Molecular polymorphism in Drosophila melanogaster and D. simulans: what have we learned from recent studies?

We present a review of recent studies of molecular polymorphism in Drosophila melanogaster and D. simulans. The availability of African and non-African samples for these species makes it possible to compare microsatellite and DNA sequence polymorphism between these species, both inside and outside their native regions. There are four main results: (i) variability is larger in African populations; (ii) variation is usually higher on the autosomes, except for African D. melanogaster; (iii) DNA sequence variation is higher on D. simulans than on D. melanogaster autosomes; (iv) the ratio of replacement to silent polymorphisms is higher for D. melanogaster autosomal loci. We summarize the main hypotheses put forward to explain these results.