Adaptation to alcoholic fermentation in Drosophila: a parallel selection imposed by environmental ethanol and acetic acid

Expression of antimicrobial peptides associated with different susceptibilities to environmental chemicals in Drosophila suzukii and Drosophila melanogaster

Drosophila suzukii is a serious agricultural pest. The evolved morphology of the female D. suzukii assists in penetrating the surface of fresh fruit and spawns eggs with its unique ovipositor. Conversely, Drosophila melanogaster, a taxonomically close species with D. suzukii, largely inhabits decaying and fermenting fruits and is consistently exposed to extensive environmental chemicals, such as 2-phenylethanol, ethanol, and acetic acid, produced by microorganisms. Considering the distinct habitats of the two flies, D. suzukii is thought to be more susceptible to environmental chemicals than D. melanogaster. We investigated the significantly higher survival rate of D. melanogaster following exposure to 2-phenylethanol, ethanol, and acetic acid. A comparison of the expression of antimicrobial peptides (AMPs) between the two flies treated with chemicals established that AMPs were generally more abundantly induced in D. melanogaster than in D. suzukii, particularly in the gut and fat body. Among the AMPs, the induction of genes (Diptericin A, Diptericin B, and Metchnikowin), which are regulated by the immune deficiency (IMD) pathway, was significantly higher than that of Drosomycin, which belongs to the Toll pathway in chemical-treated D. melanogaster. A transgenic RNAi fly (D. melanogaster) with silenced expression of AMPs and Relish, a transcription factor of the IMD pathway, exhibited significantly reduced survival rates than the control fly. Our results suggest that AMPs regulated by the IMD pathway play an important role in the chemical tolerance of D. melanogaster, and these flies are adapted to their habitats by physiological response.

Neuroecology of Alcohol Preference in Drosophila

In this review, we highlight sources of alcohols in nature, as well as the behavioral and ecological roles that these fermentation cues play in the short lifespan of Drosophila melanogaster. With a focus on neuroethology, we describe the olfactory detection of alcohol as well as ensuing neural signaling within the brain of the fly. We proceed to explain the plethora of behaviors related to alcohol, including attraction, feeding, and oviposition, as well as general effects on aggression and courtship. All of these behaviors are shaped by physiological state and social contexts. In a comparative perspective, we also discuss inter- and intraspecies differences related to alcohol tolerance and metabolism. Lastly, we provide corollaries with other dipteran and coleopteran insect species that also have olfactory systems attuned to ethanol detection and describe ecological and evolutionary directions for further studies of the natural history of alcohol and the fly.

Sensory processing during sleep in Drosophila melanogaster

During sleep, most animal species enter a state of reduced consciousness characterized by a marked sensory disconnect. Yet some processing of the external world must remain intact, given that a sleeping animal can be awoken by intense stimuli (for example, a loud noise or a bright light) or by soft but qualitatively salient stimuli (for example, the sound of a baby cooing or hearing one's own name^1-3). How does a sleeping brain retain the ability to process the quality of sensory information? Here we present a paradigm to study the functional underpinnings of sensory discrimination during sleep in Drosophila melanogaster. We show that sleeping vinegar flies, like humans, discern the quality of sensory stimuli and are more likely to wake up in response to salient stimuli. We also show that the salience of a stimulus during sleep can be modulated by internal states. We offer a prototypical blueprint detailing a circuit involved in this process and its modulation as evidence that the system can be used to explore the cellular underpinnings of how a sleeping brain experiences the world.

Identification of transcriptional responsive genes to acetic acid, ethanol, and 2-phenylethanol exposure in Drosophila melanogaster

