Far from being a prurient pastime, scrutinizing the interaction between the sexes is the only means of studying the most fundamental problem there is — the propagation of life. Two recent papers on Drosophila now highlight two of the topics tackled by this rich area of research: in one, Mattox and Dauwalder et al. describe a gene that is expressed specifically in males and is required for courting behaviour; in the other, Miller and Pitnick reveal, for the first time, how postcopulatory interactions between the sexes drives the evolution of a reproductive trait, in this case longer sperm tails.

It is the male fly that takes the initiative to mate through an elaborate courtship ritual. Like other somatic sexual features, sex-specific behaviours, such as courtship, are controlled by the combined action of the sex-specific forms of Doublesex (Dsx) and Fruitless (Fru) proteins. But how? Mattox and Dauwalder provide part of the answer to this question in their study of the takeout (to) gene, which they found — in an RNA subtractive hybridization screen — is specifically expressed in male heads. Subsequent mutant studies showed that to is required for male courting behaviour: to mutant male flies could distinguish between males and females, but courted less often. The expression of to, which encodes one in a family of 20 secreted proteins, depends on the male-specific forms of Dsx and Fru; this study therefore identifies the first target of Dsx and Fru that is involved in sex-specific behaviour, as well as providing the curious puzzle of how fat cells (the cells in the head in which to is expressed) might control courtship.

One of the driving forces behind the evolution of many male traits is female-driven sexual selection, the peacock's tail being the most famous example. Miller and Pitnick now show experimentally how female choice drives the evolution of one such sexually selected trait: longer sperm tails. Sperm tail length is highly heritable, so populations of flies could be bred that had either giant or very short sperm tails. It was clear from mating these males to females that were bred to have either very long or very short sperm storage organs (seminal receptacles, SRs) that longer sperm were more successful at fertilizing females — especially those with long SRs — when competing with shorter sperm. As well as showing that sperm length is adaptive, these results, and others, show that physical selection by females influences sperm morphology — presumably because longer sperm preferentially occupy a favourable position in longer SRs.

The biological implications of both studies go well beyond flies. For example, given that the takeout gene was previously described for promoting starvation tolerance in flies, it would be interesting to find out any existing link between the appetite for food and for sex.