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.