Behavioural responses and long chain fatty acides preferences in Drosophila melanogaster
Fatty-acids (FAs) are crucial for animal survival and reproduction since these molecules are involved in many metabolic pathways and constitute a substantial store of energy. Despite the strong relationship established between excessive FA consumption and severe human etiologies (obesity, cancer, vascular diseases), our knowledge of the mechanisms underlying FA perception and preference is limited particularly in invertebrates. This is one of the reasons why we decided to investigate, during our PhD thesis, the perception of long-chain FAs (C14:0, C16:0, C18:0, C18:1, C18:2, C18:3), using the model species Drosophila melanogaster. We have also attempted to characterize some of the biological factors involved in the modulation of this response. Our behavioral tests, carried both on individuals and groups of the two wild-type strains Dijon2000 (Di2) and Canton-S, revealed that larvae are able to distinguish these FAs. More precisely, we observed that larvae are attracted by unsaturated FAs and strongly repulsed and/or stressed by saturated FAs. The latter compounds also induce a strong behavioral reaction in groups of larvae resulting into a long-lasting aggregation figure. Another reason to study FAs is their structural proximity with cuticular hydrocarbons which act as sex pheromones in adults. Therefore, we measured the behavioral response of mutants for the desat1 and CheB42a genes, both of which are involved in the perception and/or discrimination of these pheromones. The fact that mutant desat1 larvae do not show aggregation behavior suggests that they have an abnormal perception or reaction to saturated FAs. We then targeted genetic alteration of desat1 using drivers (made with different regulatory regions of desat1) combined with a transgene carrying the interferential RNA of desat1. When targeting a specific brain region, we found that larvae do not properly react to C18:0. This result is coherent with the defect described above. On the other hand, larvae carrying the CheB42a-Gal4 transgene exhibit more subtle defects including a slightly altered reaction to saturated FAs. We also took advantage of the short generation time of D. melanogaster to perform two selection experiments using the parental Di2 strain during 50 generations. First, we selected, generation after generation, larvae showing decreased aversive response to C18:0, and we observed, after 20 generations, a weaker tendency to avoid this FA. Second, we maintained the Di2 strain on a food with a poor FA content, and we noted that resulting larvae show an altered response to C18:3. To check whether FA preference could change during life time, we also measured adult behavior. A first test performed in a olfactometer to assess olfactory response, revealed that flies are repulsed by C18:3, but are indifferent to C18:0. A second test assessing adult gustatory response (with the repression of the proboscis extension reflex) showed that unsaturated FAs repress behavior more strongly than saturated FAs. In summary, during our PhD thesis, we have shown that FA preference can change during the life of D. melanogaster, maybe in relation with different nutritional requirements. FAs seem to be perceived by olfaction, taste and maybe mechanosensation.