Regulation of sn-glycerol-3-phosphate dehydrogenase in Drosophila melanogaster larvae by dietary ethanol and sucrose

Quantitative trait loci for response to ethanol in an intercontinental set of recombinant inbred lines of Drosophila melanogaster

Alcohol, a drug widely abused, impacts the central nervous system functioning of diverse organisms. The behavioral responses to acute alcohol exposure are remarkably similar among humans and fruit flies. In its natural environment, rich in fermentation products, the fruit fly Drosophila melanogaster encounters relatively high levels of ethanol. The effects of ethanol and its metabolites on Drosophila have been studied for decades, as a model for adaptive evolution. Although extensive work has been done for elucidating patterns of genetic variation, substantially less is known about the genomic regions or genes that underlie the genetic variation of this important trait. To identify regions containing genes involved in the responses to ethanol, we used a mapping population of recombinant inbred (RIL) lines to map quantitative trait loci (QTL) that affect variation in resistance and recovery from ethanol sedation in adults and ethanol resistance in larvae. We mapped fourteen QTL affecting the response to ethanol on the three chromosomes. Seven of the QTL influence the resistance to ethanol in adults, two QTL are related to ethanol-coma recovery in adults and five affect the survival to ethanol in larvae. Most of the QTL were trait specific, suggesting that overlapping but generally unique genetic architectures underlie each trait. Each QTL explained up to 16.8% of the genetic variance among lines. Potential candidate loci contained within our QTL regions were identified and analyzed.

Direct evidence that genetic variation in glycerol-3-phosphate and malate dehydrogenase genes (Gpdh and Mdh1) affects adult ethanol tolerance in Drosophila melanogaster

Many studies of alcohol adaptation in Drosophila melanogaster have focused on the Adh polymorphism, yet the metabolic elimination of alcohol should involve many enzymes and pathways. Here we evaluate the effects of glycerol-3-phosphate dehydrogenase (Gpdh) and cytosolic malate dehydrogenase (Mdh1) genotype activity on adult tolerance to ethanol. We have created a set of P-element-excision-derived Gpdh, Mdh1, and Adh alleles that generate a range of activity phenotypes from full to zero activity. Comparisons of paired Gpdh genotypes possessing 10 and 60% normal activity and 66 and 100% normal activity show significant effects where higher activity increases tolerance. Mdh1 null allele homozygotes show reductions in tolerance. We use piggyBac FLP-FRT site-specific recombination to create deletions and duplications of Gpdh. Duplications show an increase of 50% in activity and an increase of adult tolerance to ethanol exposure. These studies show that the molecular polymorphism associated with GPDH activity could be maintained in natural populations by selection related to adaptation to alcohols. Finally, we examine the interactions between activity genotypes for Gpdh, Mdh1, and Adh. We find no significant interlocus interactions. Observations on Mdh1 in both Gpdh and Adh backgrounds demonstrate significant increases in ethanol tolerance with partial reductions (50%) in cytosolic MDH activity. This observation strongly suggests the operation of pyruvate-malate and, in particular, pyruvate-citrate cycling in adaptation to alcohol exposure. We propose that an understanding of the evolution of tolerance to alcohols will require a system-level approach, rather than a focus on single enzymes.

Does Drosophila melanogaster use ethanol as an energy source during starvation?

The influence of starvation on activities of three enzymes (ADH, ODH and alpha GPDH) was studied in Drosophila melanogaster. The changes were compared in two inbred lines which had different allelic combinations at the Odh and Aldox loci. We also studied the effect of ethanol on media which contained no sucrose ("starvation conditions"). The results show that there are large differences in the larval and adult alcohol utilization. The alcohol content of the medium, in the absence of sugar, appeared to be toxic for the larvae, while the adults appeared to utilize it as an energy source. The two strains differed little in their responses to starvation or to the ethanol treatment applied under starvation conditions. We conclude that the degree of toxicity of ethanol is highly dependent on the presence of sucrose.

The influence of the Odh-Aldox region of the third chromosome on the response of Drosophila melanogaster to environmental alcohol

Second instar larvae of Drosophila melanogaster were exposed to exogenous alcohol, which is known to influence the activities of several enzymes. In this study, the activity changes were followed in four enzymes (ADH, ODH, alpha GPDH and AOX) during ethanol exposure and compared in three inbred lines that had different allelic combinations at the Odh and Aldox loci. The results indicate that the Odh-Aldox region of the third chromosome may alter the general response to ethanol. The activity of ADH increased considerably in two strains in the larval stages in the presence of alcohol; nevertheless, strain 1, with the OdhS-AldoxF allelic combination, showed a delay in the ADH induction compared to strain 2, which had the OdhF-AldoxS combination. In strain 3 (OdhS*-AldoxS) larvae, ADH induction by environmental ethanol was not detected. Moreover, the activities of alpha GPDH and AOX in strains 2 and 3 were not affected by ethanol. In contrast, the activities of all four enzymes in strain 1 changed after exposure to ethanol.

