Evolution of Sex Chromosomes
Sex chromosomes are originally derived from a pair of autosomes. While sex chromosomes provide stable sex determination to organisms irrespective of their surrounding environments, Y chromosomes often degenerate considerably due to recombination suppression, which could potentially be deleterious to organisms. How sex chromosomes have become one of the major sex determination systems under such a potential disadvantage? To tackle this question, I have been working on Drosophila young sex chromosomes, the so-called neo-sex chromosomes that were originally autosomes but became sex chromosomes by a fusion with ordinary sex chromosomes.
1) Evolution of dosage compensation
Dosage compensation (DC) is a mechanism to equalize gene expression level between sexes as well as between chromosomes. If DC is fine-tuned during the early stage of sex chromosome evolution, pseudogenization of Y-linked genes and upregulation of their X-linked homologs are expected to be highly associated. Indeed, we found that the upregulation of neo-X-linked genes with pseudogenized neo-Y-linked homologs is significantly greater than that of the neo-X-linked genes with functional neo-Y-linked homologs in Drosophila miranda. This suggests that gene-by-gene or localized DC is operating at the initial stage of sex chromosome evolution, which may minimize the deleterious effects of losing many Y-linked genes (Nozawa et al. 2014 MBE; Nozawa et al. 2018 GBE). We are currently testing whether other neo-sex chromosomes in D. albomicans and D. americana also show a similar trend.
2) Pseudogenization process of sex chromosomes
Many genes on the Y chromosomes are known to become pseudogenes, because the Y generally do not exhibit meiotic recombination that is critical to remove deleterious mutations in long-term evolution. However, we also found that in the case of the neo-X chromosome in Drosophila miranda, not only the neo-Y but also the neo-X have been under accelerated pseudogenization [the proportion (the number in parentheses) of pseudogenes is given along the branches] (Nozawa et al. 2016 Nat Commun). We are currently testing whether other neo-sex chromosomes in D. albomicans and D. americana also show a similar trend.