Genetic architecture of a morphological shape difference between two drosophila species

The size and shape of the posterior lobe of the male genital arch differs d ramatically between Drosophila simulans and D. mauritiana. This difference can be quantified with a morphometric descriptor (PCI) based on elliptical Fourier and principal components analyses. The genetic basis of the intersp ecific difference in PC1 was investigated by the application of quantitativ e trait locus (QTL) mapping procedures to segregating backcross populations . The parental difference (35 environmental standard deviations) and the he ritability, of PC1 in backcross populations (>90%) are both very large. The use of multiple interval mapping gives evidence for 19 different QTL. The greatest additive effect estimate accounts for 11.4% of the parental differ ence bur could represent multiple closely linked QTL. Dominance parameter e stimates vary among loci from essentially Ilo dominance to complete dominan ce, and mauritiana alleles tend to be dominant over simulans alleles. Epist asis appears to be relatively unimportant as a source of variation. All but one of the additive effect estimates have the same sign, which means that one species has nearly all plus alleles and the other nearly all minus alle les. This result is unexpected under many evolutionary scenarios and sugges ts a history of strong directional selection acting on the posterior lobe.