Amino acid polymorphisms for esterase-6 in Drosophila melanogaster.

Abstract

High-resolution electrophoresis has revealed 10 allozymes of esterase-6 (EC 3.1.1.1) in Drosophila melanogaster. The sequences of 13 isolates of the Est6 gene covering all 10 allozymes were obtained and 52 nucleotide differences were found. Sixteen of these cause amino acid replacements, of which three result in charge differences whose size and direction are consistent with the electrophoretic mobilities of the allozymes in which they occur. The smeared electrophoretic phenotype of one allozyme can be explained by the loss of a cysteine residue involved in a disulfide bridge. Several minor mobility variants within the major F and S electrophoretic phenotypes differ by amino acid substitutions that are generally conservative for charge but not for some other properties (size, polarity, or hydrophobicity). Four amino acid differences are found among different isolates of the same allozymes and, overall, 12 amino acid haplotypes occur among the 13 isolates sequenced. Nevertheless, the most common variants within F and S are distinguished by only two amino acids (Asn/Asp at 237 and Thr/Ala at 247), and these are the most likely targets for the selection underlying complementary latitudinal clines in F and S frequencies.Two major difficulties have beset attempts to elucidate the adaptive significance of specific enzyme polymorphisms. The first concerns the lack of information on gametic disequilibrium between the locus of interest and variation at nearby but unknown loci; the second concerns the true amount and nature of molecular polymorphism underlying electrophoretic phenotypes (1). The first difficulty has largely been overcome for the esterase-6 (EST6; carboxylic-ester hydrolase, EC 3.1.1.1) polymorphism ofDrosophila because parallel latitudinal dines in gene frequency occur for the same allozymes in both D. melanogaster and its sibling species D. simulans (2). These species are thought to have diverged 8-10 million years ago (3), suggesting that the Est6 gene itself is probably the unit under selection, as intergenic gametic disequilibrium should have broken down since the divergence of the two species. This paper addresses the second question, concerning the number and nature of polymorphisms for Est6, by comparing the nucleotide sequences of several alleles from D. melanogaster.High-resolution cellulose acetate electrophoresis has shown five major classes of mobility variants for EST6 in D. melanogaster, with seven minor mobility variants within the most common major classes, F (fast) and S (slow) (4). Another seven variants within F and S have been resolved by in vitro thermostability criteria (5), and preliminary analyses of a small number of lines by both electrophoretic and thermostability procedures suggest that the variants detected by the two techniques are not the same (4). While the adaptive significance of the many minor mobility and thermostability variants is unknown, there is biogeographical (2), biochemical (6), and behavioral (7) evidence that the major F and S classes differ under nat...

Molecular evolution of the Est-6 gene in Drosophila melanogaster: contrasting patterns of DNA variability in adjacent functional regions

“…C, at 2372) which leads to a charge-altered amino acid replacement (amino acid position 397, neutral glycine replaced by positively charged arginine). All other detected replacement substitutions have been observed previously (Cooke and Oakeshott, 1989;Hasson and Eanes, 1996). Table 2 shows estimates of nucleotide diversity for the entire data set, and for different haplotype families separately.…”

“…The C estimator of recombination is 0.0023 for the entire sequenced region and 0.0075 for the combined sample of all published Est-6 sequences, including our data set (coding region) (Cooke and Oakeshott, 1989;Hasson and Eanes, 1996). Est-6 is located within subsection 68F7-F8, on the left arm of polytene chromosome 3, a region with high recombination.…”

“…This, together with other data on the temporal and geographic allozyme variation in natural populations and results of laboratory experiments, suggests that the EST-6 polymorphism is maintained by some form of positive selection (reviewed by Oakeshott et al, 1995;Richmond et al, 1990). Cooke and Oakeshott (1989) sequenced the complete coding region of Est-6 in 13 D. melanogaster lines in an Australian population (chosen so as to include all allozyme variants known in the population). They suggested that the main F and S allozymes differed by two amino acids (Asn/ Asp at position 237 and Thr/Ala at position 247) (but see Hasson and Eanes, 1996;Balakirev et al, 1999) and considered these two amino acid replacements as the most likely targets for selection underlying the latitudinal clines previously detected for the F and S allozymes.…”

“…They also detected a distinct peak of increased variation in the region surrounding the replacement site responsible for the EST-6 F/S allozyme polymorphism and suggested that balancing selection might be involved in the polymorphism. All these studies involve samples that are too small (Balakirev et al, 1999), non-random, or both (Cooke and Oakeshott, 1989;Hasson and Eanes, 1996) and thus unsuitable for many population genetic tests.…”

Nucleotide variation in the triosephosphate isomerase (Tpi) locus of Drosophila melanogaster and Drosophila simulans

“…The pattern and level of intraspecific variation at the nucleotide level has now been determined in D. melanogaster for seven loci bearing common allozyme polymorphisms. These are Adh (Kreitman 1983;Kreitman and Hudson 1991), 6Pgd (Begun and Aquadro 1994), G6pd (Eanes, Kirchner, and Yoon 1993;Eanes et al 1996), Gpdh (Takano, Kusakabe, and Mukai 1993), Sod (Hudson et al 1994), Est-6 (Cooke and Oakeshott 1989), and, from this study, Tpi. There is independent evidence for the potential for selection to be acting on all of these polymorphisms.…”