Differences in the effect of ethanol on fertility and viability components among laboratory strains of Drosophila melanogaster

We studied the effect of ethanol on several fitness components in six Drosophila melanogaster strains. Mating success, fecundity, egg-to-larva, egg-to-pupa and egg-to-adult survival and the number of emerging adults were estimated in a single series of experiments. The strains either had different combinations of genetic background and Adh genotypes with identical OdhF genotype or different Adh-Odh two-locus genotypes with similar genetic background. Ethanol had the greatest effect on mating success and fecundity, while its influence was lower on survival. When the experimental conditions were contrasted to the natural environment of the flies the most significant results were the ones related to fecundity and larval survival. Ethanol had the highest selective effect on fecundity. The genetic factors contributed substantially to the variation in the fertility and viability components. The Adh locus hardly influenced mating success while it had a sizable effect on fecundity and on all survival components. The influence of Adh on fecundity greatly depended on the other genetic factors. Genetic background had the largest influence on the different survival components. The influence of the Odh locus was mostly observed through the Adh-Odh interaction.

Stress tolerance and metabolic response to stress in Drosophila melanogaster

A potentially important physiological response to stress may be alteration in the gross regulation of energy metabolism. Different genotypes may respond differently to environmental stress, and the variation in these norms of reaction may be of key importance to the maintenance of genetic variation in metabolic traits. In the study reported here, a set of genetically defined lines of Drosophila melanogaster were exposed to four stresses (acetic acid, ethanol, starvation and thermal stress) in order to assess the magnitude of environmental effects and genotype x environment interactions. In addition to scoring metabolic traits, distributions of survival times under each stress were also quantified. Although both metabolic traits and survival times exhibited strong differences among genotypes, the correlations between enzyme traits and survival were generally weak. Many of the genetic correlations exhibit significant heterogeneity across environments. The results suggest that transient environmental stress may play an important role in the evolution of this highly intercorrelated set of metabolic traits.

Stress and metabolic regulation in Drosophila

Environmental changes that result in stress (defined here as decreased absolute viability and/or fecundity) result in extrinsic changes in metabolism that are to some extent compensated by altered gene expression. The fact that different genotypes may respond differently to environmental stress may be of key importance to the maintenance of genetic variation in metabolic traits. Here we quantify a set of metabolic characters in genetically defined lines of Drosophila melanogaster subjected to four stresses (3% acetic acid, 3% ethanol, starvation and thermal stress) in order to assess the magnitude of environmental effects and genotype x environment interactions. Genetic correlations were quantified, and many exhibit significant heterogeneity across environments. Pleiotropically related traits may exhibit the phenomenon of apparent selection, whose effects may be particularly strong in stressful environments. This transient apparent selection may have a large consequence on the maintenance of genetic variation.

Strains of Drosophila melanogaster differ in alcohol tolerance

The influence of environmental ethanol on different fitness components and the larval activities of some enzymes were studied in three strains of Drosophila melanogaster. All three strains carried the AdhS-alphaGpdhF allele combination on their second chromosomes while they had unique allele combinations at the Odh and Aldox loci on their third chromosomes (strain 1: OdhS-AldoxF; strain 2: OdhF-AldoxS; strain 3: OdhS-AldoxS). Normal lines and exposure lines, kept on 5% ethanol supplemented medium for at least 20 generations, were established from each strain and the responses of the two lines to different ethanol concentrations were compared. Two survival components were estimated in the juvenile life history stages. In addition, the weights of the emerging adult males were measured at various concentrations of ethanol. The changes in the activities of two enzymes (ADH and alpha GPDH) were also surveyed in the larvae after the different ethanol treatments. Strain-specific differences were observed in the responses of all investigated traits to ethanol. OdhS-AldoxF larvae seemed to be more tolerant to ethanol than the larvae of the other two strains while the utilisation of ethanol as energy source appeared to be the least effective in this strain. Larvae of the exposure lines had significantly higher tolerance to ethanol, and the adult males were heavier, than the ones from the normal lines. The enzymatic responses of the two lines to the ethanol treatments were also different. ADH activity, fresh male weight, and pupa-to-adult survival seemed only to be associated under short-term exposure to ethanol. Ethanol tolerance appeared to be independent of the utilisation of ethanol in the larva-to-pupa stage.

