Variation in activity and thermostability of alcohol dehydrogenase in Drosophila melanogaster

Rapid evolutionary change, constraints and the maintenance of polymorphism in natural populations of Drosophila melanogaster

Allele frequencies can shift rapidly within natural populations. Under certain conditions, repeated rapid allele frequency shifts can lead to the long-term maintenance of polymorphism. In recent years, studies of the model insect Drosophila melanogaster have suggested that this phenomenon is more common than previously believed and is often driven by some form of balancing selection, such as temporally fluctuating or sexually antagonistic selection. Here we discuss some of the general insights into rapid evolutionary change revealed by large-scale population genomic studies, as well as the functional and mechanistic causes of rapid adaptation uncovered by single-gene studies. As an example of the latter, we consider a regulatory polymorphism of the D. melanogaster fezzik gene. Polymorphism at this site has been maintained at intermediate frequency over an extended period of time. Regular observations from a single population over a period of 7 years revealed significant differences in the frequency of the derived allele and its variance across collections between the sexes. These patterns are highly unlikely to arise from genetic drift alone or from the action of sexually antagonistic or temporally fluctuating selection individually. Instead, the joint action of sexually antagonistic and temporally fluctuating selection can best explain the observed rapid and repeated allele frequency shifts. Temporal studies such as those reviewed here further our understanding of how rapid changes in selection can lead to the long-term maintenance of polymorphism as well as improve our knowledge of the forces driving and limiting adaptation in nature.

Fitness effects but no temperature-mediated balancing selection at the polymorphic Adh gene of Drosophila melanogaster

What factors maintain genetic variation in natural populations? Opposing selection pressures on protein stability and catalytic activity are thought to maintain variation along thermal gradients in many enzymes. We examined a classic hypothesis of temperature-mediated balancing selection, the alcohol dehydrogenase enzyme of Drosophila melanogaster, in which 2 latitudinally distributed variants are thought to be maintained by an activity/stability trade-off. Using in vitro and in vivo assays and population genetic analyses, we found no evidence of the predicted biochemical or fitness trade-offs and no signature of balancing selection. Rather, one variant confers greater activity and survival in the presence of ethanol, irrespective of temperature. Variation in Adh, and possibly other enzymes, must therefore be caused by other factors correlated with temperature.

Polymorphism in the alcohol dehydrogenase (ADH) protein of Drosophila melanogaster, like genetic variation in many other enzymes, has long been hypothesized to be maintained by a selective trade-off between thermostability and enzyme activity. Two major Adh variants, named Fast and Slow, are distributed along latitudinal clines on several continents. The balancing selection trade-off hypothesis posits that Fast is favored at high latitudes because it metabolizes alcohol faster, whereas Slow is favored at low latitudes because it is more stable at high temperatures. Here we use biochemical and physiological assays of precisely engineered genetic variants to directly test this hypothesis. As predicted, the Fast protein has higher catalytic activity than Slow, and both the Fast protein and regulatory variants linked to it confer greater ethanol tolerance on transgenic animals. But we found no evidence of a temperature-mediated trade-off: The Fast protein is not less stable or active at high temperatures, and Fast alleles increase ethanol tolerance and survivorship at all temperatures tested. Further, analysis of a population genomic dataset reveals no signature of balancing selection in the Adh gene. These results provide strong evidence against balancing selection driven by a stability/activity trade-off in Adh, and they justify caution about this hypothesis for other enzymes except those for which it has been directly tested. Our findings tentatively suggest that environment-specific selection for the Fast allele, coupled with demographic history, may have produced the observed pattern of Adh variation.

