A selective sweep associated with a recent gene transposition in Drosophila miranda

In Drosophila miranda, a chromosome fusion between the Y chromosome and the autosome corresponding to Muller's element Chas created a new sex chromoso me system. The chromosome attached to the ancestral Y chromosome is transmi tted paternally and hence is not exposed to crossing over. This chromosome. conventionally called the neo-Y, and the homologous neo-X chromosome displ ay many properties of evolving sex chromosomes, We report here the transpos ition of the exuperantial (exu1) locus from a neo-sex chromosome to the anc estral X chromosome of D, miranda. Exu1 is known to have several critical d evelopmental functions, including a male-specific role in spermatogenesis. The ancestral location of exu1 is conserved in the sibling species of D. mi randa, as well as in a more distantly related species. The transposition of exu1 can be interpreted as an adaptive fixation, driven by a selective adv antage conferred by its effect oil 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 evolu tion are discussed.