Slow evolution of sex-biased genes in the reproductive tissue of the dioecious plant Salix viminalis

The relative rate of evolution for sex-biased genes has often been used as a measure of the strength of sex-specific selection. In contrast to studies in a wide variety of animals, far less is known about the molecular evolution of sex-biased genes in plants, particularly in dioecious angiosperms. Here, we investigate the gene expression patterns and evolution of sex-biased genes in the dioecious plant Salix viminalis. We observe lower rates of sequence evolution for male-biased genes expressed in the reproductive tissue compared to unbiased and female-biased genes. These results could be partially explained by the lower codon usage bias for sex-biased genes leading to elevated rates of synonymous substitutions compared to unbiased genes. However, the stronger haploid selection in the reproductive tissue of plants, together with pollen competition, would also lead to higher levels of purifying selection acting to remove deleterious variation. Future work should focus on the differential evolution of haploid- and diploid-specific genes in order to understand the selective dynamics acting on these loci.