THE ASSOCIATION BETWEEN THE POLYMORPHISMS AT THE Adh AND αGpdh LOCI AND THE In(2L)t INVERSION IN DROSOPHILA MELANOGASTER IN RELATION TO TEMPERATURE

Substantial allele-frequency changes were observed at the Adh and αGpdh loci in a seminatural population of Drosophila melanogaster kept in a tropical greenhouse during 1972-1985. Further analysis of the changes at the Adh and αGpdh loci showed that linkage disequilibrium between these loci occurred for a prolonged period due to the presence of In(2L)t, a long inversion on the left arm of the second chromosome. We observed increases in the frequencies of In(2L)t and of short inversions on the left arm of the second chromosome in subpopulations kept at 29.5°C or 33°C. These inversion-frequency increases were accompanied by an increase in Adh^s and a decrease in αGpdh^s frequency. In populations kept at 20°C and 25°C, inversion frequencies decreased, while αGpdh^s allele frequencies decreased at 25°C and increased at 20°C. At 33°C, egg-to-adult survival of individuals possessing In(2L)t, either in the homokaryotypic or the heterokaryotypic state, was higher than that of the other karyotypes of identical allozyme constitution (i.e., Adh^s αGpdh^F ). Thus it seems that In(2L)t has a selective advantage at high temperature. We argue that the observed changes in allele frequencies at the Adh and αGpdh loci are, in part, due to genic selection and are not merely the result of selection acting on the chromosome rearrangements and hitchhiking of the allozymes. The results are discussed with respect to the latitudinal clines found for In(2L)t, Adh, and αGpdh.

Persistence of an alcohol dehydrogenase thermostable variant in a natural population of Drosophila melanogaster

A temporal survey of the alcohol dehydrogenase locus in a natural population ofDrosophila melanogasterfrom the Canary Islands has revealed the existence of an electrophoretic Fast thermostable variant, mapped within 0.8 units of the structuralAdhlocus, and which persists over a 6-year period with a frequency ranging from 0.8 to 0.5%.

The purification and biochemical properties of alcohol dehydrogenase--"fast (Chateau Douglas)" from Drosophila melanogaster

Alcohol dehydrogenase has been purified from Drosophila melanogaster lines bearing the AdhF, AdhS, and AdhFCh.D. alleles. Biochemical investigations show that the properties of the purified enzymes are very similar to those of crude enzyme extracts except that the pure enzymes are more heat stable. ADH-FCh.D. resembles ADH-S very closely in specific activity, substrate specificity, and a number of kinetic parameters including limiting values for Km(app.) for ethanol. However, it is considerably more heat stable than either of the two common variants. ADH-F differs from ADH-S and ADH-FCh.D. particularly with regard to the rate of oxidation of secondary alcohols. Atomic absorbtion spectroscopy shows that all three allozymes lack zinc or other divalent cations as active-site components. Peptide mapping experiments identify one very active cysteinyl residue; and amide residues in the NAD+ binding domain.

Alcohol dehydrogenase thermostability variants in Drosophila melanogaster: comparison of activity ratios and enzyme levels

Representatives of five allozymic classes of Drosophila alcohol dehydrogenase have been compared with respect to their activity levels on two alcohol substrates, quantities of ADH protein, and stability in crude extracts. Within each allozymic class, strains from widely diverse geographic locations differ in their enzyme activity levels but are identical for a measure known as "activity ratio," which is obtained by dividing the average activity reading on isopropanol by that obtained with ethanol. They are also similar in the rate at which ADH activity declines in crude extracts held at 25 degrees C. For several of the fast-resistant and fast-moderate strains, differences in ADH activity are associated with differences in the amount of enzyme present. The catalytic efficiencies of the fast-resistant forms are considerably lower than those of the fast-moderate allozymes. The origin and persistence of the rare but ubiquitous fast-resistant allozyme is discussed.

Selection at the alcohol dehydrogenase locus of Drosophila melanogaster: effects of ethanol and temperature

Drosophila melanogaster larvae were subjected to 10 generations of selection on 6% ethanol at 17, 25, and 30 degrees C. For each temperature there was a significant (P less than 0.01) increase in the frequency of the Adh isoallele. Controls with no ethanol showed no change in the frequency of the AdhF isoallele. Larvae subjected to stronger selection on 8% ethanol confirmed the results. When adults of various ages were subjected to 16 and 32 degrees C, the ADHF isoenzyme retained its twofold advantage in activity over ADHS regardless of the temperature. The same result was obtained with larvae at 16 and 35 degrees C. Although some effect of temperature was demonstrated, it was concluded that the effect was not strong enough for temperature to be a selective factor under the conditions studied. However, ethanol is a strong selective factor for laboratory populations.