The fruit fly, Drosophila melanogaster, is predominantly found in overripe, rotten, fermenting, or decaying fruits and is constantly exposed to chemical stressors such as acetic acid, ethanol, and 2-phenylethanol. D. melanogaster has been employed as a model system for studying the molecular bases of various types of chemical-induced tolerance. Expression profiling using Illumina sequencing has been performed for identifying changes in gene expression that may be associated with evolutionary adaptation to exposure of acetic acid, ethanol, and 2-phenylethanol. We identified a total of 457 differentially expressed genes that may affect sensitivity or tolerance to three chemicals in the chemical treatment group as opposed to the control group. Gene-set enrichment analysis revealed that the genes involved in metabolism, multicellular organism reproduction, olfaction, regulation of signal transduction, and stress tolerance were over-represented in response to chemical exposure. Furthermore, we also detected a coordinated upregulation of genes in the Toll- and Imd-signaling pathways after the chemical exposure. Quantitative reverse transcription PCR analysis revealed that the expression levels of nine genes within the set of genes identified by RNA sequencing were up- or downregulated owing to chemical exposure. Taken together, our data suggest that such differentially expressed genes are coordinately affected by chemical exposure. Transcriptional analyses after exposure of D. melanogaster with three chemicals provide unique insights into subsequent functional studies on the mechanisms underlying the evolutionary adaptation of insect species to environmental chemical stressors.

Are Drosophila preferences for yeasts stable or contextual?

Whether there are general mechanisms, driving interspecific chemical communication is uncertain. Saccharomycetaceae yeast and Drosophila fruit flies, both extensively studied research models, share the same fruit habitat, and it has been suggested their interaction comprises a facultative mutualism that is instigated and maintained by yeast volatiles. Using choice tests, experimental evolution, and volatile analyses, we investigate the maintenance of this relationship and reveal little consistency between behavioral responses of two isolates of sympatric Drosophila species. While D. melanogaster was attracted to a range of different Saccharomycetaceae yeasts and this was independent of fruit type, D. simulans preference appeared specific to a particular S. cerevisiae genotype isolated from a vineyard fly population. This response, however, was not consistent across fruit types and is therefore context-dependent. In addition, D. simulans attraction to an individual S. cerevisiae isolate was pliable over ecological timescales. Volatile candidates were analyzed to identify a common signal for yeast attraction, and while D. melanogaster generally responded to fermentation profiles, D. simulans preference was more discerning and likely threshold-dependent. Overall, there is no strong evidence to support the idea of bespoke interactions with specific yeasts for either of these Drosophila genotypes. Rather the data support the idea Drosophila are generally adapted to sense and locate fruits infested by a range of fungal microbes and/or that yeast-Drosophila interactions may evolve rapidly.

Acetic acid activates distinct taste pathways in Drosophila to elicit opposing, state-dependent feeding responses

Taste circuits are genetically determined to elicit an innate appetitive or aversive response, ensuring that animals consume nutritious foods and avoid the ingestion of toxins. We have examined the response of Drosophila melanogaster to acetic acid, a tastant that can be a metabolic resource but can also be toxic to the fly. Our data reveal that flies accommodate these conflicting attributes of acetic acid by virtue of a hunger-dependent switch in their behavioral response to this stimulus. Fed flies show taste aversion to acetic acid, whereas starved flies show a robust appetitive response. These opposing responses are mediated by two different classes of taste neurons, the sugar- and bitter-sensing neurons. Hunger shifts the behavioral response from aversion to attraction by enhancing the appetitive sugar pathway as well as suppressing the aversive bitter pathway. Thus a single tastant can drive opposing behaviors by activating distinct taste pathways modulated by internal state.

Our sense of taste is critical to our survival. Taste helps us to consume nutritious foods and avoid toxins. There are five basic taste categories: sweet, salty, bitter, sour, and umami or savory, a taste typical of protein-rich foods. Each taste category activates a distinct pathway in the brain, triggering specific feelings and behaviors. We normally find sugar, salt, and components of protein pleasant, and seek out foods with these tastes. By contrast, we often find overly bitter or sour tastes unpleasant and try to avoid them. As sour and bitter-tasting substances often contain toxins, this response helps to protect us from poisoning.

Across the animal kingdom, these preferences are largely hardwired from birth. But the relationship between taste and nutrients is not always straightforward. Some substances can be toxic despite also containing useful nutrients. Overripe fruit, for example, is broken down by yeast and bacteria to produce acetic acid, or vinegar. Like other acids, acetic acid can be toxic. But for the fruit fly Drosophila melanogaster, also known as the vinegar fly, acetic acid from rotten fruit can be a valuable source of calories. So how do flies react to the taste of acetic acid?

