Evolutionary dynamics of sex-biased gene expression in a young XY system: Insights from the brown alga genus Fucus.

• Sex-biased gene expression is considered to be an underlying cause of sexually dimorphic traits. Although the nature and degree of sex-biased expression has been well-documented in several animal and plant systems, far less is known about the evolution of sex-biased genes in more distant eukaryotic groups. • Here we investigate sex-biased gene expression in two brown algal dioecious species, Fucus serratus and Fucus vesiculosus, where male heterogamety (XX/XY) has recently emerged. • We find that in contrast to evolutionary distant plant and animal lineages, male-biased genes do not experience high turnover rates, but instead reveal remarkable conservation of bias and expression levels between the two species, suggesting their importance in sexual differentiation. Genes with consistent male-bias were enriched in functions related to gamete production, along with sperm competition and include three flagellar proteins under positive selection. • We present one of the first reports, outside of the animal kingdom, showing that male-biased genes display accelerated rates of coding sequence evolution compared to female-biased or unbiased genes. Our results imply that evolutionary forces affect male and female sex-biased genes differently on structural and regulatory levels, resulting in unique properties of differentially expressed transcripts during reproductive development in Fucus algae.