Differences in environmental temperature, ethanol and sucrose associated with enzyme activity and weight changes in Drosophila melanogaster

Activity changes of three enzymes (ADH, ODH and AOX) of Drosophila melanogaster were followed under different environmental conditions. The influences of ethanol, starvation (no carbohydrates in the medium) and ethanol stress during starvation were studied at both 18 and 26 degrees C. Two strains that were monomorphic for different alleles at the Odh and Aldox loci but otherwise identical were used. The investigated environmental conditions affected ADH induction by exogenous ethanol differently in the two strains. The different allozymes of ODH and AOX also responded differently to the treatments. We observed that the sucrose content of the medium on which ethanol exposure took place and the temperature strongly affected the responses within any single strain. Correlations were estimated among the three enzymes in the larval and adult stages of each strain separately. At both temperatures, differences between strains were observed in the patterns of associations of the response variables, in the larval, but not in the adult stages.

Enzyme polymorphism in Drosophila melanogaster populations collected in two different habitats in Hungary

The level of enzyme polymorphism was compared in ten Drosophila melanogaster populations collected in farmyards and distilleries in two regions of Hungary. The total genetic diversity was partitioned into between- and within-population components at each investigated locus using Wright's F-statistics. Population differentiation was studied in two different ways. Genetic distances between pairs of populations were calculated and a hierarchical analysis of gene diversity was performed. Based on the F values gene flow was estimated among the populations at different levels of the hierarchy. The results indicated that our 'farmyard populations' collected within a region could be considered as parallel samples from a panmictic population rather than samples of distinct populations. In distilleries, the flies might be influenced by two different evolutionary forces: (i) selection due to the extremely high concentration of ethanol in the fermenting mash and (ii) genetic drift due to the combination of repeated founder effects and fluctuating population size. Our results suggested that 'distillery populations' could not be regarded as real populations either. They could be considered as peculiar cases: founder individuals taken from the total population (region) established special populations which survived in the distilleries for many generations. Thus the dominating force acting on the 'distillery populations' was genetic drift.

Physiological genetics of the response to a high-sucrose diet by Drosophila melanogaster

A diet medium containing 10% (w/v) sucrose can be inferred to be stressful to Drosophila melanogaster from the increased developmental time and reduced size and fecundity of emerging flies. The metabolic basis for this stress and the genetic response to it are of interest from the point of view of both metabolic regulation and the evolutionary genetics of adaptation to stress. Here the effects of a high-sucrose diet on live weight, total protein, stored lipid and glycogen, and crude activities of 12 enzymes involved in energy metabolism were quantified. Assays were done on a large population of Drosophila that had been acclimated to the laboratory. A collection of eggs was divided to produce two replicate populations maintained on standard medium and two replicates maintained on high-sucrose medium for 133 generations. At the end of this period, both control and sucrose-selected populations were tested on standard and on high-sucrose medium. Results showed that the immediate effect of the high-sucrose diet (compared to standard medium) for both populations was a reduction in live weight and total protein, and activities of many of the enzymes were also reduced by the sucrose treatment, even after adjusting for the weight effect. Selection resulted in several changes on both the standard and the sucrose medium, but the direction of change was not always the same as the acute effect. In no case was there a significant medium by selection-treatment interaction. The pattern of phenotypic correlations did not resolve the reasons for the direction of the genetic responses. Correlations were generally stable across diets and after selection, but there were notable exceptions.

The toxicities of short-chain primary alcohols and the accumulation of storage bodies in the larval fat body of Drosophila melanogaster

In terms of the LD50 values for alcohols, third-instar wild-type larvae of Drosophila melanogaster had a greater tolerance to ethanol, n-propanol and n-butanol than alcohol dehydrogenase (ADH)-deficient larvae. The tolerances of the two strains to methanol were similar. Methanol, ethanol, n-propanol and n-butanol all induced higher ADH activity in wild-type larvae. Ethanol, n-propanol, methanol and n-butanol slowed the growth for ADH-deficient larvae, whereas only methanol had this effect on wild-type larvae. The proportion of wild-type pupae to eclose was increased by n-butanol, n-propanol and ethanol. Cytometric methods to measure the densities of storage bodies--glycogen rosettes, protein bodies and lipid droplets--in fat body cells indicated that all of the test alcohols exerted some negative influence on the accumulation of at least one type of storage body. Analyses of total protein, glycogen and acylglycerols indicated that ethanol and n-butanol were associated with an accumulation of acylglycerols in both wild-type and ADH-deficient larvae; whereas, the other test alcohols resulted in low glycogen and protein concentrations in both test strains. The short-chain primary alcohols may in part be toxic to larvae because of disruptions in metabolism that lead to reductions in one or more kinds of storage bodies in the larval fat body.

