Genomic and Evolutionary Features of the Brown Alga Ectocarpus UV Sex Chromosomes

The genetic determination of sex requires either non-recombining chromosome regions or complete sex chromosomes, both of which have evolved independently and repeatedly across different eukaryotic species. While the processes shaping the evolution of sex chromosomes are increasingly well understood in diploid organisms, haploid sex determination systems (U/V) have been understudied. We have recently sequenced and analysed the sex determining region of the model brown alga Ectocarpus (Ahmed et al. Curr. Biol. 2014). In Ectocarpus, sex is expressed in the gametophyte generation, during the haploid phase of the life cycle, and both the female (U) and the male (V) sex chromosomes contain non-recombining regions. The U and V of this species have been diverging for more than 100 My, yet gene degeneration has been modest, the SDR has remained relatively small with no evidence for evolutionary strata. These features may be explained by the occurrence of strong purifying selection during the haploid phase of the life cycle and low level of sexual dimorphism. The SDR is bordered by two large pseudoautosomal regions (PARs), comprising a large portion of the Ectocarpus sex chromosome. Using experimental and theoretical approaches, we have investigated these unique genomic regions. We show that recombination events are unevenly distributed along the PARs, and reveal the distinct evolutionary features of this region. These include accumulation and accelerated evolution of not only sex-biased genes but, remarkably, of genes differentially expressed during the gametophyte versus sporophyte generation of the life cycle (generation-biased genes). In agreement with the experimental data, our theoretical models predicted that the evolution of the PAR in haploid sex determining systems may be shaped by generation-antagonism, provided that different selection pressures act on males and females.