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Abstract:
Brown algae (Phaeophyceae) are exceptional models for investigating the origins and evolution of sex chromosomes, representing a diverse array of reproductive systems and life cycle types within a single taxonomic group. Majority of brown algae are characterized by haploid-diploid life cycles and a haploid U/V sex determination system. Although U/V, X/Y and Z/W systems share many common features, there are important differences between them that have broad evolutionary and genomic implications. This study explores the evolution of the U/V sex chromosomes within brown algae, an eukaryotic lineage that has independently evolved from plants and animals for billions of years. In contrast to syntenic and conserved autosomal sequences,the U/V sex chromosomes have exhibited structural dynamism since the emergence of the sex-determining region (SDR) more than 250 million years ago, hosting only seven genes. The SDR region underwent independent expansion in different algal species through nested inversions, acquiring neighboring genes from the pseudosomal regions, but also from autosomes through gene translocations. Remarkably, the dynamic evolution of SDR size aligns with the degree of gamete sexual dimorphism, with oogamous species displaying systematically larger SDRs. Intriguingly, the proportion of sex-biased gene expression did not correlate with species sexual dimorphism or the SDR size, but a significant accumulation of male-biased genes in pseudoautosomal regions on the sex chromosome was noted. Our findings contribute to a comprehensive model delineating the evolutionary trajectory of U/V sex chromosomes in brown algae.