Evolutionary genetics of the Drosophila alcohol dehydrogenase gene-enzyme system

Evolutionary genetics embodies a broad research area that ranges from the DNA level to studies of genetic aspects in populations. In all cases the purpose is to determine the impact of genetic variation on evolutionary change. The broad range of evolutionary genetics requires the involvement of a diverse group of researchers: molecular biologists, (population) geneticists, biochemists, physiologists, ecologists, ethologists and theorists, each of which has its own insights and interests. For example, biochemists are often not concerned with the physiological function of a protein (with respect to pH, substrates, temperature, etc.), while ecologists, in turn, are often not interested in the biochemical-physiological aspects underlying the traits they study. This review deals with several evolutionary aspects of the Drosophila alcohol dehydrogenase gene-enzyme system, and includes my own personal viewpoints. I have tried to condense and integrate the current knowledge in this field as it has developed since the comprehensive review by van Delden (1982). Details on specific issues may be gained from Sofer and Martin (1987), Sullivan, Atkinson and Starmer (1990); Chambers (1988, 1991); Geer, Miller and Heinstra (1991); and Winberg and McKinley-McKee (1992).

Interaction between the Adh and alpha Gpdh loci in Drosophila melanogaster: adult survival at high temperature

The role of high temperature resistance in the world-wide cline of Adh and alpha Gpdh allele frequencies of Drosophila melanogaster was investigated. Experimental strains were used with different combinations of Adh and alpha Gpdh alleles but with similar genetic background. The survival time of adult males, reared at different pre-adult temperatures, was measured at 35 degrees C. To investigate the relationship between survival and protein content, triglycerides content and ADH and alpha GPDH enzyme activity, the latter characters were measured before and after 12 h exposure to 35 degrees C. Three-way ANOVA of survival at 35 degrees C showed significant effects of rearing temperature and Adh genotype, interaction between Adh and alpha Gpdh, interaction between Adh and rearing temperature and interaction between Adh, alpha Gpdh and rearing temperature, alpha GPDH enzyme activity did not change significantly. Although protein content, triglycerides content and ADH enzyme activity decreased significantly at 35 degrees C, no correlation was observed between survival and these traits. It is concluded that the world-wide cline of Adh and alpha Gpdh allele frequencies cannot simply be explained by the genotypic differences in resistance to high temperature of adult Drosophila melanogaster.

Polymorphism at the Adh and alpha Gpdh loci in Drosophila melanogaster: effects of rearing temperature on developmental rate, body weight, and some biochemical parameters

The role of developmental time in the world-wide cline of Adh and alpha Gpdh allele frequencies of Drosophila melanogaster, and the relationship with weight and some biochemical characters, were investigated. Experimental strains were constructed with different combinations of Adh and alpha Gpdh alleles but with similar genetic background. Developmental time, adult weight, protein-and triglyceride-content, and ADH and alpha GPDH enzyme activity were measured at a rearing temperature of 20, 25 and 29 degrees C. Genotype effects were found in all studied characters. In general the developmental times of genotypes were: AdhFF less than AdhFS less than AdhSS and alpha GpdhFF greater than alpha GpdhFS = alpha GpdhSS. Developmental time and adult weight were strongly affected by rearing temperature. Triglyceride content and ADH and alpha GPDH enzyme activity were slightly affected by temperature. Interactions between genotype and temperature effects were found for developmental rate, adult weight and protein content. No trade off was observed between developmental time on the one hand and adult weight, protein- and triglyceride-content, and ADH and alpha GPDH enzyme activity on the other hand. It is argued that developmental rate differences might be one of the underlying mechanisms of the world-wide cline of the Adh and alpha GPdh allele frequencies.

The involvement of catalase in alcohol metabolism in Drosophila melanogaster larvae

The involvement of catalase (H2O2:H2O2 oxidoreductase, EC 1.11.1.6) in the metabolism of alcohols was investigated by comparing Drosophila melanogaster larvae in which catalase was inhibited by dietary 3-amino-1,2,4-triazole (3AT) to larvae fed a diet without 3AT. 3AT inhibited up to 80% of the catalase activity with concordant small increases in the in vitro activities of sn-glycerol-3-phosphate dehydrogenase, fumarase, and malic enzyme, but with a 16% reduction in the in vivo incorporation of label from [14C]glucose into lipid. When the catalase activity was inhibited to different degrees in ADH-null larvae, there was a simple linear correlation between the catalase activity and flux from [14C]ethanol into lipid. By feeding alcohols simultaneously with 3AT, ethanol and methanol were shown to react efficiently with catalase in wild-type larvae at moderately low dietary concentrations. Drosophila catalase did not react with other longer chain alcohols. Catalase apparently represents a minor pathway for ethanol degradation in D. melanogaster larvae, but it may be an important route for methanol elimination from D. melanogaster larvae.

