Fig. 4.

Hypothetical model for sex system turnovers in Salicaceae. The W chromosomes of Populus alba and Salix purpurea both carry several intact RR genes and are likely to serve as a dominant promoter of female function. On the Y chromosomes of P. euphratica, P. trichocarpa, P. deltoides, and P. tremula, partial duplicates of the RR gene likely act as suppressors of female development by encoding an siRNA that targets the intact RR gene through RNA-directed DNA methylation. The SDR information for S. purpurea (Zhou, Macaya-Sanz, Carlson, et al. 2020), P. trichocarpa (Zhou, Macaya-Sanz, Schmutz, et al. 2020), P. deltoides, and P. tremula (Müller et al. 2020) comes from recently published studies.