HISTORICAL SELECTION, AMINO-ACID POLYMORPHISM AND LINEAGE-SPECIFIC DIVERGENCE AT THE G6PD LOCUS IN DROSOPHILA-MELANOGASTER AND DROSOPHILA-SIMULANS

The nucleotide diversity across 1705 bp of the G6pd gene is studied in 50 Drosophila melanogaster and 12 D. simulans lines. Our earlier repo rt contrasted intraspecific polymorphism and interspecific differences at silent and replacement sites in these species. This report expands the number of European and African lines and examines the pattern of polymorphism with respect to the common A/B allozymes. In D. melanogas ter the silent nucleotide diversity varies 2.8-fold across localities. The B allele sequences are two- to fourfold more variable than the de rived A allele, and differences between allozymes are twice as among B alleles. There is strong linkage disequilibrium across the G6pd regio n. in both species the level of silent polymorphism increases from die 5' to 3' ends, while there is no comparable pattern in level of silen t site divergence or fixation. The neutral model is not rejected in ei ther species. Using D. yakuba as an outgroup, the D, melanogaster line age shows a twofold greater rate of silent fixation, but less than hal f die rate of amino acid replacement. Lineage-specific differences in mutation fixation ar-e inconsistent with neutral expectations and sugg est the interaction of species-specific population size differences wi th both weakly advantageous and deleterious selection.