Evolution of sex chromosomes in seed plants.

In seed plants (angiosperms and gymnosperms), sex is typically determined in the diploid sporophyte phase of the life cycle by sex chromosomes. This contrasts with bryophytes (mosses, liverworts, and hornworts), in which sex chromosomes act in the haploid gametophyte, whereas vascular non-seed plants (ferns and their allies) are not currently known to possess sex chromosomes. In seed plants, sex is usually controlled by one or several dominant factors in a non-recombining region. The causes of recombination suppression remain debated, but its consequences are increasingly well understood. These include X-Y (or Z-W) divergence, reduced effective population size, enhanced drift, degeneration of non-recombining regions through amino acid substitutions, pseudogenization, gene loss, and transposable-element accumulation. We first review these consequences and then consider their downstream effects on rates of molecular evolution, gene expression, dosage compensation, and reproductive isolation. Although many features of seed-plant sex chromosomes parallel those of animals, seed-plant systems often remain homomorphic and may experience slower degeneration, possibly because selection during the haploid phase and pollen competition help maintain Y-linked gene function. The repeated origins, wide age range, and varied dynamics of seed-plant sex chromosomes make them especially valuable models for understanding their evolution and how recombination suppression affects the genome.