is the ¢rst study to show directly the involvement of the female nervous system in sperm storage.

Ruedi Stoop and Benjamin I. Arthur, Jr. Institute for Neuroinformatics ETHZ/UNIZH, Winterthurerstr. We use symbolic dynamics to describe Drosophila courtship communication. We posit that behavior should be defined in terms of irreducible periodic orbits of fundamental acts. This leads to a first operational definition of behavior, which allows for a fine grained quantitative analysis of behavior. We obtain evidence that during Drosophila courtship, individual characteristics of the protagonists are exchanged (predominantly from the male to the female) and that males in the presence of fruitless males perform a behavioral switch from male to female behavior

Female yellow dung flies, Scathophaga stercoraria, can influence the traffic of sperm stored in their spermathecae to the site of fertilization in the bursa copulatrix. However, the anatomical mechanisms employed are largely unknown. We investigated the anatomy of the female genital tract, seeking structures involved in sperm transfer and egg fertilization. We found a membranous structure descending from the ends of the spermathecal and accessory gland ducts into the bursa copulatrix. We call this the prolatus. Sperm accumulate in the prolatus during oviposition. When an egg is in the bursa the egg micropyle, rather than being aligned towards the dorsal openings of the spermathecal ducts, lies on the opposite, ventral side. We also confirm the presence, and suggest a function for, a cuticularized pouch on the ventral wall of the anterior bursa copulatrix. This pouch, plus a previously undescribed chamber, may be homologous to the ventral receptacle/fertilization chamber found in other dipterans. Further, we describe a translucent cap, apparently transversed by channels, covering the micropyle. Sperm were observed to aggregate on and in the micropyle cap, which appears to attract and hold sperm. We interpret the prolatus as a structure that allows an ovipositing female to transfer a few sperm onto the ventral bursal wall and thus, indirectly, onto the micropyle cap. Such anatomy potentially gives the female a large degree of control over sperm traffic from storage to the site of fertilization.