Sex chromosomes and speciation in Drosophila

Two empirical rules suggest that sex chromosomes play a special role in speciation. The first is Haldane's rule the preferential sterility and inviability of species hybrids of the heterogametic (XY) sex. The second is the disproportionately large effect of the X chromosome in genetic analyses of hybrid sterility. Whereas the causes of Haldane's rule are well established, the causes of the 'large X-effect' have remained controversial. New genetic analyses in Drosophila, confirm that the X is a hotspot for hybrid male sterility factors, providing a proximate explanation for the large X-effect. Several other new findings - on faster X evolution, X chromosome meiotic drive and the regulation of the X chromosome in the male-germline - provide plausible evolutionary explanations for the large X-effect.