NUCLEOTIDE VARIATION AT THE HYPERVARIABLE ESTERASE 6 ISOZYME LOCUS OFDROSOPHILA-SIMULANS

Esterase 6 (Est-6/EST6) is polymorphic in both Drosophila melanogaster and D. simulans for two common allozyme forms, as well as for several other less common variants. Parallel latitudinal dines in the frequen cies of the common EST6-F and EST6-S allozymes in these species have p reviously been interpreted in terms of a shared amino acid polymorphis m that distinguishes the two variants and is subject to selection. Her e we compare the sequences of four D. simulans Est-6 isolates and show that overall estimates of nucleotide heterozygosity in both coding an d 5' flanking regions are more than threefold higher than those obtain ed previously for this gene in D. melanogaster. Nevertheless, the rati o of replacement to exon silent-site polymorphism in D. simulans is le ss than the ratio of replacement to silent divergence between D. simul ans and D. melanogaster, which could be the result of increased effici ency of selection against replacement polymorphisms in D. simulans or to divergent selection between the two species. We also find that the amino acid polymorphisms separating EST6-F and EST6-S in D. simulans a re not the same as those that separate these allozymes in D. melanogas ter, implying that the shared dines do not reflect shared molecular ta rgets for selection. All comparisons within and between the two specie s reveal a remarkable paucity of variation in a stretch of nearly 400 bp immediately 5' of the gene, indicative of strong selective constrai nt to retain essential aspects of Est-6 promoter function.