An Introgression Analysis of Quantitative Trait Loci That Contribute to a Morphological Difference Between Drosophila simulans and D. mauritiana

Drosophila simulans and D. mauritiana differ markedly in morphology of the posterior lobe, a male-specific genitalic structure. Both size and shape of the lobe can be quantified by a morphometric variable, PC1, derived from principal components and Fourier analyses. The genetic architecture of the species difference in PC1 was investigated previously by composite interval mapping, which revealed largely additive inheritance, with a minimum of eight quantitative trait loci (QTL) affecting the trait. This analysis was extended by introgression of marked segments of the mauritiana third chromosome into a simulans background by repeated backcrossing. The two types of experiment are consistent in suggesting that several QTL on the third chromosome may have effects in the range of 10–15% of the parental difference and that all or nearly all QTL have effects in the same direction. Since the parental difference is large (30.4 environmental standard deviations), effects of this magnitude can produce alternative homozygotes with little overlap in phenotype. However, these estimates may not reflect the effects of individual loci, since each interval or introgressed segment may contain multiple QTL. The consistent direction of allelic effects suggests a history of directional selection on the posterior lobe.