Devineni et al. show that, unlike other chemicals, acetic acid triggers different taste responses in flies depending on whether the insects are hungry. Well-fed flies find the taste repulsive, probably because it signals toxicity. But hungry flies find it attractive, presumably because of their overriding need for calories. Devineni et al. show that acetic acid activates both sugar-sensing and bitter-sensing pathways in the fly brain. Hunger increases activity in the sugar pathway and reduces it in the bitter pathway. As a result, hungry flies are attracted to acetic acid, whereas fully fed flies are repulsed.

Flexibility in the taste system enables animals to react to the same substance in different ways depending on their current needs. Related to this, evidence suggests that obesity may be associated with altered sensitivity to certain tastes, such as sweet, as well as a blunted response to satiety signals. Understanding how the brain combines information about taste and hunger to control food consumption may ultimately help us to understand and treat obesity.

Conservation of social effects (Ψ) between two species of Drosophila despite reversal of sexual dimorphism

Indirect genetic effects (IGEs) describe the effect of the genes of social partners on the phenotype of a focal individual. Here, we measure indirect genetic effects using the “coefficient of interaction” (Ψ) to test whether Ψ evolved between Drosophila melanogaster and D. simulans. We compare Ψ for locomotion between ethanol and nonethanol environments in both species, but only D. melanogaster utilizes ethanol ecologically. We find that while sexual dimorphism for locomotion has been reversed in D. simulans, there has been no evolution of social effects between these two species. What did evolve was the interaction between genotype‐specific Ψ and the environment, as D. melanogaster varies unpredictably between environments and D. simulans does not. In this system, this suggests evolutionary lability of sexual dimorphism but a conservation of social effects, which brings forth interesting questions about the role of the social environment in sexual selection.

Olfactory detection of a bacterial short-chain fatty acid acts as an orexigenic signal in Drosophila melanogaster larvae

Microorganisms inhabiting fermenting fruit produce chemicals that elicit strong behavioral responses in flies. Depending on their ecological niche, individuals confer a positive or a negative valence to a chemical and, accordingly, they trigger either attractive or repulsive behaviors. We studied the case of bacterial short-chain fatty acids (SCFA) that trigger opposite behaviors in adult and larvae of Drosophila melanogaster. We determined that SCFA-attractive responses depend on two larval exclusive chemoreceptors, Or30a and Or94b. Of those SCFA, propionic acid improves larval survival in suboptimal rearing conditions and supports growth. Olfactory detection of propionic acid specifically is sufficient to trigger feeding behaviors, and this effect requires the correct activity of Or30a⁺ and Or94b⁺ olfactory sensory neurons. Additionally, we studied the case of the invasive pest Drosophila suzukii that lives on undamaged ripe fruit with less SCFA production. Contrary to D. melanogaster, D. suzukii larvae show reduced attraction towards propionic acid, which does not trigger feeding behavior in this invasive species. Our results demonstrate the relevance of propionic acid as an orexigenic signal in D. melanogaster larvae. Moreover, this study underlines that the changes on ecological niche are accompanied with alterations of olfactory preferences and vital olfactory driven behaviors.

Fermentation for Disinfesting Fruit Waste From Drosophila Species (Diptera: Drosophilidae)

Economic losses in a range of fruit crops due to the Drosophila suzukii (Matsumura) have become severe. Removal and treatment of fruit waste, which may harbor D. suzukii, is a key step in preventing reinfestation of fruit production. Natural fermentation for disinfesting fruit wastes from D. suzukii was examined at ambient air temperatures of 12-20 °C. Soft and stone fruit wastes infested with eggs, larvae, and pupae of Drosophila melanogaster (Meigen) or D. suzukii were placed in sealed vessels containing fruit wastes, and samples were retrieved at intervals and tested for the emergence of adults. Mean temperatures of the fruit waste in the sealed vessels during fermentation were 15-23 °C. Fermentation for 3 d was effective in disinfesting waste from different life stages of D. suzukii. Treatment for 4 d also ensured that the waste was free of viable life stages of D. melanogaster, which could be used as an indicator species for disinfestation of waste from D. suzukii owing to its greater tolerance of fermentation. The O2 concentration of the headspace air in the vessels became undetectable after 13-16 h, with a corresponding increase in CO2 concentration, which exceeded 80% vol/vol. The resulting hypoxia and hypercapnia may explain the efficacy of the fermentation treatment in disinfesting the waste. Fermented fruit remained attractive to D. suzukii and retained its capacity to rear a life cycle. Covering or mixing fermented fruit with a sufficient depth (0.1 m) or volume (×9) of soil or coir prevented the reinfestation of treated waste.

