Incompatibilities between Y chromosome and autosomes are responsible for male hybrid sterility in crosses between Drosophila virilis and Drosophila texana.

Crosses between Drosophila virilis and D. texana produce viable and fertile F1 males and females. When F1 males are backcrossed to either parental species they also produced fertile sons. However, about one-third of F1 males carrying the D. texana Y chromosome are sterile. When fertile F1 males with the D. texana Y chromosome are crossed to D. virilis, about three quarters of the sons are sterile. We show that these sterilities result from incompatibilities between the D. texana Y chromosome and at least two of the D. virilis autosomes. X/Y incompatibilities can be excluded in this pair of species, and X/autosome incompatibilities appear to be either absent or to play a minor role in the sterility of male progeny from backcrosses of F1 males to females from either species. It is suggested that Y/autosome incompatibilities may be among the first to appear in the development of postzygotic isolation in Drosophila.