Ethanol-guided behavior in Drosophila larvae

Abstract

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.

The function of ethanol in olfactory associative behaviors inDrosophila melanogasterlarvae

“…During development, D. melanogaster larvae are exposed to increasing concentrations of ethanol in their environment when growing on fruits colonized with yeast [16]. The larvae perceive ethanol as an odorant [6]. Larvae also develop withdrawal-like symptoms after long-term ethanol exposure, suggesting that ethanol elicits rewarding properties [14].…”

“…This is supported by the observation that Canton S larvae show a positive association with odorants paired with 5% ethanol in one learning situation as determined by calculating the learning index, whereas w 1118 larvae mostly develop a negative association with odorants with 5% ethanol. A positive association with ethanol has been recently described for olfactory learning and memory experiments for Canton S larvae using higher ethanol concentrations of 8% and 20% [6]. However, this raises the question of ecological relevance, since in natural environments, ethanol concentrations are rarely so high [18].…”

“…Embryos and larvae develop in an environment with steadily increasing ethanol concentrations. The ethanol-enriched environment provides nutrients, increases fitness and protects against predatory insects [3–6]. In the natural environment, ethanol rarely reaches concentrations greater than 5% [7].…”

“…Drosophila larvae move toward a source of ethanol [9]. The attraction to the ethanol odorant does not depend on preexposure and varies slightly at different larval stages and ethanol concentrations [6]. Larvae are attracted to a wide variety of odorants [10], and it is not clear whether the larvae respond with exploratory approach behavior to the presence of an odorant in an odorant- and food-free environment when searching for food or whether larvae associate a positive valence with ethanol.…”

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“…They naturally feed on decaying fruits, which contain varying concentrations of alcohol-a by-product of fermentation. Consumption of food with low to moderate levels of alcohol [(alcohol) < 4%) can result in increased larval fitness, while higher concentrations [(alcohol) > 4%] can cause significant developmental delays and cognitive dysfunction (McKenzie and Parsons, 1972;Robinson et al, 2012b;Schumann et al, 2021). Thus, it is not surprising that adult D. melanogaster females prefer to lay eggs on foods containing low to moderate alcohol concentrations while avoiding concentrations higher than 5% (Kacsoh et al, 2013).…”