Haldane’s rule usually applies only to early stages of speciation. Building reproductive barriers is a snowball process29, or, as Darwin (1866) put it, a series of “graduated steps from very slightly lessened fertility to utter and absolute sterility”30. Therefore, at advanced stages of reproductive isolation, both sexes are sterile. We are currently unclear, however, as to whether males and females proceed to complete sterility by the same or by different routes. Sex differences in meiotic disruption within hybrids, then, can tell us about the genetic and cytological bases of these steps. Unfortunately, due to technical difficulties female meiosis in the hybrids has rarely been analysed. We are aware of only one such study31, which found that female hybrids between Mus musculus and M. domesticus were fertile, even though their oocytes displayed the same pairing abnormalities as male hybrids - with half the frequency, though.