Inbreeding, heterozygote advantage and the evolution of neo–X and neo–Y sex chromosomes

Associations between heterozygosities at different loci are generated by inbreeding. This can cause a fusion or translocation involving a sex chromosome and an autosome to have a selective advantage, when there is selection in favour of heterozygotes. Population genetic models of Y-autosome and X–autosome rearrangments in populations mating by a mixture of full sib–matings and random mating are described, in which the rearrangements cause an autosomal locus with heterozygote advantage to become linked to the true sex chromosomes. Such rearrangements gain a selective advantage under a wide range of conditions. If they can invade, Y–autosome rearrangements always spread to fixation, whereas X–autosome rearrangements may be maintained as stable polymorphisms. The results are discussed in relation to data on breeding systems and karyotypic evolution in termites.