A genome-wide survey of hybrid incompatibility factors by the introgression of marked segments of Drosophila mauritiana chromosomes into Drosophila simulans

In hybrids between Drosophila simulans and D. mauritiana, males are sterile and females are fertile, in compliance with HALDANE's rule. The genetic basis of this phenomenon was investigated by introgression of segments of the mauritiana genome into a simulans background. A total of 87 positions throughout the mauritiana genome were marked with P-element insertions and replicate introgressions were made by repeated backcrossing to simulans for 15 generations. The fraction of hemizygous X chromosomal introgressions that are male sterile is ~50% greater than the fraction of homozygous autosomal segments. This results suggests that male sterility factors have evolved at a higher rate on the X, but chromosomal differences in segment length cannot be ruled out. The fraction of homozygous autosomal introgressions that are male sterile is several times greater than the fraction that are either female sterile or inviable. This observation strongly indicates that male sterility factors have evolved more rapidly than either female sterility or inviability factors. These results, combined with previous work on these and other species, suggest that HALDANE'S rule has at least two causes: recessivity of incompatibility factors and differential accumulation of sterility factors males and females.