Reproductive sexual dimorphisms in two willow species, Salix exigua Nutt. and S. nigra Marshall.

Premise of the Research The prevalence of sexual dimorphisms, which evolve due to contrasting strategies to maximize reproductive success in males and females, is variable among dioecious plant species. In the Salicaceae, many traits have been assessed across many studies, but direct or indirect associations between these traits and reproductive allocation are often neglected. Given the dynamic evolution of sex determination systems and the strong interest in wood production in the family, we wondered whether sexual dimorphisms related to reproduction may have gone unreported. Here, we assess sexual dimorphism in reproductive traits in two species of Salix. Recognition of reproductive sexually dimorphic traits will contribute to understanding the evolution of sex determination systems in the Salicaceae. Methodology We conducted observational studies in natural populations to assess the presence of sexual dimorphisms in early spring bud density, catkin number, and flower number per catkin across four sampling periods in Salix exigua. We also analyzed flower number and catkin number per flower in Salix nigra. Pivotal Results We observed no sexual dimorphism in pre-season buds per branch in S. exigua but did find that males produced more flowers per catkin and more catkins per branch than females in both S. exigua and S. nigra. Conclusions Higher flower numbers in males compared to females is consistent with expectations from intra-sexual selection among males. The presence of reproductive sexual dimorphisms in Salix suggests that sexual selection may affect the evolution of mating strategies in Salix species, and the evolution of the sex determination system within this genus.