Abstract
In Drosophila miranda, a chromosome fusion between the Y chromosome and the autosome corresponding to Muller's element C has created a new sex chromosome system. The chromosome attached to the ancestral Y chromosome is transmitted paternally and hence is not exposed to crossing over. This chromosome, conventionally called the neo-Y, and the homologous neo-X chromosome display many properties of evolving sex chromosomes. We report here the transposition of the exuperantia1 (exu1) locus from a neo-sex chromosome to the ancestral X chromosome of D. miranda. Exu1 is known to have several critical developmental functions, including a male-specific role in spermatogenesis. The ancestral location of exu1 is conserved in the sibling species of D. miranda, as well as in a more distantly related species. The transposition of exu1 can be interpreted as an adaptive fixation, driven by a selective advantage conferred by its effect on dosage compensation. This explanation is supported by the pattern of within-species sequence variation at exu1 and the nearby exu2 locus. The implications of this phenomenon for genome evolution are discussed.
