FIGURE 2.

Single-gene sex determination enables turnovers between XY and ZW systems. (A) The feminizing sex switch (magenta box), i.e., ARR17 and MeGI in poplar and persimmon, respectively, is located outside of the sex-determining region (SDR, indicated by gray shading), because a hairpin RNA-encoding Y-chromosomal sequence (blue box) controls its activity in trans via small RNAs, via a dominant repressing action (ARR17 inv. rep: ARR17 inverted repeats). (B) Intriguingly, the same sex gene appears to be a sex-determining gene in a ZW system in white poplar (P. alba). Copies of the gene are found in the SDR of this species, and its absence from the Z chromosome (dashed gray box) leads to recessive loss of female and activation of male function.