Figure 1.

Schematic diagram of different phases in ZW sex chromosome evolution. We propose a hypothetical evolutionary model to illustrate the origin and evolution of ZW sex chromosomes. First, owing to strong selection of an evolutionary hotspot region, an ancestral autosomal pair undergoes mutation to become a sex determination region, and transformation into homomorphic proto-sex chromosomes. This is followed by heteromorphic differentiation resulting in formation of a proto-W chromosome with cessation of recombination and gain of female beneficial sequences for fitness and adaptation. The proto-W chromosome subsequently undergoes structural changes, such as rearrangements, gene degradation, repeat accumulations, and heterochromatinization, to form a neo-ZW chromosome system with limited differentiation. In some cases, during this stage turnover cycles might convert the partially differentiated heteromorphic sex chromosomes into homomorphic sex chromosomes, as in certain snake species, such as Ptyas species. To achieve full heteromorphy the neo-ZW chromosomes escape this evolutionary trap, and the young W chromosome undergoes severe degeneration with lineage-specific sequence variation and evolves into a mature and stable sex chromosome.