Sexual dimorphism in the dioecious willow Salix purpurea

Authors: Gouker, Fred; CARLSON, CRAIG - Cornell University; ZOU, JUNZHU - Cornell University; EVANS, LUKE - University Of Colorado; CROWELL, CHASE - Cornell University; SMART, CHRISTINE - Cornell University; DIFZIO, STEPHEN - West Virginia University; SMART, LAWRENCE - Cornell University

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2021
Publication Date: 8/18/2021
Citation: Gouker, F.E., Carlson, C.H., Zou, J., Evans, L.M., Crowell, C.R., Smart, C.D., Difzio, S.P., Smart, L.B. 2021. Sexual dimorphism in the dioecious willow Salix purpurea. American Journal of Botany. https://doi.org/10.1002/ajb2.1704.
DOI: https://doi.org/10.1002/ajb2.1704

Interpretive Summary: While sexual dimorphism in animals is common, sexual dimorphism in dioecious plant species is much less obvious or is absent. The short generation time of Salix (willow species) makes this genus suitable for studying sexual dimorphism and its genetic control in plants. The purple osier willow, Salix purpurea, is a naturalized species in North America and an important species in breeding shrub willow bioenergy crops in North America as it has been used in over 30% of all intra- and interspecific hybrids produced to date. Scientists at Cornell University evaluated the effects of sexual dimorphism in S. purpurea and progeny by comparing pest and disease resistance, drought tolerance, nitrogen and water use efficiency, and biomass yield. Results provide the first evidence of sexual dimorphism in S. purpurea for secondary sexual characteristics where male plants were generally larger in morphological traits and had greater biomass. Also, this study confirms the prevalence of female-biased sex ratios previously found in other Salix species and suggests that this phenomenon may be more genetically controlled and less influenced by environment, which has immediate implications for breeding of this crop.

Technical Abstract: While sexual dimorphism in animals is common, sexual dimorphism in dioecious plant species is much less obvious or is absent. When quantified, it has been shown to arise from differences in reproductive investments. Dioecious species populations often exhibit deviations from expected sex ratios, but little is known about why this occurs. Previous studies on members of the Salicaceae family have shown strong, partial, and no sexual dimorphism. Some studies have shown sex-biased ratios in several Salix spp., however, S. purpurea has never been examined for evidence of sexual dimorphism in non-reproductive traits or for the presence of sex-ratio bias, and therefore a comprehensive phenotypic study is needed to fill this gap in our knowledge of this emerging bioenergy crop species. This study examined a suite of morphological, phenological, physiological and wood composition traits from multi-environment and multi-year replicated field trials in a diversity panel of unrelated S. purpurea accessions and in full-sib F1 and F2 families produced through controlled cross pollinations to test for sexual dimorphism and sex ratio bias. Significant evidence of sexual dimorphism was found in vegetative traits with greater means for many traits in male genotypes compared to females across three populations of S. purpurea, including many of the morphological phenotypes measured across multiple years that were highly predictive of biomass yield. Male plants exhibited greater nitrogen accumulation under fertilizer amendment as measured by SPAD in the diversity panel, and male genotypes showed greater susceptibility to fungal infection by Melampsora spp in the F2 family. There were also consistent female-biased sex ratios in both the F1 and F2 families. These results provide the first evidence of sexual dimorphism in S. purpurea for secondary sexual characteristics where male plants were generally larger in morphological traits and had greater biomass, suggesting differences in the allocation of resources in males and females. Also, this study confirms the prevalence of female-biased sex ratios previously found in other Salix spp. and suggests that this phenomenon may be more genetically controlled and less influenced by environment.