Evolution of Sex Determination and Sex Chromosomes: A Novel Alternative Paradigm.

Sex chromosomes can differ between species as a result of evolutionary turnover, a process that can be driven by evolution of the sex determination pathway. Canonical models of sex chromosome turnover hypothesize that a new master sex determining gene causes an autosome to become a sex chromosome or an XY chromosome pair to switch to a ZW pair (or vice versa). Here, a novel paradigm for the evolution of sex determination and sex chromosomes is presented, in which there is an evolutionary transition in the master sex determiner, but the X chromosome remains unchanged. There are three documented examples of the novel paradigm, and it is hypothesized that a similar process could happen in a ZW sex chromosome system. Three other taxa are also identified where the novel paradigm may have occurred, and how it could be distinguished from canonical trajectories in these and additional taxa is also described.