Evidence for the adaptive significance of enzyme activity levels: interspecific variation in alpha-GPDH and ADH in Drosophila

Ethanol-guided behavior in Drosophila larvae

Chemosensory signals allow vertebrates and invertebrates not only to orient in its environment toward energy-rich food sources to maintain nutrition but also to avoid unpleasant or even poisonous substrates. Ethanol is a substance found in the natural environment of Drosophila melanogaster. Accordingly, D. melanogaster has evolved specific sensory systems, physiological adaptations, and associated behaviors at its larval and adult stage to perceive and process ethanol. To systematically analyze how D. melanogaster larvae respond to naturally occurring ethanol, we examined ethanol-induced behavior in great detail by reevaluating existing approaches and comparing them with new experiments. Using behavioral assays, we confirm that larvae are attracted to different concentrations of ethanol in their environment. This behavior is controlled by olfactory and other environmental cues. It is independent of previous exposure to ethanol in their food. Moreover, moderate, naturally occurring ethanol concentration of 4% results in increased larval fitness. On the contrary, higher concentrations of 10% and 20% ethanol, which rarely or never appear in nature, increase larval mortality. Finally, ethanol also serves as a positive teaching signal in learning and memory and updates valence associated with simultaneously processed odor information. Since information on how larvae perceive and process ethanol at the genetic and neuronal level is limited, the establishment of standardized assays described here is an important step towards their discovery.

Experimental test and refutation of a classic case of molecular adaptation in Drosophila melanogaster

Identifying the genetic basis for adaptive differences between species requires explicit tests of historical hypotheses concerning the effects of past changes in gene sequence on molecular function, organismal phenotype and fitness. We address this challenge by combining ancestral protein reconstruction with biochemical experiments and physiological analysis of transgenic animals that carry ancestral genes. We tested a widely held hypothesis of molecular adaptation-that changes in the alcohol dehydrogenase protein (ADH) along the lineage leading to Drosophila melanogaster increased the catalytic activity of the enzyme and thereby contributed to the ethanol tolerance and adaptation of the species to its ethanol-rich ecological niche. Our experiments strongly refute the predictions of the adaptive ADH hypothesis and caution against accepting intuitively appealing accounts of historical molecular adaptation that are based on correlative evidence. The experimental strategy we employed can be used to decisively test other adaptive hypotheses and the claims they entail about past biological causality.

Regulatory differences in developmental expression of alcohol dehydrogenase are related to interspecies differences in ethanol tolerance of Drosophila

Biochemical properties and expression patterns of alcohol dehydrogenase were compared among members of the virilis phylad of Drosophila. Quantitative differences in stage- and tissue-specific expression of ADH of D. virilis, D. novamexicana, D. texana and D. americana were observed. These differences seem to correlate with the temporal differences in ethanol tolerance among these species. Quantitative differences in ADH protein levels account for most, if not all, of the activity differences between species. Analysis of the interspecific hybrids revealed that these differences arise by a mechanism affecting monomer synthesis. A comparison with earlier studies, where independently evolved regulatory differences were related to adaptations, revealed contrasting underlying mechanisms.

Effects on ADH activity and distribution, following selection for tolerance to ethanol in Drosophila melanogaster

Strains of Drosophila melanogaster homozygous for either the AdhF or the AdhS allele were kept on food supplemented with ethanol for 20 generations. These strains (FE and SE) were tested for tolerance to ethanol and compared with control strains (FN and SN). The E strains showed increased tolerance to ethanol both in the adult and in the juvenile life stages. In adults the increase in tolerance was not accompanied by an increase in overall ADH activity. However, there were changes in the distribution of ADH over the body parts. Flies of the FE strain possessed significantly more ADH in the abdomen, compared with FN. Another set of FN and SN populations were started both on standard food and on ethanol food with reduced yeast concentrations. After 9 months ADH activities were determined in flies from these populations which had been placed on three different media: the food the populations had been kept on, regular food and regular food supplemented with ethanol. The phenotypic effects of yeast reduction on ADH activity were considerably, but longterm genetic effects were limited.

