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Examination of Allelic Variation at the Hexokinase Loci of DROSOPHILA PSEUDOOBSCURA and D. PERSIMILIS by Different Methods.

Recently a number of electrophoretic techniques have been applied to reveal the presence of additional genetic variation among the electrophoretic mobility classes of the highly polymorphic xanthine dehydrogenase (XDH ) and esterase-5 (est-5) loci. We examined the hexokinase loci of Drosophila pseudoobscura and D. persimilis using a variety of techniques to determine whether further allelic variation could be revealed for these much less polymorphic loci and to analyze the nature of the known variation at the hexokinase-1 (hex-1) locus. The following studies were conducted: 135 strains of the two species from six localities were examined with buffer pH ranging from 5.5 to 10.0; 40 strains of D. pseudoobscura and 9 strains of D. persimilis from Mather were studied using starch gel concentrations ranging from 8.5 to 15.5% and were examined for differences in heat stability and reactivity to the thiol reagent pCMSA; strains were also tested for susceptibility to urea denaturation and differences in relative activities. Major findings of the work are: (1) No additional allelic variation could be detected at any of the hexokinase loci by applying these techniques. The finding of abundant hidden genetic variation in XDH and est-5 does not extend to all enzyme loci. (2) Evidence from studies using pCMSA indicates that the hex-1 alleles 0.6, 0.8, 1.0 and 1.2 of the two species form a series of unit charge steps. Since the 0.94 allele of D. persimilis has mobility intermediate between 0.8 and 1.0, it is argued that routine electrophoretic techniques are sensitive to at least some conservative amino acid substitutions. (3) Strong correlations were found at the hex-1 locus between low allelic frequency, reduced relative activity and reduced stability to heat and urea denaturation. Since the three sibling species, D. pseudoobscura, D. persimilis and D. miranda, all appear to share a common high frequency allele (1.0) at that locus, these findings are taken as evidence that the observed allelic frequencies are a result of directional selection and mutation, rather than any form of balancing selection.