Egg-laying preference for ethanol involving learning has adaptive significance inDrosophila melanogaster

To study the influence of negative or positive experience of the ingestion of ethanol in femaleDrosophila with thefast form of the Adh gene on the acceptance of a medium containing this substance as site for egg laying, flies raised on a medium without alcohol were kept (1) on a regular medium, (2) on a medium supplemented with ethanol, (3) on a medium supplemented with 4-methylpyrazole (4-MP, an inhibitor of the enzyme ADH), and (4) on a medium containing both ethanol and this inhibitor. Whereas theDrosophila exhibited no spontaneous egg-laying preference for ethanol, a preference was induced by the ingestion of this substance, which also led to an increase in the number of eggs laid. By contrast, ingestion of ethanol, associated with the inhibitor, reduced the number of eggs laid and led to an avoidance of ethanol-containing media. Flies treated with ethanol and 4-MP exhibited signs of sickness (reduction in locomotor activity and loss of balance). The egg-laying preference for the ethanol results from learning linked to the metabolic utilization of this alcohol. The adaptative significance of this learning is discussed.