A genetic tool kit for cellular and behavioral analyses of insect sugar receptors

<div><p>Arthropods employ a large family of up to 100 putative taste or gustatory receptors (Grs) for the recognition of a wide range of non-volatile chemicals. In <i>Drosophila melanogaster</i>, a small subfamily of 8 <i>Gr</i> genes is thought to mediate the detection of sugars, the fly's major nutritional source. However, the specific roles for most <i>sugar Gr</i> genes are not known. Here, we report the generation of a series of mutant <i>sugar Gr</i> knock-in alleles and several composite <i>sugar Gr</i> mutant strains, including a sugar blind strain, which will facilitate the characterization of this gene family. Using Ca²⁺ imaging experiments, we show that most gustatory receptor neurons (GRNs) of sugar blind flies (lacking all 8 <i>sugar Gr</i> genes) fail to respond to any sugar tested. Moreover, expression of single <i>sugar Gr</i> genes in most sweet GRNs of sugar-blind flies does not restore sugar responses. However, when pair-wise combinations of <i>sugar Gr</i> genes are introduced to sweet GRNs, responses to select sugars are restored. We also examined the cellular phenotype of flies homozygous mutant for <i>Gr64a</i>, a <i>Gr</i> gene previously reported to be a major contributor for the detection of many sugars. In contrast to these claims, we find that sweet GRNs of <i>Gr64a</i> homozygous mutant flies show normal responses to most sugars, and only modestly reduced responses to maltose and maltotriose. Thus, the precisely engineered genetic mutations of single <i>Gr</i> genes and construction of a sugar-blind strain provide powerful analytical tools for examining the roles of <i>Drosophila</i> and other insect <i>sugar Gr</i> genes in sweet taste.</p>