Revisiting the model for the evolution of plant sex chromosomes heteromorphy with C. sativa. (A) The current model for the evolution of plant sex chromosomes heteromorphy is as follows: (1) Sex chromosomes originate from autosomes on which sex-determining genes evolve; (2) the region encompassing the sex-determining genes stops recombining; (3) the non-recombining region grows larger due to additional events of recombination suppression; (4) the nonrecombining region of the Y Chromosome accumulates repeats and can become larger than the corresponding region on the X Chromosome; (5–6) the Y Chromosome undergoes large deletions and ultimately becomes smaller than the X Chromosome. Steps 1–4 have been previously documented in plants (e.g., Charlesworth et al. 2005; Ming et al. 2011; Muyle et al. 2017 for review), while steps 5–6 are speculative. Our study is supportive of this scenario if we assume that the C. sativa Y Chromosome has been larger in the past. (B) It is possible, however, that the accumulation of repeats has been slow in the Y Chromosome of the C. sativa lineage and that X and Y Chromosomes have always been of similar size. Here, step 4 does not imply the elongation of the Y Chromosome.