Feeding on ripening and over-ripening fruit: interactions between sugar, ethanol and polyphenol contents in a tropical butterfly

In ripe fruit, energy mostly derives from sugar, while in over-ripe fruit, it also comes from ethanol. Such ripeness differences may alter the fitness benefits associated with frugivory if animals are unable to degrade ethanol when consuming over-ripe fruit. In the tropical butterfly Bicyclus anynana, we found that females consuming isocaloric solutions mimicking ripe (20% sucrose) and over-ripe fruit (10% sucrose, 7% ethanol) of the palm Astrocaryum standleyanum exhibited higher fecundity than females consuming a solution mimicking unripe fruit (10% sucrose). Moreover, relative to butterflies consuming a solution mimicking unripe fruit, survival was enhanced when butterflies consumed a solution mimicking either ripe fruit supplemented with polyphenols (fruit antioxidant compounds) or over-ripe fruit devoid of polyphenols. This suggests that (1) butterflies have evolved tolerance mechanisms to derive the same reproductive benefits from ethanol and sugar, and (2) polyphenols may regulate the allocation of sugar and ethanol to maintenance mechanisms. However, variation in fitness owing to the composition of feeding solutions was not paralleled by corresponding physiological changes (alcohol dehydrogenase activity, oxidative status) in butterflies. The fitness proxies and physiological parameters that we measured therefore appear to reflect distinct biological pathways. Overall, our results highlight that the energy content of fruit primarily affects the fecundity of B. anynana butterflies, while the effects of fruit consumption on survival are more complex and vary depending on ripening stage and polyphenol presence. The actual underlying physiological mechanisms linking fruit ripeness and fitness components remain to be clarified.

ADAPTATION TO FERMENTING RESOURCES IN DROSOPHILA MELANOGASTER: ETHANOL AND ACETIC ACID TOLERANCES SHARE A COMMON GENETIC BASIS

Ethanol and acetic acid tolerances were compared in a French, highly tolerant population, and in a Congolese, very sensitive population. For both tolerances, chromosome substitutions demonstrated a major effect on chromosome 3, a lesser effect on chromosome 2, and no effect on chromosome 1, except in interactions. Directional selection experiments led to significant increases of tolerance to both toxics. Of greater interest, a strong correlated response was observed in each line: increased ethanol tolerance was accompanied by higher acetic acid tolerance and vice versa. A high genetic correlation (average value r = 0.77) was found between the two traits. These data suggest that alcohol dehydrogenase (ADH) activity does not play a major role in explaining the physiological differences known between Afrotropical and European populations. The metabolic flux permitting the detoxification of ethanol and acetic acid seems to be mainly controlled by acetyl-coA synthetase (ACS) at least in adult flies. Acetic acid adaptation could be as important as ethanol adaptation in the ecology of Drosophila melanogaster and other Drosophila species.

The Context of Chemical Communication Driving a Mutualism

Recent work suggests that Drosophila and Saccharomyces yeasts may establish a mutualistic association, and that this is driven by chemical communication. While individual volatiles have been implicated in the attraction of D. melanogaster, the semiochemicals affecting the behavior of the sibling species D. simulans are less well characterized. Here, we scrutinized a broad range of volatiles produced by attractive and repulsive yeasts to experimentally evaluate the chemical nature of communication between these species. When grown in liquid or on agar-solidified grape juice, attraction to S. cerevisiae was driven primarily by 3-methylbutyl acetate (isoamyl acetate) and repulsion by acetic acid, a known attractant to D. melanogaster (also known as vinegar fly). By using T-maze choice tests and synthetic compounds, we showed that these responses are strongly influenced by compound concentration. Moreover, the behavioral response is impacted further by the chemical context of the environment. Thus, chemical communication between yeasts and flies is complex, and is not driven simply by the presence of single volatiles, but modulated by compound interactions. The ecological context of chemical communication needs to be taken into consideration when testing for ecologically realistic responses.

