Differential strength of sex-biased hybrid inferiority in impeding gene flow may be a cause of Haldane's rule.

In animals, if one sex of the F1 hybrid between two species is sterile or inviable, it is usually the heterogametic (XY or WZ) sex. This phenomenon, known as Haldane's rule, is currently thought to be coincidentally caused by different mechanisms in separate entities. The following questions have never been asked: Are heterogametic and homogametic inferiority (sterility or inviability) equivalent as isolating mechanisms? Could discrepancies between them, if existing, produce Haldane's rule? Here I consider sex-biased hybrid inferiority strictly as an isolating mechanism, and quantitatively evaluate its strength in impeding gene flow. The comparison reveals that the ability of sex-biased inferiority to impede gene flow varies according to the sex and chromosome involved. Heterogametic inferiority is a weaker barrier when unidirectional and a much stronger one when in compound reciprocal directions, compared with homogametic inferiority. Such differential strength may affect divergence in speciation and produce Haldane's rule.