Figure 5.

Sexually antagonistic selection can cause an autosome to hijack sex determination from the ancestral sex chromosomes. Left panel: In the ancestral sex chromosomes, a dominant male-determining factor (the circle) is carried by the Y chromosome. On a pair of autosomes, a locus under sexually antagonistic selection segregates for 2 alleles, 1 beneficial to females (F) and the other to males (M). Middle panel: A dominant sex-reversal mutation that makes all carriers develop as male (the diamond) appears on the autosome near to the sexually antagonistic locus. Selection causes the mutation to become associated (in linkage disequilibrium) with the male-beneficial allele (M). During the transition, both linkage groups act as sex chromosomes, but individual males carry only 1 of the 2 Y chromosomes. Right panel: If sexually antagonistic selection is sufficiently strong, the chromosome with the new sex-determining mutant spreads. It becomes a neo-Y, its homolog becomes a neo-X, and the ancestral Y is lost. The linkage group that previously determined sex is now a pair of autosomes consisting of the ancestral X chromosomes. Details of the model are given in van Doorn and Kirkpatrick (2007).