POLYMORPHISM AND DIVERGENCE AT THE PRUNE LOCUS IN DROSOPHILA-MELANOGASTER AND DROSOPHILA-SIMULANS

The prune locus of Drosophila melanogaster lies at the tip of the X ch romosome, in a region of reduced recombination in which nearby loci sh ow reduced variation relative to evolutionary divergence from D. simul ans. DNA sequencing of prune alleles from D. melanogaster and D. simul ans reveals extremely low variation in D. melanogaster but greater var iation in D. simulans. Divergence between the two species is not reduc ed. This pattern may be explained by either positive selection leading to hitchhiking of neutral variation or background selection against d eleterious mutations. The pattern of silent versus replacement polymor phism and divergence at prune is consistent with either a model of wea kly deleterious selection against amino acid substitutions or balancin g selection.