Recent Gene Capture By The UV Sex Chromosomes Of Ceratodon purpureus

Non-recombining sex chromosomes differ from autosomes by having decreased gene density, and the genes that are present are often specialized to function in the sex carrying the non-recombining chromosome. The pronounced gene loss from Y-chromosomes of mammals and Dipterans is a consequence of the combined effects of (i) suppressed recombination, which reduces the efficacy of purifying selection, (ii) heterozygosity, since recessive deleterious mutations on the Y, including gene loss, are sheltered by the X chromosome in males, and (iii) transmission exclusively through males, reducing the effective population size and leading to the chromosome retaining few genes other than ones with male functions. In the sex chromosomes of bryophytes, which are haploid plants, the effects of these three processes can be distinguished, because recombination is suppressed on both the female-transmitted (U) sex chromosome and the male-transmitted (V) chromosome. Genes on both chromosomes therefore should show the deleterious effects of suppressed recombination and sex-limited transmission, while purifying selection should maintain homologs of genes essential for both sexes on both sex chromosomes. Here we use a combination of genetic mapping and male and female transcriptomic data from the moss Ceratodon purpureus to test these predictions.