Micro-spatial population differentiation in activity of glycerol-3-phosphate oxidase (GPO) from mitochondria of Drosophila melanogaster

Replicate mass-bred laboratory populations of D. melanogaster were derived from females collected in the Tahbilk winery cellar and from females collected outside but from within two kilometers of the cellar. When mitochondrial extracts from larvae were assayed for specific activity of glycerol-3-phosphate oxidase the cellar populations had levels only 50% of those from the outside area, confirming an earlier report of such a difference among isofemale lines derived from these same areas. This micro-spatial differentiation occurred when larvae were raised on a medium supplemented with both sucrose (5% w/v) and ethanol (4% v/v), known to effect high GPO activity, but was not detected when the larvae were raised on unsupplemented medium. A heritable basis for larval GPO activity variation was confirmed in a set of 32 isogenic second chromosome substitution lines and measured in a subset of 4 of these lines about 25 generations later. A reciprocal cross using two isogenic substitution lines with the highest and lowest activities suggested the difference was attributable to genes acting additively and that there were no maternal or paternal effects. The detection of a collection site difference in GPO enzyme activity in the isogenic lines suggests that polymorphic variation on the second chromosome is responsible for the differentiation at the winery. Variation in adult GPO activity did not show a dependence on the winery location from where the isogenic lines were derived nor was there an effect of line. Adult GPO activity was significantly higher than that detected in larval tissues and did not show a dependence on the sugar/ethanol level in the growth medium.

Effect of dietary carbohydrates and ethanol on expression of genes encoding sn-glycerol-3-phosphate dehydrogenase, aldolase, and phosphoglycerate kinase in Drosophila larvae

The genes encoding glycolytic enzymes in Drosophila form a group of functionally related genes that may be coordinately regulated and thus controlled by common factors. We have examined the effect of dietary carbohydrates and ethanol on expression of the genes encoding glycerol-3-phosphate dehydrogenase (GPDH), aldolase (ALD), and phosphoglycerate kinase (PGK) in D. melanogaster larvae. GPDH activity and transcript abundance increased in response to ethanol and additional amounts of several different carbohydrates. In addition, the levels of two alternatively processed Gpdh transcripts were differentially regulated by the treatments. The nutritional conditions tested had little or no effect on the activities and transcript levels of ALD and PGK. These results indicate that changes in dietary conditions affect expression of specific genes and do not evoke a general response from genes involved in cellular metabolism. The observation that dietary carbohydrates and ethanol increase Gpdh expression without affecting expression of Ald and Pgk reinforces previous suggestions that dietary carbon can be diverted by GPDH from glycolytic catabolism into lipid biosynthesis.

Genotype-by-environment and epistatic interactions in Drosophila melanogaster: the effects of Gpdh allozymes, genetic background and rearing temperature on larval developmental time and viability

The possible role of temperature as a component of natural selection generating the latitudinal clines in Gpdh allele frequencies in natural populations of Drosophila melanogaster was examined. Effects of rearing temperature (16 degrees, 22 degrees and 29 degrees) and of Gpdh allozymes (S and F) on larval developmental time and viability were measured. Eight genetic backgrounds from each of three populations (continents) were used to assess the generality of any effects. Analyses of variance indicated significant temperature effects and allozyme-by-genetic background interaction effects for both characters. Viability showed significant genetic background effects, as well as significant temperature-by-allozyme and temperature-by-allozyme-by-population interactions. In general, the S/S genotype was significantly lower in viability than the F/F and F/S genotypes at extreme temperatures (16 degrees and 29 degrees), with no significant differences at 22 degrees. However, each population had a slightly different pattern of viability associated with temperature, and only the Australian population showed a pattern that could contribute to the observed cline formation. Although the same two interactions were not significant for developmental time, examination of the means showed that the S/S genotype had a slightly faster rate of development at 16 degrees than the F/F genotype in all populations (by an average of 0.25 day or 1.1%). The low temperature effect on developmental time is consistent with the clines observed in nature, with the S allele increasing in frequency with higher latitudes. The results for both viability and developmental time are consistent with the interpretation of Gpdh as a minor polygene affecting physiological phenotypes, as indicated by previous work with adult flight metabolism. Finally, it is proposed that the temperature-dependent antagonistic effects of the allozymes on viability vs. developmental time and flight metabolism may be the underlying force giving rise to the worldwide polymorphism.