A cytological and molecular analysis of Adh gene expression in Drosophila melanogaster polytene chromosomes

Analysis of puffing patterns in Drosophila melanogaster salivary gland chromosomes indicates the existence of a developmentally specific puff in the 35B region. This puff seems to originate from bands 35B2 or 35B3, where Adh is located, and it is expanded in more than 60% of the nuclei examined. The presence of RNA polymerase II in this puff as well as its ability to incorporate tritiated uridine shows that it corresponds to a transcriptionally active site. RNA blotting and in situ hybridization experiments indicate that Adh is transcribed, although not very actively, in salivary glands during the third larval instar. However, this tissue does not display detectable levels of ADH activity. By contrast, we have found that in midgut polytene chromosomes the 35B region is not visibly puffed in spite of the high levels of Adh transcripts detected. These results seem to suggest that puffing at the 35B region could be mainly promoted by genes closely linked to Adh, possibly with a minor contribution of this gene.

Functional and biochemical features of alcohol dehydrogenase in four species of the obscura group of Drosophila

The biochemical features of ADH of four Drosophila species of the obscura group have been studied. The relationship between ethanol tolerance and ADH activity has been investigated. Propan-2-ol and acetone concentrations have been determined in propan-2-ol treated flies and ADH activity has been followed during 96 h of propan-2-ol treatment. Data on the ADH system confirm constructed phylogenies based on electrophoretic variation and chromosome homologies.

Developmental rate and viability of rainbow trout with a null allele at a lactate dehydrogenase locus

We show that a previously described isozyme polymorphism in rainbow trout (Salmo gairdneri) is the result of an enzymatically inactive (i.e., null) allele (n). Ldh3 null homozygotes (n/n) and heterozygotes (100/n) have reductions of about 20 and 12% in total lactate dehydrogenase (LDH) activity at hatching, respectively. As juveniles, (100/n) fish have reductions in LDH activity of 15, 37, and 21% in brain, heart, and white muscle, respectively. Embryos with different Ldh3 phenotypes from 11 families do not differ significantly in either survival or hatching time. However, a second measure of developmental rate, the amount of malate dehydrogenase (MDH) and phosphoglucomutase (PGM) activity in 33-day-old embryos, suggests that (100/n) embryos develop more slowly than (100/100) embryos. In three of four families examined, (100/n) embryos have significantly lower amounts of total MDH activity (8-10%). In one of these, (100/n) embryos also have significantly lower total PGM activity (15%). These data suggest that the reduction in total LDH activity is associated with small but detectable delays in developmental rate but nondetectable differences in survival to hatching.

Interpreting the behavior of enzymes: purpose or pedigree?

To interpret the growing body of data describing the structural, physical, and chemical behaviors of biological macromolecules, some understanding must be developed to relate these behaviors to the evolutionary processes that created them. Behaviors that are the products of natural selection reflect biological function and offer clues to the underlying chemical principles. Nonselected behaviors reflect historical accident and random drift. This review considers experimental data relevant to distinguishing between nonfunctional and functional behaviors in biological macromolecules. In the first segment, tools are developed for building functional and historical models to explain macromolecular behavior. These tools are then used with recent experimental data to develop a general outline of the relationship between structure, behavior, and natural selection in proteins and nucleic acids. In segments published elsewhere, specific functional and historical models for three properties of enzymes--kinetics, stereospecificity, and specificity for cofactor structures--are examined. Functional models appear most suitable for explaining the kinetic behavior of proteins. A mixture of functional and historical models appears necessary to understand the stereospecificity of enzyme reactions. Specificity for cofactor structures appears best understood in light of purely historical models based on a hypothesis of an early form of life exclusively using RNA catalysis.

Intergenotypic effect of isopropanol ingestion in the further detoxification of ethanol and isopropanol in Drosophila melanogaster

The effect of isopropanol ingestion on a further tolerance to ethanol and isopropanol, and its relationship with the Adh locus, have been studied using Drosophila melanogaster selected for tolerance to ethanol. For this purpose, AdhF AdhF, AdhF AdhS and AdhS AdhS flies were independently pretreated with 2 per cent isopropanol and then further exposed to solutions of 10 per cent ethanol or of 2 per cent isopropanol. Afterwards, the ability to tolerate both alcohols, and the ADH activities of the surviving flies were compared with those of flies not pretreated with isopropanol. After isopropanol ingestion, the flies of all three Adh genotypes shown much higher sensitivity to ethanol than to isopropanol although the opposite results were observed in flies not pretreated with isopropanol. Isopropanol treatment decreased the ADH activity in flies of all three genotypes within a range varying from 73 per cent (females FF) to 93 per cent (males FS), the remaining ADH activity being between 2 to 3 times higher in FF than in FS and SS flies. The reduction in ADH activity was associated with the phenomenon of ADH isozyme interconversion. After the isopropanol pretreatment, the most isopropanol tolerant flies (FF) were also the most ADH active ones. Therefore, the adaptative significance of the isozyme conversion is questioned.