Mechanisms of naturally evolved ethanol resistance in Drosophila melanogaster

The decaying fruit in which Drosophila melanogaster feed and breed can contain ethanol in concentrations as high as 6-7%. In this cosmopolitan species, populations from temperate regions are consistently more resistant to ethanol poisoning than populations from the tropics, but little is known about the physiological basis of this difference. I show that when exposed to low levels of ethanol vapor, flies from a tropical African population accumulated 2-3 times more internal ethanol than flies from a European population, giving evidence that faster ethanol catabolism by European flies contributes to the resistance difference. Using lines differing only in the origin of their third chromosome, however, I show that faster ethanol elimination cannot fully explain the resistance difference, because relative to African third chromosomes, European third chromosomes confer substantially higher ethanol resistance, while having little effect on internal ethanol concentrations. European third chromosomes also confer higher resistance to acetic acid, a metabolic product of ethanol, than African third chromosomes, suggesting that the higher ethanol resistance conferred by the former might be due to increased resistance to deleterious effects of ethanol-derived acetic acid. In support of this hypothesis, when ethanol catabolism was blocked with an Alcohol dehydrogenase mutant, there was no difference in ethanol resistance between flies with European and African third chromosomes.

A holidic medium for Drosophila melanogaster

A critical requirement for research using model organisms is an appropriate, well-defined and consistent diet. There is currently no complete chemically defined (holidic) diet available for Drosophila melanogaster. We describe a holidic medium that is equal in performance to an oligidic diet optimized for adult fecundity and lifespan. It is also sufficient to support development over multiple generations, but at a reduced rate. During seven years of experiments, the holidic diet yielded more consistent experimental outcomes than oligidic food for adult fitness traits. Furthermore, nutrients and drugs are more accessible to flies in holidic medium and, similar to dietary restriction on oligidic food, amino acid dilution increases fly lifespan. We also report amino acid specific effects on food choice behavior and that folic acid from the microbiota is sufficient for development. These insights could not be gained using oligidic or meridic diets.

Divergent behavioral and electrophysiological taste responses in the mid-legs of adult butterflies, Vanessa indica and Argyreus hyperbius

Adult nymphalid butterflies possess sensilla trichodea (ST) that perceive taste in their walking legs. We examined whether the gustatory responses to mid-leg tarsal stimulation were different between Vanessa indica (rotting-food feeder) and Argyreus hyperbius (flower-nectar feeder). Sucrose, fructose, and glucose elicited behavioral responses (proboscis extension reflex: PER) and electrophysiological responses (spikes) from ST. Sugar responsiveness was similar in both species, where sucrose was the most stimulatory. Two fermentation products, ethanol and acetic acid, never induced PERs but elicited large-amplitude spikes at a concentration of >1% (w/v). The two species significantly differed in responsiveness to the binary mixtures of sucrose and the fermentation products. Ethanol enhanced the sugar responses of V. indica but slightly inhibited those of A. hyperbius. Although acetic acid suppressed the sugar responses of both species, V. indica was less susceptible than A. hyperbius. When concentration of the fermentation products increased, binary mixtures evoked large spikes together with small ones regarded as the sucrose responses. Unlike the proboscal sensilla in our previous study, the tarsal ST of both species unambiguously responded to fermentation products. These results demonstrate that the tarsal gustatory sense of V. indica is adaptive to the use of rotting foods.

Alcoholism: A Systems Approach From Molecular Physiology to Addictive Behavior

Alcohol consumption is an integral part of daily life in many societies. The benefits associated with the production, sale, and use of alcoholic beverages come at an enormous cost to these societies. The World Health Organization ranks alcohol as one of the primary causes of the global burden of disease in industrialized countries. Alcohol-related diseases, especially alcoholism, are the result of cumulative responses to alcohol exposure, the genetic make-up of an individual, and the environmental perturbations over time. This complex gene × environment interaction, which has to be seen in a life-span perspective, leads to a large heterogeneity among alcohol-dependent patients, in terms of both the symptom dimensions and the severity of this disorder. Therefore, a reductionistic approach is not very practical if a better understanding of the pathological processes leading to an addictive behavior is to be achieved. Instead, a systems-oriented perspective in which the interactions and dynamics of all endogenous and environmental factors involved are centrally integrated, will lead to further progress in alcohol research. This review adheres to a systems biology perspective such that the interaction of alcohol with primary and secondary targets within the brain is described in relation to the behavioral consequences. As a result of the interaction of alcohol with these targets, alterations in gene expression and synaptic plasticity take place that lead to long-lasting alteration in neuronal network activity. As a subsequent consequence, alcohol-seeking responses ensue that can finally lead via complex environmental interactions to an addictive behavior.