The metabolism of ethanol-derived acetaldehyde by alcohol dehydrogenase (EC 1.1.1.1) and aldehyde dehydrogenase (EC 1.2.1.3) in Drosophila melanogaster larvae

Both aldehyde dehydrogenase (ALDH, EC 1.2.1.3) and the aldehyde dehydrogenase activity of alcohol dehydrogenase (ADH, EC 1.1.1.1) were found to coexist in Drosophila melanogaster larvae. The enzymes, however, showed different inhibition patterns with respect to pyrazole, cyanamide and disulphiram. ALDH-1 and ALDH-2 isoenzymes were detected in larvae by electrophoretic methods. Nonetheless, in tracer studies in vivo, more than 75% of the acetaldehyde converted to acetate by the ADH ethanol-degrading pathway appeared to be also catalysed by the ADH enzyme. The larval fat body probably was the major site of this pathway.

Alcohol dehydrogenase and ethanol tolerance at the cellular level in Drosophila melanogaster

Exposure of early third instar larvae of Drosophila melanogaster to a nonlethal dose of ethanol was detrimental to larvae lacking alcohol dehydrogenase (ADH) but beneficial to wild-type larvae in terms of surviving a later ethanol tolerance test, indicating that one of the important functions of the ADH system is to supply derivatives of ethanol to larvae that in turn promote ethanol tolerance. High intracellular concentrations of ethanol in ADH-deficient (Adhn2) larvae fed ethanol were accompanied by a decrease in the cell membrane infoldings of fat body cells, suggesting that the capacities to absorb and release molecules were reduced. Marked effects of ethanol on the endoplasmic reticulum and mitochondria of ADH-deficient larvae were also evident. The absence of similar changes in wild-type larvae that were fed moderate levels of ethanol showed that the ADH system kept the intracellular level of ethanol at a concentration low enough to avoid cell damage. A cytometric analysis of electron micrographs showed that there were ethanol-induced reductions in glycogen, lipid, and protein stores in the fat body cells of ADH-deficient larvae fed 1.25% ethanol (v/v) compared with null larvae fed an ethanol-free diet. This finding implied that the capacities to synthesize or store these compounds may be limited by high intracellular concentrations of ethanol. The cytometric analysis also revealed that the consumption of diets containing 2.5% and 4.5% ethanol by Canton-S wild-type larvae for 3 days after 4 days of feeding on an ethanol-free diet resulted in decreases in glycogen and protein deposits in fat body cells, but increased the amount of lipid deposits compared to larvae fed an ethanol-free diet. This observation, coupled with the greater weight of wild-type adults that were fed a growth-limiting concentration of ethanol compared with control adults, suggested that a metabolic defense mechanism in larvae is to convert toxic ethanol to nontoxic storage products. Dietary ethanol alone and in combination with isopropanol stimulated an increase in the size of the NAD-pool in larvae, a condition that may favor the activity of ADH. A low dietary level of isopropanol (1%) completely blocked glycogen deposition in wild-type larvae, whereas ethanol did not. Thus ethanol and isopropanol exert some different toxic effects on larval fat bodies.

The epistasis of Adh and Gpdh allozymes and variation in the ethanol tolerance of Drosophila melanogaster larvae

The role of epistatic interaction of allozymes in the determination of variation in larval ethanol tolerance ofDrosophila melanogasterwas examined. Isofemale lines from the Tahbilk Winery were made homozygous for different common alleles of alcohol dehydrogenase (Adh) and sn-glycerol-3-phosphate dehydrogenase (Gpdh). When fed 6% ethanol, all the lines had reduced survival and, in the survivors, reduced body weight and lengthened development time. A strong positive correlation between tolerance and development time suggested that alleles responsible for slowing development on ethanol also increased ethanol tolerance. Analysis of larval ethanol tolerance over four generations showed that larvae of theAdhffGpdhff, andAdhssGpdhssallelic combinations were more tolerant than larvae with the other combinations. However, these genotypes were not associated with the slowing of development nor the weight loss on ethanol. Hence, larvae with certain combinations ofAdhandGpdhallozymes may have a greater capacity to metabolize ethanol and be more tolerant to its toxic effects.

