Two wild Spinacia species, S. turkestanica and S. tetrandra, genomes reveal a sex chromosome turnover in the genus

Spinach (Spinacia oleracea) is a dioecious species (with male and female flowers on separate individuals). Spinach and its closest wild relative, S. turkestanica, has homomorphic sex chromosomes, but the more distant relative S. tetrandra has heteromorphic sex chromosomes. We report high-quality genome assemblies for S. turkestanica and S. tetrandra. These diverged approximately 6.3 million years ago (Mya), while S. turkestanica split from S. oleracea much more recently, around 0.8 Mya, supporting previous suggestions that S. turkestanica is the direct progenitor of cultivated spinach. Using a combination of genomic approaches, we identified a sex-linked region (SLR) of [~]133 Mb in S. tetrandra. In all three species, the SLRs are within a large pericentromeric region of chromosome 4. We describe evidence that, in S. tetrandra, this region has completely stopped recombining in male meiosis, creating a large Y-linked region (YLR) that has partially degenerated; loss of recombination appears to have evolved in two events that created two "evolutionary strata", one of which and is highly rearranged, relative to the X. The SLRs of S. turkestanica and S. oleracea are much smaller: both include only a 10 Mb Y-specific region which is not detected in S. tetrandra. This was duplicated into a 14 Mb inverted region, and is termed the Y-duplicated region, or "YDR". These findings suggest that a turnover event created the YDR before these species diverged, replacing an extensive ancestral Y-linked region like the S. tetrandra YLR.