Alcohol dehydrogenase activities and ethanol tolerance in Anastrepha (Diptera, Tephritidae) fruit-fly species and their hybrids

The ADH (alcohol dehydrogenase) system is one of the earliest known models of molecular evolution, and is still the most studied in Drosophila. Herein, we studied this model in the genus Anastrepha (Diptera, Tephritidae). Due to the remarkable advantages it presents, it is possible to cross species with different Adh genotypes and with different phenotype traits related to ethanol tolerance. The two species studied here each have a different number of Adh gene copies, whereby crosses generate polymorphisms in gene number and in composition of the genetic background. We measured certain traits related to ethanol metabolism and tolerance. ADH specific enzyme activity presented gene by environment interactions, and the larval protein content showed an additive pattern of inheritance, whilst ADH enzyme activity per larva presented a complex behavior that may be explained by epistatic effects. Regression models suggest that there are heritable factors acting on ethanol tolerance, which may be related to enzymatic activity of the ADHs and to larval mass, although a pronounced environmental effect on ethanol tolerance was also observed. By using these data, we speculated on the mechanisms of ethanol tolerance and its inheritance as well as of associated traits.

Membrane lipid physiology and toxin catabolism underlie ethanol and acetic acid tolerance in Drosophila melanogaster

Drosophila melanogaster has evolved the ability to tolerate and utilize high levels of ethanol and acetic acid encountered in its rotting-fruit niche. Investigation of this phenomenon has focused on ethanol catabolism, particularly by the enzyme alcohol dehydrogenase. Here we report that survival under ethanol and acetic acid stress in D. melanogaster from high- and low-latitude populations is an integrated consequence of toxin catabolism and alteration of physical properties of cellular membranes by ethanol. Metabolic detoxification contributed to differences in ethanol tolerance between populations and acclimation temperatures via changes in both alcohol dehydrogenase and acetyl-CoA synthetase mRNA expression and enzyme activity. Independent of changes in ethanol catabolism, rapid thermal shifts that change membrane fluidity had dramatic effects on ethanol tolerance. Cold temperature treatments upregulated phospholipid metabolism genes and enhanced acetic acid tolerance, consistent with the predicted effects of restoring membrane fluidity. Phospholipase D was expressed at high levels in all treatments that conferred enhanced ethanol tolerance, suggesting that this lipid-mediated signaling enzyme may enhance tolerance by sequestering ethanol in membranes as phophatidylethanol. These results reveal new candidate genes underlying toxin tolerance and membrane adaptation to temperature in Drosophila and provide insight into how interactions between these phenotypes may underlie the maintenance of latitudinal clines in ethanol tolerance.

Evolution of the AMP-forming acetyl-CoA synthetase gene in the Drosophilidae family

Analysis of the AMP-forming ACS gene was performed in 12 species of the Drosophilidae family. Systematically four introns, aligned at the same positions, were detected, but none of them showed a position similar to those known for species outside the Drosophilidae family. The average length of introns varied from 63 to 75 bp but in two species Drosophila takahashii and D. kikkawai the length of the second intron was 343 and 210 bp, respectively. In coding regions, about 80% of the third codon positions were substituted while first and second positions showed, respectively, 14% and 6% substitutions. Interestingly, the divergence observed at the protein level between species was very low. The phylogenetic tree based on the DNA sequences of the exons was mainly in agreement with taxonomic classification and previous molecular phylogenies except for D. ananassae, which appeared more closely related to D. subobscura and D. funebris than to the species of the melanogaster group.

