Fig. 1.

Sex chromosome cycle (inside) and rough frequency of species in seven groups of organisms at each stage of the cycle (outside). The sex chromosome cycle illustrates the evolutionary process where new sex chromosomes emerge, differentiate, degenerate and eventually disappear (circumferential black arrows). In the cycle, sex chromosome turnover can occur at any stage while maintaining stable sex determination (inside blue arrows). Colors corresponding to each taxonomic group are shown in the top left. Color intensity roughly corresponds to the relative frequency of species at the stage of the sex chromosome cycle in each group. The illustration was originally inspired by Furman et al. (3).