Gustatory Perception of Sugars in Drosophila melanogaster Larvae

Drosophila melanogaster larvae have historically been targeted as favorable models for the study of chemosensation and chemosensory learning due to the relative simplicity of their neuroanatomy. Sugars are traditionally used as a ‘reward’ in learning assays, though the neurons and genes involved in larval sugar sensation are not fully known. Analysis of transgenic D. melanogaster larvae coexpressing GAL4 promoter constructs with UAS-GFP confirmed the expression of one putative sugar gustatory receptor (psGr) gene in the larval stage of the life cycle, along with a novel sugar receptor, Gr43a, which is not a member of the traditional psGr gene family. Gr43a was found to be expressed in six neurons; two in the dorsal pharyngeal sense organ, and four further down the oesophagus, and Gr5a shows expression in 18 bilaterally symmetric neurons distributed along the length of the larva (segmental nerves.) Analysis of the Gr64a, Gr64f, and Gr61a genes has yielded no evidence of expression. Larval two-choice assays (LTCA) revealed a decreased preference for the sugar fructose in ΔGr43a larvae, indicating that it functions as a high-affinity fructose receptor, as it does in the adult. Experiments with multiple-knockout strains indicate one or more genes in the Gr64a-f cluster or Gr61a also play a minor role in the perception of fructose. These experiments demonstrate the utility of the LTCA in deorphanizing chemoreceptors