Microspatial structure of Drosophila melanogaster populations in Brazzaville: evidence of natural selection acting on morphometrical traits

Two genetically distinct habitat races of Drosophila melanogaster coexist in Brazzaville (Congo). One is the typical field type of Afrotropical populations, the other mainly breeds in beer residues in breweries. These two populations differ in their ethanol tolerance, in their allelic frequencies at several enzyme and microsatellite loci and in the composition of their cuticular hydrocarbons. The brewery population is quite similar to European temperate populations with regard to all these traits. Previous investigations of two morphological traits (ovariole number and sternopleural bristle number) failed to detect any difference between the two habitat races. Here we investigated other morphological traits (wing and thorax length, thorax pigmentation and female abdomen pigmentation). The reaction norms of these traits according to growth temperature were compared in the two Afrotropical habitat races and in a French temperate population. As expected, the French population was very different from the field African population: as a general rule, the brewery population (Kronenbourg) was intermediate in several aspects between the other two. We conclude that the strong selective forces that maintain the genetic divergence between the two habitat races also act on morphometrical traits, and the possible selective mechanisms are discussed.

Molecular evolution of the AMP-forming Acetyl-CoA synthetase

Acetyl-CoA-Synthetase (ACS) is involved in the production of acetate, a major metabolite in numerous organisms. There are two forms of this enzyme: ADP-forming ACS and ATP-forming ACS. We focus mainly on the AMP-forming ACS gene, which is relatively well conserved in eubacteria, archeaebacteria, and eukaryotes. BLAST searches in databases showed 30 protein sequences significantly related to the ACS. Most of these sequences were identified as ACS but three of them, belonging to the mammalian species, were annotated as another gene named: the SA gene, which is involved in the essential hypertension. The ACS and SA genes probably derived from a duplication of an ancestral gene but have acquired different functions. Six conserved regions of the ACS protein were defined across the three domains of life. While the precise function of the conserved regions remains unknown, they are probably involved in the enzymatic activity. Among eukaryotes, we found a high variability with respect to the number and the position of introns. However, some positions are conserved between fungi and a nematode. A maximum likelihood tree based upon the conserved regions showed that all sequences except the one from B. subtilis, belong to two basic groups: one the SA-like group including sequences from Archaeoglobus fulgidus and Streptomyces coelicolor, and second, the ACS group. The later can be further divided in two parts: a prokaryotic one including eubacteria and an archaebacterium, and a eukaryotic group within which two proteobacterial sequences branch including ACS from the alpha-proteobacterium Rhodobacter capsulatus. Within the eukaryotic group, bootstrap support is very low, but overall the data are consistent with the view that eukaryotes acquired their ACS gene from the ancestors of mitochondria. The localization of this enzyme in eukaryotic mitochondria is the additional evidence in favor of this interpretation.

Deletion of a conserved regulatory element in the Drosophila Adh gene leads to increased alcohol dehydrogenase activity but also delays development

In vivo levels of enzymatic activity may be increased through either structural or regulatory changes. Here we use Drosophila melanogaster alcohol dehydrogenase (ADH) in an experimental test for selective differences between these two mechanisms. The well-known ADH-Slow (S)/Fast (F) amino acid replacement leads to a twofold increase in activity by increasing the catalytic efficiency of the enzyme. Disruption of a highly conserved, negative regulatory element in the Adh 3' UTR also leads to a twofold increase in activity, although this is achieved by increasing in vivo Adh mRNA and protein concentrations. These two changes appear to be under different types of selection, with positive selection favoring the amino acid replacement and purifying selection maintaining the 3' UTR sequence. Using transgenic experiments we show that deletion of the conserved 3' UTR element increases adult and larval Adh expression in both the ADH-F and ADH-S genetic backgrounds. However, the 3' UTR deletion also leads to a significant increase in developmental time in both backgrounds. ADH allozyme type has no detectable effect on development. These results demonstrate a negative fitness effect associated with Adh overexpression. This provides a mechanism whereby natural selection can discriminate between alternative pathways of increasing enzymatic activity.

Larval tolerance in the Drosophila melanogaster species complex toward the two toxic acids of the D. sechellia host plant

The toxicity of hexanoic (C6) and octanoic (C8) acids, the two major components of the host plant of Drosophila sechellia, was investigated upon larvae of the four species included in the D. melanogaster complex and on interspecific hybrids between D. sechellia and D. simulans. Specific methods had to be devised for obtaining reproducible toxicity results. The three generalist species (D. melanogaster, D. mauritiana and D. simulans) were found to be very sensitive, as indicated by low lethal concentrations and an increase in development duration. By contrast D. sechellia was much more tolerant, especially toward C8 which is the most abundant product in the natural resource. Interspecific hybrids (F1 and backcrosses) exhibited intermediate characteristic, but a dominance of D. simulans sensitivity was observed for both acids and especially for C8. Data on larvae are quite different from those previously obtained on adults, and are more likely to reflect the natural selective pressures existing in the wild.

