Abstract
Three types of sex chromosome system exist in nature: diploid XY and ZW systems and haploid UV systems. For many years, research has focused exclusively on XY and ZW systems, leaving UV chromosomes and haploid sex determination largely neglected. Here, we perform a detailed analysis of DNA sequence neutral diversity levels across the U and V sex chromosomes of the model brown alga Ectocarpus using a large population dataset. We show that the U and V non-recombining regions of the sex chromosomes (SDR) exhibit about half as much neutral diversity as the autosomes. This difference is consistent with the reduced effective population size of these regions compared with the rest of the genome, suggesting that the influence of additional factors such as background selection or selective sweeps is minimal. The pseudoautosomal region (PAR) of this UV system, in contrast, exhibited surprisingly high neutral diversity and there were several indications that genes in this region may be under balancing selection. The PAR of Ectocarpus is known to exhibit unusual genomic features and our results lay the foundation for further work aimed at understanding whether, and to what extent, these structural features underlie the high level of genetic diversity. Overall, this study fills a gap between available information on genetic diversity in XY/ZW systems and UV systems and significantly contributes to advancing our knowledge of the evolution of UV sex chromosomes.
Highlights
Distinct sex chromosomes have evolved multiple times independently in both plants and animals [1]
Knowledge of the exact position of the borders of the SDR and pseudoautosomal region (PAR) is important in the context of this study because theoretical models predict increased neutral diversity at the borders of the SDR and PAR regions [13,17]
To conduct the population genetic tests on the different genomic compartments, we needed to confirm that sister species E. siliculosus and Ectocarpus 7 shared the same PAR-SDR boundary
Summary
Distinct sex chromosomes have evolved multiple times independently in both plants and animals [1]. Sex chromosome evolution has been mainly studied in male-heterogametic (XX/XY) and female heterogametic (ZZ/ZW) sex determination systems. A typical sex chromosome pair derives from a pair of autosomes through the acquisition of genes involved in sex determination. If more than one locus involved in sex determination is located on the chromosome, recombination between these loci is expected to be suppressed, leading to the establishment of a non-recombining. Genes 2018, 9, 286 region on the nascent sex chromosome, the sex-determining region or SDR. The formation of this non-recombining region has important consequences for the evolution of this part of the genome. Repetitive DNA can accumulate, leading to an increase in SDR size (see review by Bachtrog [1])
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