Reduced sequence variability on the neo-Y chromosome of Drosophila americana americana

Sex chromosomes are generally morphologically and functionally distinct, bu t the evolutionary forces that cause this differentiation are poorly unders tood. Drosophila americana americana was used in this study to examine one aspect of sex chromosome evolution, the degeneration of nonrecombining Ychr omosomes. The primary X chromosome of D. a. annericana is fused with a chro mosomal element that was ancestrally an autosome, causing this homologous c hromosomal pair to segregate with the sex chromosomes. Sequence variation a t the Alcohol Dehydrogenase (Adh) gene was used to determine the pattern of nucleotide variation on the neo-sex chromosomes in natural populations. Se quences of Adh were obtained for neo-X and neo-Y chromosomes of D, a. ameri cana, and for Adh of D, a. texana, in which it is autosomal. No significant sequence differentiation is present between the neo-X and neo-Y chromosome s of D. a. americana or the autosomes of D. a. texana. There is a significa ntly lower level of sequence diversity on the neo-Y chromosome relative to the neo-X in D. a. americana. This reduction in variability on the neo-Y do es not appear to have resulted from a selective sweep. Coalescent simulatio ns of the evolutionary transition of an autosome into a Y chromosome indica te there may be a low level of recombination between the neo-X and neo-Y al leles of Adh. and that the effective population size of this chromosome may have been reduced below the expected Value of 25% of the autosomal effecti ve size, possibly because of the effects of background selection or sexual selection.