Contrasting patterns of X-linked and autosomal nucleotide variation in Drosophila melanogaster and Drosophila simulans

Surveys of molecular variation in Drosophila melanogaster and Drosophila si mulans have suggested that diversity outside of Africa is a subset of that within Africa. It has been argued that reduced levers of diversity in non-A frican populations reflect a population bottleneck, adaptation to temperate climates, or both. Here, I summarize the available single-nucleotide polym orphism data for both species. A simple "out of Africa" bottleneck scenario is consistent with geographic patterns for loci on the X chromosome but no t with loci on the autosomes. Interestingly, there is a trend toward lower nucleotide diversity on the X chromosome relative to autosomes in non-Afric an populations of D. melanogaster, but the opposite trend is seen in Africa n populations. In African populations, autosomal inversion polymorphisms in D. melanogaster may contribute to reduced autosome diversity relative to t he X chromosome. To elucidate the role that selection might play in shaping patterns of variability, I present a summary of within- and between-specie s patterns of synonymous and replacement variation in both species. Overall , D. melanogaster autosomes harbor an excess of amino acid replacement poly morphisms relative to D. simulans. Interestingly, range expansion from Afri ca appears to have had little effect on synonymous-to-replacement polymorph ism ratios.