In a recent study published in Science Advances, researchers present ecological and evolutionary perspectives for alcohol attraction among Drosophila melanogaster flies.
These flies, which have evolved in environments rich in alcohol, such as fermenting fruit, are driven toward alcohol to increase their mating success rather than as a coping mechanism to deal with partner rejection.
Study: Neuroecology of alcohol risk and reward: Methanol boosts pheromones and courtship success in Drosophila melanogaster. Image Credit: NuayLub/Shutterstock.com
Introduction
Male Drosophila flies compete for female attention, often courting the same female. Therefore, any advantage in sexual signals or pheromones (e.g., detection, content, or abundance) can affect sexual selection and courtship success.
Researchers have observed a strong attraction to ethanol in adults and larvae of the Drosophila fly. Previous studies suggested that alcohol consumption helps cope with mate rejection.
The authors propose that attraction to alcohol is not just a response to negative experiences but an evolutionary adaptation that improves mating success in their natural environment. This perspective shifts the focus from a psychological interpretation to an ecological and evolutionary one.
About the Study
In the present study, researchers explored the behavioral ecology aspects of alcohol attraction in Drosophila flies.
The researchers conducted Flywalk assays to analyze fly behavior in the presence of alcohol. This assay allowed simultaneous tracking of flies exposed to controlled odorant pulses. The team assessed fly behavior before, during, and after odor stimulation to measure attraction to ethanol and methanol.
Capillary feeder (CAFÉ) assays tested flies for feeding preferences using sucrose solutions with and without methanol. Tube assays assessed fly responses to various alcohols. Further, researchers quantified the survival rates of flies exposed to varying concentrations of alcohol to evaluate toxicity. They tested attraction at low (3.0%) and high (15%) ecologically relevant methanol concentrations.
Gas chromatography-mass spectrometry (GC-MS) enabled the sampling of fermenting resources. Researchers investigated natural host resources producing high levels of ethanol and methanol during fermentation.
They identified fruits that virgin and mated males preferred during the trials. They generated axenic flies to determine the influence of microbes on alcohol attraction.
Courtship assays evaluated mating preferences based on dietary exposure. The team investigated whether males exposed to fermenting citrus had any competitive advantage over those fed standard laboratory diets. They used deuterated isotopes of methanol (i.e., CD3OD instead of CH3OH) to examine the role of alcohol in pheromone biosynthesis.
Two-photon calcium imaging in olfactory sensory neurons (OSNs) and projection neurons (PNs) revealed neuronal responses to alcohol stimuli in flies. Three-dimensional reconstructions enabled the analysis of neuronal pathways linked to alcohol detection.
Since the flies were motivated to contact alcohol-rich substances, researchers investigated the mechanisms of odor detection in flies using whole antenna electroantennograms (EAG) and maxillary palp recordings.
Single-sensillum recordings (SSRs) revealed the odorant receptors (ORs) involved in alcohol attraction. The team revisited attraction paradigms using transgenic flies, testing flies with individually silenced olfactory sensory neurons that activate alcohol-detecting olfactory receptors.
Results
Virgin males exhibited higher attraction to both alcohols (ethanol and methanol) compared to mated males. However, methanol elicited a stronger attraction.
Males exposed to methanol-rich sources, such as citrus fruits, increased production of aggregation and courtship pheromones such as methyl laurate (ML), methyl palmitate (MP), and methyl myristate (MM). This increase resulted from fatty acids converting to their ester forms on exposure to alcohol.
Citrus-exposed males showed lower copulation latency (517 seconds vs. 889 seconds) and higher copulation success (63% vs. 37%) than those on standard diets. Exposure to methanol increased courtship success; however, the alcohol is toxic at high concentrations. Thus, flies must weigh the risk-benefit ratio of alcohol attraction.
Low alcohol concentrations detected by Or42b (expressed by OSN type ab1A) and Or59b (ab2A) activate attraction to alcohol, whereas higher concentrations activating Or42a (pb1A) lead to alcohol aversion.
The findings support previous data on the neural pathways of the brain that link alcohol risk and reward, particularly those related to neuropeptide F. Two glomeruli in the antennal lobe (AL) detect ethanol and methanol. They include DM1 and DM4, innervated by OSN ab1A and ab2A, respectively.
Conclusions
The researchers highlight a biological basis for alcohol preference among Drosophila melanogaster flies. These flies are attracted to fruits with high alcohol content, especially methanol, where exposure rapidly amplifies the production of pheromones that improve mating success.
The findings suggest that exposure to ecologically relevant and alcohol-containing natural host substrates provides a courtship and mating advantage. Methanol exposure led to stronger attraction; however, the alcohol can be fatal at high concentrations.
The findings highlight the trade-offs between risk and reward related to the alcohol preference of flies. The rewards may, at times, offset the risks, particularly for virgin and rejected males who seek a competitive advantage for mating.
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