Evolutionary trajectories of three independent neo-sex chromosomes in Drosophila
Dosage compensation (DC) on the X chromosome is a mechanism to counteract the deleterious effects by gene loss from the Y chromosome. However, DC cannot work efficiently if the X chromosome also degenerates. This indeed occurs in the neo-sex chromosomes in Drosophila miranda, where neo-X as well as neo-Y chromosomes are under accelerated pseudogenization. To examine the generality of this pattern, we investigated the evolution of two additional neo-sex chromosomes that independently emerged in D. albomicans and D. americana and compared their evolutionary processes with that in D. miranda. Comparative genomic and transcriptomic analyses revealed that the pseudogenization rate on neo-X is also accelerated in the two species (though lesser extent in D. americana). We also found that neo-X-linked genes whose neo-Y homologs are pseudogenized tend to be upregulated more stringently than those whose neo-Y homologs remain functional. Moreover, the genes under strong functional constraints and highly expressed in the testis tended to remain functional on neo-X and neo-Y, respectively. Focusing on the D. miranda and D. albomicans neo-sex chromosomes that independently emerged from the same autosome, we further found that the same genes tend to have been pseudogenized in parallel on neo-Y. Those genes include Idgf6 and JhI-26 whose functions seem to be unnecessary or could be even harmful for males. These results indicate that neo-sex chromosomes in Drosophila share a common evolutionary trajectory after their emergence, which may be applicable to other sex chromosomes in a variety of organisms to avoid being an evolutionary dead-end.