Gene Expression and Introgression of Two Genes Implicated in Behavioral Reproductive Isolation between Drosophila simulans and D. sechellia

Reproductive isolation maintains species barriers and can cause population divergence. Pre-mating isolation prevents courtship between species and can be caused by changes in gene expression resulting in sex- and species-specific phenotypes. In Drosophila, courtship is heavily influenced by the production and reception of cuticular hydrocarbons (CHCs), which act as pheromones that elicit or repel courtship from a potential mate. Differences in expression of genes involved in CHC biosynthesis between species can produce different types of CHCs, resulting in species-specific CHC profiles. Differential CHC profiles can result in courtship barriers between species. Drosophila simulans and D. sechellia are asymmetrically reproductively isolated from each other in part because of differential production of CHCs. D. simulans males and females produce 7-tricosene (7-T) whereas D. sechellia males produce 7-T and D. sechellia females produce 7,11-heptacosadiene (7,11-HD). 7,11-HD acts as an anti-aphrodisiac to D. simulans males, which only court D. simulans females. D. sechellia males court both D. simulans and D. sechellia females. Thus reproductive isolation occurs between D. simulans males and D. sechellia females. Genomic regions containing desatF and eloF, among other genes, have been identified by quantitative trait locus (QTL) studies as potentially contributing to production of the D. sechellia pheromone 7,11-HD. In this study I tested the hypothesis that desatF and eloF influence reproductive isolation between D. simulans and D. sechellia. In the first set of experiments, using gene expression analysis, I measured the mRNA production of desatF and eloF, as well as other desaturases and elongases associated with QTL affecting CHC differences between D. simulans and D. sechellia. Both eloF and desatF were expressed only in females of D. sechellia. The other genes had variable expression patterns that did not suggest involvement in sex-specific CHC production. Using allele-specific qPCR in D. simulans/D. sechellia hybrids, I found that only the D. sechellia alleles of desatF and eloF are expressed, implying that desatF and eloF expression differences between females of D. simulans and D. sechellia are likely caused by a cis-regulatory change. To further examine the differences in desatF and eloF expression, in the second set of experiments I introgressed the D. simulans alleles of desatF and eloF into a D. sechellia background. Introgression lines did not express the D. simulans desatF and/or eloF alleles. To determine the effect of desatF and eloF expression on courtship, I measured the courtship behavior of D. simulans and D. sechellia males toward the introgression lines. The time required for a male to start courting did not depend upon the target females, whether it was an introgression line, conspecific or heterospecific. In contrast, the courtship effort of males differed by target female. Male D. simulans courted all introgression lines, and some lines received significantly more courtship than D. sechellia females, though not as much as D. simulans females, indicating that the change in allele removed some of the reproductive barrier between D. simulans males and D. sechellia females. The type of female had less of an effect for D. sechellia males, which was expected because D. sechellia males are not completely reproductively isolated from D. simulans females. Altogether these results imply that desatF and eloF expression differences between D. simulans and D. sechellia directly affect reproductive isolation between the species, presumably through the genes’ roles in 7,11-HD production