The effect of temperature on biochemical and molecular properties of Drosophila alcohol dehydrogenase

The gene products of the two major alleles of alcohol dehydrogenase (ADH-F and ADH-S) have been subjected to kinetic and biochemical analyses over a range of temperatures. Although temperature was found to have a significant effect on both kinetic and biochemical properties of Drosophila ADH, no significant differential effect was observed between the major ADH allozymes. The results are discussed within the context of the selective maintenance of Adh polymorphism in natural populations.

Acetaldehyde utilization and toxicity in Drosophila adults lacking alcohol dehydrogenase or aldehyde oxidase

Metabolic utilization and toxicity of acetaldehyde were studied in flies lacking alcohol dehydrogenase (ADH), aldehyde oxidase (AO), or both functions. Prior to the experiments, mutant alleles Adhn4 and mal were transferred to the same genetic background by 10 successive backcrosses. By comparison with wild-type flies, various deleterious, pleiotropic effects could be attributed to the mal allele but not to Adhn4. Of the four genotypes studied (mal, Adhn4, mal Adhn4, and wild), all were able to use acetaldehyde as a resource in a similar way. In spite of its high toxicity, acetaldehyde appeared a better resource than ethanol. Flies treated with intermediate acetaldehyde concentrations (around 0.5%) exhibited a very high interindividual heterogeneity which could reflect a physiological adaptation occurring as a consequence of the aldehyde treatment. Toxicity tests showed that ADH-negative flies were more sensitive to acetaldehyde than wild type, but this is most likely explained by the transformation of the aldehyde into alcohol. Our results show that the aldehyde metabolizing enzyme (AME) system in Drosophila is neither ADH nor AO. The existence of an aldehyde dehydrogenase is plausible.

Alcohol dehydrogenase from Drosophila funebris and Drosophila immigrans: molecular and evolutionary aspects

Alcohol dehydrogenase from Drosophila funebris and D. immigrans is evident at all developmental stages. The highest activity level appears in third-instar larvae and declines to a lower level at all later stages of development. Both species are monomorphic. The enzyme is a dimer consisting of two identical subunits with molecular weight 27,600. The pI values are 8.6 for D. funebris and 9.02 for D. immigrans. The optimum pH is 8.6 and 8.7 for D. funebris and D. immigrans, respectively. The Km values for NAD+, propan-2-ol, and butan-2-ol are 0.15, 2.90, and 2.08 mM, respectively, for D. funebris and 0.16, 1.53, and 1.49 mM, respectively, for D. immigrans. The half-life for the purified enzyme is 45 days for D. funebris and 18 days for D. immigrans at 4 degrees C. Data on the amino acid composition of both enzymes and peptide maps of alcohol dehydrogenase of D. immigrans reveal that they have marked homologies between them and also with alcohol dehydrogenases of other species. D. funebris shows reduced levels of alcohol dehydrogenase synthesis but has the highest specific activity reported to date for a Drosophila species. D. immigrans synthesises six times more enzyme but the specific activity is comparable to that of other species of Drosophila. This evidence could explain their different alcohol tolerance. The molecular properties of these alcohol dehydrogenases together with other species of Drosophila suggest that the alcohol dehydrogenase of Drosophila has arisen by divergent evolution from a common ancestral gene.

Regulatory gene evolution: adaptive differences in expression of alcohol dehydrogenase in Drosophila melanogaster and Drosophila simulans

In Drosophila melanogaster X D. simulans hybrids, the alcohol dehydrogenase (ADH) electromorphs characteristic of the two parents display tissue- and stage-specific differences in relative level of expression. This implies distinct cis-acting regulatory elements associated with the respective Adh alleles. These cis-acting elements account in part, but not completely, for markedly different overall patterns of ADH expression in the two species. The regulatory patterns seem to be adaptively significant since they correlate with species-specific patterns of ethanol tolerance. The activity differences are accounted for by different levels of enzyme protein, but the underlying mechanisms have not been fully analysed and may be complex. Independent evolution of various aspects of the ADH developmental programme may relate to use of different promoters for transcription of the Adh locus in different developmental contexts. This system illustrates the potential importance of regulatory genes in evolution and provides a model for investigating the molecular basis of evolved regulatory differences.