LUSH odorant-binding protein mediates chemosensory responses to alcohols in Drosophila melanogaster

The molecular mechanisms mediating chemosensory discrimination in insects are unknown. Using the enhancer trapping approach, we identified a new Drosophila mutant, lush, with odorant-specific defects in olfactory behavior. lush mutant flies are abnormally attracted to high concentrations of ethanol, propanol, and butanol but have normal chemosensory responses to other odorants. We show that wild-type flies have an active olfactory avoidance mechanism to prevent attraction to concentrated alcohol, and this response is defective in lush mutants. This suggests that the defective olfactory behavior associated with the lush mutation may result from a specific defect in chemoavoidance. lush mutants have a 3-kb deletion that produces a null allele of a new member of the invertebrate odorant-binding protein family, LUSH. LUSH is normally expressed exclusively in a subset of trichoid chemosensory sensilla located on the ventral-lateral surface of the third antennal segment. LUSH is secreted from nonneuronal support cells into the sensillum lymph that bathes the olfactory neurons within these sensilla. Reintroduction of a cloned wild-type copy of lush into the mutant background completely restores wild-type olfactory behavior, demonstrating that this odorant-binding protein is required in a subset of sensilla for normal chemosensory behavior to a subset of odorants. These findings provide direct evidence that odorant-binding proteins are required for normal chemosensory behavior in Drosophila and may partially determine the chemical specificity of olfactory neurons in vivo.

Light-induced vitamin deficiency in Drosophila melanogaster

Illumination by visible light (400 Ix) of cultures containing larvae of Drosophila melanogaster can reduce survival (Bruins et al., Insect Biochemistry 21:535-539, 1991). Here we show that the effect of light depends on the presence of propionic or acetic acid in the food medium. We also show that survival is far more affected by illumination of the yeast food media than by direct illumination of the eggs and developing larvae. It is shown that addition of antioxidants to the food prevents light induced mortality. The action of antioxidants suggests that free radicals are important in light induced mortality. We also showed that both yeast and riboflavin (vitamin B2) solutions illuminated with visible light (400 Ix) generate hydrogen peroxide. Other vitamin and amino acid solutions do not produce peroxide in measurable amounts. However, the concentration of photogenerated hydrogen peroxide is far too low to explain the death of eggs and developing larvae upon exposure to light. A 400 Ix light treatment destroys the capability of yeast food media to support survival of larvae. Addition of vitamin C, carotene, tryptophan, nipagin, uric acid, or sucrose to the light treated medium does not restore viability. It is restored when riboflavin is added to the photo-inactivated yeast. A high concentration of pyridoxine also produced an improvement in survival. When riboflavin is treated with light, it cannot support survival on synthetic food media nor can it restore survival on light treated yeast food media. These results show that riboflavin (or a derivative) is a major light sensitive compound of yeast, which can be degraded by light. Light induced loss of riboflavin leads to mortality, because this is an essential dietary vitamin. The vitamin degradation can be prevented by dietary antioxidants. A chromatographic analysis confirms this conclusion.

Phenotypic expression of ADH regulatory genes in Drosophila melanogaster: a comparative study between a paleartic and a tropical population

In vitro ADH activity was studied in D. melanogaster males from two sets of third chromosome substitution lines, one from a paleartic population (Gigean, France), the other from a tropical population (Brazzaville, Congo). As a linear model with raw ADH activity dependent on fresh weight was significant in both sets of lines, the raw activity was adjusted by regression on weight. Two main results were found: (a) the well-known substantial intrapopulation variability; and (b) third chromosome geographical origin did not affect the mean ADH activity. Unlike the structural Adh gene polymorphism which allows the two populations to be distinguished, the polymorphism of the third chromosome ADH regulatory genes (or more exactly their phenotypic expression) does not allow to discriminate between them. These results are discussed in the context of the adaptation of D. melanogaster to the alcoholic substrates in light of a model proposed by Hedrick and McDonald (1980) in order to interpret variations in both structural and regulatory gene polymorphisms.