Brain Structures Involved in the Locomotor Behaviour

Drosophila brainThe Drosophila brain is divided in four parts: the optic lobes, the antennal lobes, the central complex and the mushroom bodies. The mushroom bodies play an important role in mediating complex behaviors, such as olfactory learning and memory, courtship, locomotor activity, place memory, visual tasks such as context generalization and in decision and choice behavior.

These structures (and especially the gamma lobes) have also been shown to control the centrophobism/thigmotaxis behaviour. This behaviour refers to the preference of peripheral zones and the avoidance of central areas. Evolutionarily and ecologically, it may be considered as an antipredator strategy because it allows animals to avoid open spaces and decreases their probability of meeting a predator.

Basically, when a neurotransmitter activates a cell, the signal is relayed by a second messenger which amplifies it and induces the response. In the mushroom bodies, the cyclic Adenosine MonoPhosphate (cAMP) is one of the most important second messenger and acetylcholine is the main excitatory neurotransmitter in the Drosophila brain.

Using mutant Drosophila strains, I analysed the effects of the cAMP pathway as well as the cholinergic transmission in the centrophobism behaviour.

Mutations affecting both the cAMP transduction pathway and the cholinergic transmission disrupt the centrophobism behaviour. Flies lacking the acetylcholine receptors less avoid the central zones of an arena. A similar effect is found when the intracellular level of cAMP is disturbed (increased or decreased) or when the main target of the cAMP (Protein Kinase A) is lacking.

Regulation of pheromone communication and sexual behavior

Drosophila matingIn the wild, Drosophila flies aggregate on decaying fruit where they feed, mate and lay eggs. In order to find a mate, it is thus important for a male to find fruit on which sexually receptive females are present.

Food-related odours are responsible for attracting flies from a long distance by mediating upwind flight. Then, when getting close to the food source, flies are attracted by the presence of other flies. However, only odours (pheromones) produced by copulating flies are attractive.

The pheromone responsible for this attraction is the male-produced pheromone cis-vaccenyl acetate (cVA). Males transfer large amounts of cVA onto females during mating, which repels other males from courting these recently mated females. In addition, exposure to such high quantity of pheromones during mating subsequently abolish cVA attraction in females. We identified the neuronal mechanisms involved in the suppression of pheromone attraction in mated females. Attraction to cVA is mediated by the activation of a specific odorant receptor (Or67d) in the fly antennae. When females are exposed to large amount of pheromones, a second odorant receptor (Or65a) is activated, which decreases the signal coming from Or67d-expressing olfactory neurones in a brain structure involved in olfactory processing called antennal lobe.

Sexual receptivity, as well as pheromone perception, is strongly regulated by the fly nutritional state. When starved, females give priority to food searching over mate finding and reproduction. Consequently, female attraction to cVA is strongly decreased when deprived of food. This is partly due to the effect of insulin, a peptide hormone inviloved in the regulation of feeding and metabolism, on Or67d olfactory neurones. In addition, another hormone involved in sugar and lipid metabolism, the adipokinetic hormone, also regulates pheromone perception and sexual behavior depending on the fly feeding state.

Evolution of reproductive behavior in Drosophila flies

In contrast to other Drosophila species that lay their eggs on rotten fruit, the spotted wing Drosophila (Drosophila suzukii) shifted its preference for fresh berries. This change in ecology is accompanied with a modification in their pheromone communication system.

Indeed, D. suzukii does not produce cVA, while most of other Drosophila species do, and has undergone structural modification in the part of the antennal lobe processing pheromone signals. Consequently, instead of promotting sexual receptivity, cVA has become a repellent and inhibits mating in D. suzukii.

The change in egg-laying preference in this species is also probably due to a modification of its olfactory system. My current project is to understand what are the chemical signals and odorant receptors involved in the preference for laying eggs on fresh fruit in D. suzukii.


Copyright © 2011 Lebreton