Sex-biased gene expression in Fucus

The sex-dependent regulation of gene expression is considered the underlying cause of often extensive, sexually dimorphic traits between males and females. Although the nature and degree of sex-biased expression has been well-documented in several animal systems, including humans, far less is known about the commonality, conservation, recruitment mechanisms and evolution of sex-biased genes in more distant eukaryotic groups. Brown algae are of particular interest for empirical studies of sex-biased gene expression evolution, because they have been evolving independently from animals and plants for over 1 billion years. Here we focus on two dioecious species of Fucus serratus and Fucus vesiculosus, which recently developed male heterogamety (XX/XY) after transition from haploid-diploid to fully diplontic life cycle. We study sex-biased gene expression and discuss different evolutionary forces responsible for the evolution of sex-biased genes. We find that both species evolved masculinized transcriptomes, with sex-biased genes allocated mainly to male reproductive tissue and nearly absent in vegetative tissues. Conserved male-biased genes are enriched in functions related to gamete production, along with sperm competition and include two flagellar proteins under positive selection. In contrast to female-biased genes which show high turnover rates, male-biased genes reveal remarkable conservation of bias and expression levels between the two species. Similarly to other XY systems, male-biased genes also display accelerated rates of coding sequence evolution compared to female-biased or unbiased genes. Our results imply that evolutionary forces are acting differently on male and female sex-biased genes on structural and regulatory levels and suggest that sex-biased gene expression evolves during the transition to dioecy to resolve intralocus sexual conflict arising from anisogamy.