Drosophilas feed on microorganisms such as yeasts, that thrive on decaying fruits, but they also eat various nutrients from these fruits, such as carbohydrates, that they appreciate. They are able to digest starch thanks to an enzyme, alpha-amylase, but they cannot sense starch itself. Their sensory system is known to enable Drosophila melanogaster to detect monomeric sugars like glucose or fructose, or dimers like maltose or saccharose using 9 gustatory receptors (GR) over 60 that are present.
We have carried out a series of behavior and electrophysiology observations to assess the capacity of detecting glucose polymers (up to G12) in D. melanogaster adults. While we were expecting the highest sensitivity for glucose and maltose and then a progressive decrease with increasing polymer length, we observed that sensitivity increased up to G5, then decreased from G6 to G12. This result cannot be easily interpreted, peculiarly regarding the detection mechanism. Such a detection would require that the flies be endowed with GRs harboring a detection site of sufficient size to accomodate a bulky molecule such as maltopentaose, or a cooperation between neighboring receptors, or the quick action of an enzyme degrading the glucose polymers into smaller sugars inside gustatory sensillas. We also observed that the preference for G5 over G1 is general, but variable depending on the species of Drosophila studied, with sometimes surprising consumption kinetics.
In order to decipher the mechanisms at work, we propose a genomic approach based on the study of the extant variability of a drosophila population. This takes advantage of the Drosophila genetic reference panel (DGRP), a collection of 200 iso-female lines from a single american population collected on a market, each of which with its genome sequenced. Characterizing the behavioral response to G5 vs. G2, we will assign lines into responsive or non-responsive to G5.
To carry out this projet, the student will make use a device set up in the laboratory, named MultiCaFe (multiple capillary feeding), that measures the individual consumption of food and food preference choice for nearly 100 flies simultaneously in a semi-automated way.
Then, by a GWAS (genome-wide association study) analysis, contrasting the most responsive vs. the less responsive lines towards G5, we will be able to identify genes potentially involved in the sensitivity to this glucose oligomere. Once some candidate genes will be identified, their role in G5 detection will be assessed using a genetic analysis that consists in inactivationg the said genes in control flies.
It will be also advisable to extend the number of species compared, especially contrasting different ecological preferences.