Stress, altered energy availability and larval fitness in Drosophila melanogaster

This paper reports some effects of temperature variation, nutritional stress and a novel alteration in energy availability upon larval fitness in Drosophila melanogaster. The cofactor nicotinamide adenine dinucleotide (NAD) has been chosen as a novel energy source to be supplemented in food during larval development. The effects have been assessed at the phenotypic level for larval survival and development time and at the genotypic level for the alcohol dehydrogenase (Adh) and glycerol-3-phosphate dehydrogenase (Gpdh) loci. Supplemented NAD was found to increase survival at the lower temperatures and decrease survival at the higher temperatures. Further, for each temperature, NAD decreased development time although this effect diminished as temperature increased. There were no significant effects at the genotypic level. Hence a phenotypic approach studying the effects of environmental stresses and novel energy availability may be useful in understanding fitness variation in Drosophila populations.

The effects of a choline deficiency on the lipid composition and ethanol tolerance of Drosophila melanogaster

1. A reduction in the dietary concentration of choline, an essential nutrient for Drosophila melanogaster, from the optimal concentration of 80 micrograms/ml of defined medium to 8 micrograms/ml diminished the level of tissue phosphatidylcholine to less than one-third the normal level in third instar larvae without significantly altering the amount of phosphatidylethanolamine. 2. The rates of synthesis of phospholipids, triglycerides, diglycerides and monoglycerides were reduced by the choline-deficiency, and the chain length of fatty acids in lipids was shortened. 3. The activity of succinic dehydrogenase, a mitochondrial enzyme, was decreased by the deficiency, but the activities of fumarase, sn-glycerol-3-phosphate dehydrogenase, alcohol dehydrogenase, sn-glycerol-3-phosphate oxidase and fatty acid synthetase were unaffected. A choline-deficiency did not alter the ultrastructure of mitochondria of larval fat body cells. 4. Choline-deficient individuals were more susceptible to the toxic effects of ethanol during larval and pupal development, and less adept at utilizing ethanol as a substrate for adult tissue synthesis.

Selection affecting enzyme polymorphisms in enclosed Drosophila populations maintained in a natural environment

Allele frequencies for the Adh, Gpdh, and Est6 enzyme polymorphisms of Drosophila melanogaster show large-scale latitudinal clines, whereas those for Pgm do not vary systematically with latitude. To elucidate possible mechanisms of selection underlying these distributions, large collections of the species were made from five Australasian localities spanning 24 degrees of latitude. Two replicate experimental populations were established from each collection, and each replicate was then released into an enclosure surrounding a natural habitat at a central-latitude locality. Genotype frequencies at the four loci were monitored for 15 months, covering 12 discrete generations, and selection coefficients on each polymorphism were then estimated by maximum likelihood procedures. For Est6 no coefficients were found to be significantly different from zero. For Pgm some nonzero coefficients were estimated, but these were heterogeneous across experimental populations of different geographic origins. For both Adh and Gpdh, nonzero selection coefficients were estimated that were homogeneous across populations and indicated heterozygote advantage. Predicted Adh and Gpdh equilibrium allele frequencies were consistent with those found in adjacent free-living populations. It is concluded that, at such intermediate latitudes at least, selection operates on the Adh and Gpdh polymorphisms to the advantage of heterozygotes.

The effect of dietary ethanol on the composition of lipids of Drosophila melanogaster larvae

At a moderate concentration (2.5%, v/v) dietary ethanol reduced the chain length of total fatty acids (FA) and increased the desaturation of short-chain FA in Drosophila melanogaster larvae with a functional alcohol dehydrogenase (ADH). The changes in length in total FA were postulated to be due to the modulation of the termination specificity of fatty acid synthetase. Because the ethanol-stimulated reduction in the length of unsaturated FA was blocked by linoleic acid, it was thought to reflect the properties of FA 9-desaturase. Although the ethanol-stimulated reduction in chain length of unsaturated FA was also observed in ADH-null larvae, ethanol promoted an increase in the length of total FA of the mutant larvae. Thus, the ethanol-stimulated change in FA length was ADH dependent but the ethanol effect on FA desaturation was not. Ethanol also stimulated a decrease in the relative amount of phosphatidylcholine and an increase in phosphatidylethanolamine. Because similar ethanol-induced changes have been found in membrane lipids of other animals, ethanol may alter the properties of membranes in larvae. It is proposed that ethanol tolerance in D. melanogaster may be dependent on genes that specify lipids that are resistant to the detrimental effects of ethanol.

Structural analysis of glycerol-3-phosphate dehydrogenase from several Drosophila species

This report describes preliminary protein structural studies of glycerol-3-phosphate dehydrogenase (alpha-GPDH) from Drosophila spp. and an important innovative feature of our enzyme purification protocol. The scheme involves the coupling of substrate (alpha-glycerophosphate) elution from CM-Sephadex and cofactor (NADH) elution from Affi-Gel blue resin. Using this method a 32.7% yield and a 111-fold purification were obtained from a D. melanogaster line carrying the alpha-GpdhS allele at the alpha-Gpdh locus. The product obtained from 0 to 3-day-old adult flies was electrophoretically homogeneous and consisted mainly of the adult alpha-GPDH-1 isozyme. The method was used to obtain alpha-GPDH protein from D. melanogaster (two lines), D. hydei, D. immigrans, and D. mercatorum. Peptide mapping revealed structural differences among the enzymes from the different species, and amino acid sequencing showed many similarities between D. melanogaster alpha-GPDH and the rabbit muscle enzyme.

