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

Ethanol Behavioral Responses in Drosophila

Drosophila melanogaster is a powerful genetic model for investigating the mechanisms underlying ethanol-induced behaviors, metabolism, and preference. Ethanol-induced locomotor activity is especially useful for understanding the mechanisms by which ethanol acutely affects the brain and behavior. Ethanol-induced locomotor activity is characterized by hyperlocomotion and subsequent sedation with increased exposure duration or concentration. Locomotor activity is an efficient, easy, robust, and reproducible behavioral screening tool for identifying underlying genes and neuronal circuits as well as investigating genetic and molecular pathways. We introduce a detailed protocol for performing experiments investigating how volatilized ethanol affects locomotor activity using the fly Group Activity Monitor (flyGrAM). We introduce installation, implementation, data collection, and subsequent data-analysis methods for investigating how volatilized stimuli affect activity. We also introduce a procedure for how to optogenetically probe neuronal activity to identify the neural mechanisms underlying locomotor activity.

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.

Male sexual behaviour and ethanol consumption from an evolutionary perspective: A comment on "Sexual Deprivation Increases Ethanol Intake in Drosophila"

Shohat-Ophir et al. (1) demonstrate a connection between sexual behaviour and ethanol consumption in male Drosophila flies, and how the neuropeptide F system regulates ethanol preference. Their results are rightly discussed only in a physiological context, but this has facilitated erroneous anthropomorphic interpretations by the media. Here we discuss the link between male sexual behaviour and ethanol consumption from an evolutionary perspective, providing a broader context to interpret their results.

Ethanol preference in Drosophila melanogaster is driven by its caloric value

BACKGROUND

Perhaps the most difficult thing to ascertain concerning the behavior of another animal is its motivation. The motivation underlying the preference of Drosophila melanogaster for ethanol (EtOH)-rich food has long been ascribed to its value as a food. A recently introduced idea is that, as in humans, the pharmacological effects of EtOH also motivate the fly to choose EtOH-rich food over nonalcoholic food.

METHODS

Flies are given a choice between pipets that contain liquid food and liquid food supplemented with EtOH. In some experiments, carbohydrates are added to the non-EtOH-containing food to balance the calories for EtOH.

RESULTS

We confirm that D. melanogaster indeed prefer food that is supplemented with EtOH. However, if the alternative food choice is isocaloric, D. melanogaster usually do not show any preference for a 10% EtOH solution. Even after EtOH preference has been established, it can be completely reversed if the alternative food is calorically supplemented. This occurs even when the carbohydrate solution used to balance calories is not gustatorily attractive. Furthermore, if the alternative food contains more calories than the EtOH food, the flies will prefer the non-EtOH food. We go on to show that during the preference assay that EtOH in the fly does not exceed 4 mM, which in mammals is a nonintoxicating dose.

CONCLUSIONS

We conclude that preference for EtOH in this assay arises not from the pharmacological effects of EtOH but rather because of its nutritive value.

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.

Detection of individual variation in enzyme activity in natural populations of Drosophila melanogaster

The method allows the determination of the activity level of enzymes in a single fly and assessing the genetic composition of the given individual at these enzyme loci. Three isofemale lines were constructed which were monomorphic at several enzyme loci. Samples were prepared in two different ways: (i) individual samples--individuals were homogenised separately; (ii) collective samples--a common homogenate was prepared from several individuals. Oregon-R strain was also used to prepare a standard homogenate. The activities of alcohol dehydrogenase (ADH), alpha-glycerophosphate dehydrogenase (alpha GPDH), isocitrate dehydrogenase (IDH), and 6-phosphogluconate dehydrogenase (6PGDH), were measured in each sample on starch gel after the proteins were separated by electrophoresis. Enzyme activities were assessed by the optical density of the bands. Gel and position weights were estimated on the basis of the statistical analyses of the activities measured in the standard samples. Gel weights were then used to account for the activity differences among the gels while position weights were applied to correct for the general tendencies in the activities observed within the gels. The gel and position weighted activities of individual and collective samples were compared in the isofemale lines. The individual samples had approximately two times as much variation as the collective samples for all four enzymes. The electrophoretic method is sensitive enough to study the structure of the phenotypic variation in enzyme activity in the natural populations. The total variation among the standard samples was close to the within subline component of variation obtained for the collective samples (measurement error). This shows that the standard samples can be used to estimate the size of the measurement error.

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.