A test of reciprocal X-Y interactions as a cause of hybrid sterility in Drosophila.

Elucidation of the nature of the gene interactions that underly the sterility of interspecific hybrids is important in evolutionary biology. The interactions between the heterospecific X and Y (or Z and W) chromosomes are often used as an explanation for two reasons. First, the fertility of the hybrids of the heterogametic sex is much more often affected than that of the homogametic sex (Haldane's rule) and X-Y interactions are specific to the heterogametic sex. Second, sex chromosomes, especially the X chromosome, are often considered to be of special importance in determining the fertility of hybrids. X-Y interactions have been addressed in studies of males with a heterospecific Y chromosome in a mixed genetic background. A more stringent test of the X-Y interaction model requires each X chromosome sterility factor to be tested separately for its interaction with the Y chromosome in a homogeneous background of the pure species. Here we report such a test of the X-Y interaction model and conclude that X-Y interactions should not be assumed to be the only or even the most common cause of hybrid sterility.