Dietary ethanol and lipid synthesis in Drosophila melanogaster

When cultured on a defined diet, ethanol was an efficient substrate for lipid synthesis in wild-type Drosophila melanogaster larvae. At certain dietary levels both ethanol and sucrose could displace the other as a lipid substrate. In wild-type larvae more than 90% of the flux from ethanol to lipid was metabolized via the alcohol dehydrogenase (ADH) system. The ADH and aldehyde dehydrogenase activities of ADH were modulated in tandem by dietary ethanol, suggesting that ADH provided substrate for lipogenesis by degrading ethanol to acetaldehyde and then to acetic acid. The tissue activity of catalase was suppressed by dietary ethanol, implying that catalase was not a major factor in ethanol metabolism in larvae. The activities of lipogenic enzymes, sn-glycerol-3-phosphate dehydrogenase, fatty acid synthetase (FAS), and ADH, together with the triacylglycerol (TG) content of wild-type larvae increased in proportion to the dietary ethanol concentration to 4.5% (v/v). Dietary ethanol inhibited FAS and repressed the accumulation of TG in ADH-deficient larvae, suggesting that the levels of these factors may be subject to a complex feedback control.

Dual function of the alcohol dehydrogenase of Drosophila melanogaster: ethanol and acetaldehyde oxidation by two allozymes ADH-71k and ADH-F

Until recently the alcohol dehydrogenase of Drosophila melanogaster was thought to act only in the first step of primary alcohol oxidation, producing an aldehyde. Instead, acetic acid is the main product of a two-step process. A rapid procedure was developed for the isolation and purification of two allozymes. The thermostability of the purified enzymes was found to be very different, t 1/2 at 35 degrees C, being 45 min and 130 min for ADH-F and ADH-71k respectively. The kinetic parameters of ethanol oxidation by the two purified allozymes were determined within physiological substrate and coenzyme ranges. The use of artificial electron acceptors has a notable influence on the ethanol oxidation: the apparent Michaelis constants increase; the oxidation rate with ADH-71k increases, whereas it decreases with ADH-F. Purified ADH is shown to be able to catalyze the oxidation of acetaldehyde solely in the presence of NAD+, and PMS and MTT as artificial electron acceptors. From the kinetic data the relative in vivo oxidation rates of ethanol by both ADH allozymes were calculated. ADH-F turned out to be somewhat less effective (30%-40%) than ADH-71k. The physiological consequences of these differences are discussed.

Genetic variation in the dietary sucrose modulation of enzyme activities in Drosophila melanogaster

Genetic variation in the modulating effect of dietary sucrose was assessed inDrosophila melanogasterby examining 27 chromosome substitution lines coisogenic for theXand second chromosomes and possessing different third isogenic chromosomes derived from natural populations. An increase in the concentration of sucrose from 0·1% to 5% in modified Sang's medium C significantly altered the activities of 11 of 15 enzyme activities in third instar larvae, indicating that dietary sucrose modulates many, but not all, of the enzymes ofD. melanogaster. A high sucrose diet promoted high activities of enzymes associated with lipid and glycogen synthesis and low activities of enzymes of the glycolytic and Krebs cycle pathways, reflecting the physiological requirements of the animal. Analyses of variance revealed significant genetic variation in the degrees to which sucrose modulated several enzyme activities. Analysis of correlations revealed some relationships between enzymes in the genetic effects on the modulation process. These observations suggest that adaptive evolutionary change may depend in part on the selection of enzyme activity modifiers that are distributed throughout the genome.

Purification and characterization of the naturally occurring allelic variants of sn-glycerol-3-phosphate dehydrogenase in Drosophila melanogaster

The naturally occurring electrophoretic variants of sn-glycerol-3-phosphate dehydrogenase and a heterodimeric form of the enzyme resulting from a genetic cross of two variant strains of Drosophila were purified to homogeneity by a combination of DEAE-cellulose chromatography and 8-(6-aminohexyl)-amino-ATP-Sepharose affinity chromatography. Each purified protein was compared with respect to a number of physicochemical and kinetic properties. All forms of the enzyme were found to be similar, except for pI differences associated with the electrophoretic variation observed.