An unusual origin of a ZW sex chromosome system

Transitions in the heterogamety of sex chromosomes (e.g. XY to ZW or vice versa) are rare and fundamentally alter the genetic basis of sex determination. Although in an XY->ZW transition the W is more likely to arise from the X because they both carry feminizing genes and the X is expected to harbor less genetic load than the Y, here we show that both the W and the Z have arisen from the Y in willows (Salix). The new Z chromosome shared multiple homologous putative masculinizing factors with the ancestral Y, whereas the new W lost these masculinizing factors and gained feminizing factors. The origination of both the W and Z from the Y was permitted by an unexpectedly low genetic load on the Y and indicates that the origins of sex chromosomes during homologous transitions may be more flexible than previously considered.