The fate of neo-sex chromosomes in Drosophila albomicans-nasuta hybrid populations

Drosophila albomicans (2n = 6), bearing a pair of neo-sex chromosomes, can be crossed to an allopatric sibling species D. nasuta (2n = 8). We previously proposed a "stepwise chromosome evolution" hypothesis to elaborate 2 stages in the karyotype evolution of D. albomicans derived from the ancestral nasuta-like state. The 1st stage included the formation and maintenance of the fused 3-X chromosome, and the 2nd stage included the fusion of 3-Y and the increased frequency of both fused chromosomes. The 2-stage model is supported by a previous study using experimental populations with either the X- or Y-type sex chromosome fixed. In this study, we attempted to explore the fates of sex chromosomes in populations initially consisting of all 4 types of sex chromosomes (i.e., the derived 3-X and 3-Y and the ancestral X and Y). Our present results revealed that the 3-X chromosome is capable of reaching a high frequency as 3-Y reaches fixation, and the frequency of 1 neo-sex chromosome depends on that of the other in the same population. In another set of experimental populations, the existence of a meiotic driver located on the neo-X chromosome showed no significant effect on the fate of this chromosome. The efficacy of extrapolating these results obtained from hybrid populations to the initial chromosomal dynamics of D. albomicans is discussed. http://zoolstud.sinica.edu.tw/Journals/47.1/84.pdf.