Extent of genetic variation at a highly polymorphic esterase locus in Drosophila pseudoobscura.

Abstract

The esterase-5 locus of Drosophila pseudoobscura has been screened for variation by using a wide variety of electrophoretic conditions and by testing heat sensitivity.Among 50 isogenic lines from 17 popua ions, 21 genetic variants were found at the locus, 18 by electrophoresis and 3 by heat denaturation. Including alleles previously known, there are a total of 30 alleles at this locus in natural populations. This represents a doubling of the previous estimates of polymorphism at this locus. The study confirms that, in contrast to monomorphic loci, loci judged polymorphic under one detection procedure show large increases in polymorphism when examined extensively.The attempt to give a satisfactory explanation for the large amount of genetic variation now known to exist for structural gene loci coding for enzymes and soluble proteins in most organisms has largely been frustrated by a lack of correspondence between the variables of population genetic theory and the observed variables from experiments. Population genetic theory is constructed in terms of the frequencies of alternative alleles, the rates of origin and loss of these alleles by mutation and random sampling events, and the fitnesses of genotypes specified by their allelic composition. Observations, however, have been of phenotypic classes distinguished from each other almost entirely by gel electrophoresis carried out in a very restricted range of conditions. While these phenotypic classes, called electromorphs by King and Ohta (1) much in D. persimilis (4). In contrast, the nearly monomorphic locus for octanol dehydrogenase, Odh, showed only a slight increase in variation in both species, the new alleles also being low in frequency (4). The virtually completely monomorphic locus for malic dehydrogenase, Mdh, showed no change whatever (only D. pseudoobscura was tested) (4), and Beckenbach and Prakash (5) report a lack of hidden variation at the hexokinase locus in D. pseudoobscura and D. persimilis. Thus, there was some suggestion that more intensive examination of allelic differences using heat denaturation and different conditions of electrophoresis may simply produce a roughly proportional increase in the genetic variation already observed by simple methods. It therefore became important to use our newer methodology to survey what had previously been by far the most polymorphic locus in D. pseudoobscura and D. persimilis, esterase-5, a locus coding for a dimeric a-esterase of unknown physiological function. Esterase loci are highly polymorphic in most species of plants and animals surveyed (6). In early surveys of allozyme polymorphism, the esterase-5 locus was shown to have 12 electromorphs in D. pseudoobscura, with an average heterozygosity within populations of 0.75 (7), while in D. persimilis there were 6 electromorphs with an average heterozygosity of 0.45 (8). Moreover, Cobbs (9) has found three different alleles that produce proteins that either fail to dimerize or do so inefficiently. In addition, McDowell and Prakash (10) (ii) 99 es...

Sea Urchin Dna Sequence Variation and Reduced Interspecies Differences of the Less Variable Dna Sequences

“…With the standard technique they used, they may have seen only a fraction of the total variants (Coyne et al, 1978). Nevertheless it is completely clear that the coding sequence differences leading to amino acid replacement in even highly polymorphic proteins share only the smallest part of the single copy DNA variation.…”

“…No estimate of silent differences comes from these measurements. The total variation (Quid be somewhat larger since the technique detects only a fraction of the total amino acid differences (Coyne et al, 1978).…”

An experimental study of neutrality at the malic dehydrogenase and esterase-5 loci in Drosophila pseudoobscura

“…The observed unevenness of electromorph frequencies (COYNE et al 1978), however, puts this explanation as well as other explanations of balancing selection into serious difficulties . We, thus, have a paradox.…”

“…This simple heuristic model of random drift of neutral or nearly-neutral mutations within isofitness alleles of a balanced polymorphism needs to be quantified in order to test its predictions against predictions of the neutral and nearly-neutral models of classical or monomorphic loci. By providing a mechanism for reaching a neutral-theory equilibrium, this model may explain better the great number of rare electromorphs revealed at Group I1 loci (COYNE et al 1978) as well as the greater dichotomy between Group I and Group I1 loci revealed by variable electrophoresis conditions (COYNE and FELTON 1977;BECKENBACH and PRAKASH 1977) than do neutrality models of monomorphic loci.…”

“…When functional and electromorphic heterozygosities are combined, the differences between peripheral and central populations are cancelled, yielding uniform nonspecific total electromorphic heterozygosities in different populations throughout the species' range. These predictions can be tested, for example, with electrophoresis of variable conditions (e.g., COYNE et al 1978) on samples from peripheral and central populations.…”