HALDANE RULE HAS MULTIPLE GENETIC CAUSES

HALDANE'S rule states that ''When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic or XY] sex''1. This rule represents one of the few patterns characterizing animal speciation2,3. Traditional explanations of Haldane's rule1,4-6 claim that heterogametic hybrids are unfit because they lack an X chromosome that is 'compatible' with the autosomes of one species. Recent work2,7 shows that this explanation is incorrect for hybrid sterility: contrary to prediction, homogametic hybrids carrying both X chromosomes from the same species remain fertile. Until now, similar tests have not been performed for hybrid inviability. Here I show that homogametic hybrids who carry both X chromosomes from the same species are inviable. These results show that the genetic causes of Haldane's rule differ for hybrid sterility versus inviability. Haldane's rule does not, therefore, have a single genetic basis.