Biochemical and molecular analysis of naturally occurring Adh variants in Drosophila melanogaster

The results of a detailed analysis of the biochemical and molecular basis of alcohol dehydrogenase (ADH) activity variation existing among six naturally occurring and one ethyl methanesulfonate-induced Adh variant strain of Drosophila melanogaster are presented. Significant specific activity differences exist among the strains but the majority of ADH activity variation can be accounted for by differences in levels of ADH protein. These protein level differences can, in turn, be accounted for by ADH synthesis rate variation, which positively correlates with in vivo levels of cytoplasmic Adh mRNA. The functional variability is correlated with known structural variation in and around the area of the Adh gene.

The effect of alcohol stress on nicotinamide adenine dinucleotide (NAD+) levels in Drosophila

Previous studies carried out in mammalian systems indicated that an organism's NAD+/NADH balance is carefully regulated but can be destabilized by dietary stresses. Since Drosophila alcohol dehydrogenase (ADH) uses NAD+ to remove a hydrogen from ethanol in the first step of alcohol catabolism, it is possible that under alcohol stress conditions the in vivo NAD+ levels in Drosophila may decrease. In this study genetically homozygous flies were stressed with maximally sublethal concentrations of ethanol (10%) for periods of up to 24 hr. The results indicate that NAD+ levels do in fact drop by at least 20% in response to ethanol stress. Evidence is presented that suggests that this decrease is the direct result of ADH-mediated catabolism.

Drosophila alcohol dehydrogenase: developmental studies on cryptic variant lines

Thirty-five cryptic variant lines were used to examine the mechanisms involved in genetic modulation of alcohol metabolism in Drosophila. Late third-instar larval, preemergence pupal, and adult stages cultured at 18 and 28 C were examined. Spectrophotometric analyses for native alcohol dehydrogenase (ADH) activity and residual ADH activity after treatment with guanidine hydrochloride and heat were performed. Differential response of cryptic variants to treatment with the denaturants during development suggested that this variation may have an adaptive significance.

Changes in levels of alcohol dehydrogenase during the development of Drosophila melanogaster

The results of an analysis of the biochemical basis of changes in alcohol dehydrogenase (E.C.1.1.1.1) activity over Drosophila development are presented. The data indicate that (1) the characteristic changes that occur in ADH activity over development are predominantly, it not exclusively, the result of quantitative changes in the amount of enzyme present rather than qualitative changes affecting the enzyme's specific activity and (2) the fluctuations in amount of ADH which occur during development are not the result of the only known form of post-translational modification capable of affecting the biochemical properties of the enzyme. We conclude that developmental changes in amount of ADH are most likely the result of fluctuations in the turnover of the ADH protein.

Thermal stability of alpha-glycerophosphate dehydrogenase in Drosophila

Thermal stability of alpha-glycerophosphate dehydrogenase-1 (alpha-Gpdh-1) in nine Drosophila species was studied at pH's ranging form 6.4 to 8.5. This was done by measuring the changes in the activity of enzymes during the heat denaturation process. In addition to temperature, the rate of denaturation is highly dependent on the pH of the incubation buffer. The results of this study show that the thermal stability of enzyme molecules is different in different species. This holds true also in the species in which the enzymes have been found to be identical by other means. The differences between species of the Drosophila virilis group are discussed.

Effect of environmental alcohol on in vivo properties of Drosophila alcohol dehydrogenase

The effects of environmental 2-propanol on the in vivo properties of Drosophila alcohol dehydrogenase (E.C. 1..1.1.1.) are presented. Exposed flies were found to exhibit a significant decrease in ADH specific activity with a concomitant increase in the enzyme's relative in vivo stability and concentration. The possible adaptive significance of the observed responses is discussed.

