Candidate ionotropic taste receptors in the Drosophila larva

Abstract

We examine in Drosophila a group of ∼35 ionotropic receptors (IRs), the IR20a clade, about which remarkably little is known. Of 28 genes analyzed, GAL4 drivers representing 11 showed expression in the larva. Eight drivers labeled neurons of the pharynx, a taste organ, and three labeled neurons of the body wall that may be chemosensory. Expression was not observed in neurons of one taste organ, the terminal organ, although these neurons express many drivers of the Gr (Gustatory receptor) family. For most drivers of the IR20a clade, we observed expression in a single pair of cells in the animal, with limited coexpression, and only a fraction of pharyngeal neurons are labeled. The organization of IR20a clade expression thus appears different from the organization of the Gr family or the Odor receptor (Or) family in the larva. A remarkable feature of the larval pharynx is that some of its organs are incorporated into the adult pharynx, and several drivers of this clade are expressed in the pharynx of both larvae and adults. Different IR drivers show different developmental dynamics across the larval stages, either increasing or decreasing. Among neurons expressing drivers in the pharynx, two projection patterns can be distinguished in the CNS. Neurons exhibiting these two kinds of projection patterns may activate different circuits, possibly signaling the presence of cues with different valence. Taken together, the simplest interpretation of our results is that the IR20a clade encodes a class of larval taste receptors.taste | ionotropic receptors | taste receptors | larva

Drosophila Chemoreceptors: A Molecular Interface Between the Chemical World and the Brain

“…Some IRs are expressed in gustatory organs of adult or larval Drosophila [88, 97, 98], where investigations into their roles in taste perception have only recently been initiated. A large clade of IRs, the IR20a clade, consists of ~35 members that are more distantly related to the iGluRs.…”

“…Neurons expressing these drivers project to taste centers in the CNS. Eleven drivers are expressed in the larva, mostly in single pairs of pharyngeal neurons [97]. …”

“…IR76b has also been proposed to be a co-receptor with other IRs [93]. IR25a may be a co-receptor in both olfactory and taste systems [88, 93, 97], providing another interesting link between the two sensory modalities.…”

Molecular neurobiology of Drosophila taste

“…As in the adult, members of Gr , ppk , and Ir families are expressed in larval taste neurons [19-21], defining sub-populations that are likely to have distinct functional roles. Gr and Ir genes that are expressed include some that are specific to larvae and others that are also in the adult [19,20]. Only a single sweet receptor, Gr43a, is expressed in the pharynx and brain [22].…”

The role of the sugar receptor IR60b for Drosophila melanogaster: A hypothesis

“…We note that Drosophila larvae also use external digestion: they live in their food and engage in social digestion as they feed collectively (Gregg et al, 1990;Dombrovski et al, 2017) and actively regulate external digestion (Sakaguchi & Suzuki, 2013). Larvae express IR60b in their dorsal pharyngeal sensilla (Stewart et al, 2015).…”