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Abstract

The genetic determination of sex requires either non- recombining chromosome regions or complete sex chromosomes, both of which have evolved indepen- dently and repeatedly across different eukaryotic spe- cies. While the processes shaping the evolution of sex chromosomes are increasingly well understood in diploid organisms, haploid sex determination systems (U/V) have been under-studied. We have recently sequenced and analysed the sex determining region of the brown alga Ectocarpus, where 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 sex-determin- ing regions (SDRs; Ahmed et al. 2014, Current Biology). Here, we use experimental and theoretical approaches to investigate the extensive pseudoauto- somal regions (PARs) that border the SDR on the Ectocarpus sex chromosome. Despite a considerable amount of theoretical work on PAR genetics and evo- lution, these genomic regions have remained poorly characterized empirically, even in classic model organisms. We show that although the PARs of the U/V sex chromosomes of Ectocarpus recombine at a similar rate to autosomal regions of the genome, they exhibit many features typical of non-recombining regions. The pseudoautosomal regions also preferen- tially accumulate sporophyte-biased genes, which tend to occur in physically linked clusters. A model- ling-based approach was used to investigate possible evolutionary mechanisms underlying this enrichment in sporophyte-biased genes. We provide the first detailed analysis of the recombining regions of a hap- loid sex chromosome system and propose a mechan- ism that may explain some of the exceptional evolutionary features of these regions compared with autosomes.