Relationship between ADH activity and behavioral response to environmental alcohol in Drosophila

Three alcohol dehydrogenase genotypes, homozygous for either the electrophoretically fast, slow, or null allele at the Adh locus in D. melanogaster, were tested for relative larval alcohol preference behavior (APB) over a range of ethanol concentrations. Differences in behavior between genotypes were not significant at concentrations below 10%. At concentrations greater than 10%, avoidance behavior was negatively correlated with the relative ADH activity levels of the genotypes tested. A model based on the differential buildup of toxic acetaldehyde is proposed to explain the avoidance response.

Oviposition site preference in Drosophila

Comparative studies of oviposition site preference (OSP) in Drosophila suggest that choice of oviposition site is an important adaptive behavior which influences individual fitness and the potential of populations for speciation. OSP has been investigated under conditions which provided females with a choice of standard medium or medium containing ethanol for oviposition. OSP is an extremely labile behavior in the laboratory, but a technique has been developed which minimizes variation between replicates and allows the detection of OPS differences between semispecies of a single species. An analysis of the OSP of 14 Drosophila species shows that this behavior is not correlated with phylogenetic relationships. OSP with respect to ethanol may be correlated with the presence of ethanol in the environment and the activity of alcohol dehydrogenase in the species tested.

Biochemical properties of esterase 6 in Drosophila melanogaster

Biochemical properties of esterase 6 in Drosophila melanogaster were investigated using partially purified preparations from three genotypes, 1/1, 1/2, and 2/2. The molecular weight of the enzyme is estimated to be about 90,000, and treatment with sodium dodecylsulfate cleaves the enzyme into four units with a molecular weight of about 22,000. The activity toward 28 naturally occurring esters was assayed and shown to vary considerably with substrate, the 1/1 preparation having in general higher activity than 1/2 and 2/2, which were very similar. Heat sensitivity, the effect of metal ions, and the effects of the presence or absence of an end product were also studied. The differences demonstrated between allozymes would allow considerable scope, under appropriate conditions, for differential selection to operate between genotypes.

Alcohol-oxidizing enzymes in 13 Drosophila species

Starch and polyacrylamide gel electrophoresis were used to ascertain the substrate specificities of alcohol-oxidizing enzymes in 13 Drosophila species. The substrates used were a variety of long- and short-chain aliphatic alcohols, one aromatic alcohol, and benzaldehyde. Only one enzyme (product of a single-gene locus) showed significant NAD+-dependent alcohol dehydrogenase activity with short-chain aliphatic alcohols. The 13 species, belonging to four different Drosophila groups, all showed a similar complement of alcohol-oxidizing enzymes, although differences in electrophoretic mobility and in levels of activity existed from species to species. These findings are relevant to the adaptation of Drosophila to alcohol environments.

Gene regulation in adaptive evolution

It has been suggested that gene regulation may play a critical role in adaptive evolution. However, gene regulation has proved to be most refractory to experimental investigation in multicellular organisms. Using specially constructed stocks of Drosophila melanogaster, we have demonstrated the following. (1) The existence in natural populations of ample variation in regulatory genes that modify the activity of alcohol dehydrogenase (ADH), an enzyme coded by a structural gene locus, Adh, located on the second chromosome; the regulatory genes are located on the third chromosome, and thus are not adjacent to the structural locus. (2) The regulatory genes act not by means of post-transcriptional or post-translational modification of the gene product, but rather by controlling the number of ADH molecules; this is consistent with the hypothesis of gene regulation by means of macromolecules specifically binding at control sites adjacent to the structural gene locus. (3) The variation in regulatory genes is adaptively significant; adaptation to higher levels of environmental alcohol takes place not by changes in the Adh structural locus, but by changes in regulatory genes that control the number of ADH molecules in the organisms. Our results provide direct evidence of the importance of gene regulation in eucaryotic evolution.

Selection acting upon slow-migrating ADH alleles differing in enzyme activity

Relative egg to adult viability was compared in three stocks of Drosophila melanogaster differing in alcohol dehydrogenase (ADH) activity. Two of these, Grell's standard ADHS and a mutant ADHS strain producing half the normal number of ADH molecules, had the same electrophoretic mobility. The experiment demonstrated a correlation between survival in alcohol-supplemented media and enzyme activity. This supports the hypothesis that enzyme activity per se, rather than some other attribute of allelic differences, may be responsible for the previously observed selective advantage of high activity alleles on alcohol media.