Achiasmy and sex chromosome evolution

Abstract

Abstract Achiasmy is a phenomenon whereby chiasma (“chromosomal cross–over”) formation is absent during meiosis in one or both sexes. It is one of the strategies that have evolved to cease recombination between sex chromosomes in various taxa. Absence of recombination makes Y (W) chromosome diverge from X (Z) chromosome via natural selection on linked genes. In the present review, we first review the research history of achiasmy in Drosophila species and summarize the prevalence of achiasmy in a wide range of animal taxa and the rapid turnover of achiasmy. We then briefly describe the mechanism of achiasmatic meiosis in Drosophila melanogaster and the evolution of meiotic genes in Drosophila species. Second, we discuss sex chromosome evolution in achiasmatic species. If meiotic recombination had been suppressed between sex chromosomes, heterogametic sex chromosomes have no chance of meiotic recombination. Therefore, the neo–Y chromosome which originated from Y–autosome fusion in achiasmatic species has no chance of recombination and no pre-existing genetic diversity since its origination. On the contrary, if the neo-Y chromosome had chance of recombination, it can maintain ancestral polymorphisms. We compare three different evolutionary trajectories of neo–Y chromosome in Drosophila americana, Drosophila miranda, and Drosophila albomicans. Lastly, we discuss the origin of Y chromosome in achiasmatic species. In particular, we introduce hypotheses proposed in three insect species that achiasmatic meiosis may facilitate the recruitment of an accessory B chromosome to a Y (W) chromosome.