Sex chromosomes and speciation.

Studies of reproductive isolation between animal species have shown (i) that if one sex of the hybrids between two species is sterile or inviable, it is usually the heterogametic sex (Haldane's rule), and (ii) the genes on the sex chromosomes play a particularly large role in hybrid sterility and inviability. We propose an explanation for these two observations which is based on the changes in chromosome conformation which take place during gametogenesis. These changes are far greater in sex chromosomes than in autosomes. They are also greater in the heterogametic than in the homogametic sex. We suggest that the sensitivity of hybrids of the heterogametic sex to the genetic divergence that occurs during periods of population isolation is partly the result of the failure of their sex chromosomes to undergo appropriate conformational changes. This hypothesis explains why the sex chromosomes play a disproportionate role in post-zygotic, but not in pre-zygotic, isolation, and why often only the germ line is sensitive to hybridization.