Using resources generated by our group as part of the Ectocarpus genome project (Cock et al., 2010 Nature), we identified, mapped and characterized the male and female sex chromosomes. The sex-determining regions of these UV chromosomes share features those of XY and ZW systems but also exhibit specific evolutionary characteristics (Ahmed et al., 2014 Current Biology). In addition, analysis of the pseudo-autosomal (PAR) region of the sex chromosomes identified several unique structural and evolutionary features, including clusters of recently evolved, sporophyte-
expressed genes (Luthringer et al., 2015 Molecular Biology and Evolution). Our results indicated that the evolution of the PAR in haploid systems is influenced by differential selection pressures in males and females acting on alleles that are advantageous during the sporophyte generation of the life cycle. A large RNA-seq dataset was generated to analyse the transcriptional network involved in sex determination and differentiation and we showed that both female- and male-biased genes are evolving rapidly, at least partly as a result of adaptive evolution, and that rates were similar for the two sexes. This contrasts with XY (or ZW) systems, where male (or female)-biased genes evolve more rapidly. The patterns of sex-biased gene expression in Ectocarpus are consistent both with predicted characteristics of UV (haploid) sexual systems and with the distinctive aspects of this organism’s reproductive biology, that involves separate sexes at the haploid stage of the life cycle and low levels of sexual dimorphism. More recently, these analyses have been extended to additional brown algal species to understand sex chromosome evolution and sex differentiation across this group. This work has provided the first analysis of gene traffic in a haploid UV system and has identified several features of general relevance to the evolution of sex chromosomes (Lipinska et al., 2017 Genome Biology). Sex-markers for kelps are being used as valuable tools for algal cultivation practices and algal breeding programs.
Our further analysis using more brown algal sex chromosomes will not only reveal the fundamental forces that shape sex chromosome evolution in the scope of the tree of life, but will also uncover the mechanisms underlying important evolutionary transitions between major reproductive and life cycle modes and shed new light on the origin and evolution of the sexes.
