The first steps of animal speciation are thought to be the development of sexual isolating mechanisms. In contrast to recent progress in understanding the genetic basis of postzygotic isolating mechanisms, little is known about the genetic architecture of sexual isolation. Here, we have subjected Drosophila melanogaster to 29 generations of replicated divergent artificial selection for mating speed. The phenotypic response to selection was highly asymmetrical in the direction of reduced mating speed, with estimates of realized heritability averaging 7%. The selection response was largely attributable to a reduction in female receptivity. We assessed the whole genome transcriptional response to selection for mating speed using Affymetrix GeneChips and a rigorous statistical analysis. Remarkably, >3,700 probe sets (21% of the array elements) exhibited a divergence in message levels between the Fast and Slow replicate lines. Genes with altered transcriptional abundance in response to selection fell into many different biological process and molecular function Gene Ontology categories, indicating substantial pleiotropy for this complex behavior. Future functional studies are necessary to test the extent to which transcript profiling of divergent selection lines accurately predicts genes that directly affect the selected trait.

Nevertheless, genes exhibiting parallel changes in transcript abundance between replicate Fast and Slow selected lines are candidate genes affecting mating behavior. Could 21% of the genome really be responsible for regulating mating speed? Recent studies assessing subtle quantitative effects of P element insertional mutations on numbers of sensory bristles (83) and resistance to starvation stress (84) have concluded that >20% of the genome affects each of these traits. These results imply massive pleiotropy: the same genes affect multiple complex traits. Thus, genes regulating mating behavior are as likely to be genes involved in neurogenesis, metabolism, development, and general cellular processes as genes with specific effects on behavior (85). In fact, the same loci may affect multiple behaviors. Pigment dispersing factor (Pdf) and cyrptochrome (cry) were defined based on the involvement in circadian rhythm but were up-regulated in lines selected for positive geotaxis and confirmed to affect geotaxis behavior in functional tests (71). We note that Pdf and cry are also differentially expressed between the Fast and Slow mating speed selection lines, implicating them in mating behavior.