Molecular basis for the detection of nutrients and toxins by the honeybee

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Our sense of taste is our primary means of detecting nutrients and toxins in food, and its function is important for our health and well-being. The principles of gustation are shared in mammals and insects making it possible to use insects as model organisms to understand how chemical information is detected and encoded by the gustatory system. When animals ingest food, nutrients like sugars are detected by cells in taste buds on the tongue, and in insects by neurons in chemosensory sensilla. In general, the gustatory system is organized such that a subset of gustatory cells or neurons is excited by sugars, whereas others are excited by toxic (bitter) compounds, by amino acids, by salts, or by water. In insects, gustatory receptors (Grs) on the membranes of taste neurons selectively bind to classes of chemical compounds (e.g. sugars) and their activation indicates which compounds are present in food. Animals may have as many as a few hundred Grs, but millions of potential ligands exist. We do not know how Gr diversity affords the detection and classification of chemical compounds. Decoding gustation, therefore, first requires the identification of the nutrients or toxins that activate specific Grs in one animal species. This could then be related to the response properties of its Gr neurons and to its taste perception and acuity. The research proposed here will develop the honeybee as a model system for understanding the logic of the gustatory code. The honeybee has only 10 Gr genes - the least reported from insects with sequenced genomes. Based on sequence homology with Drosophila and what we know about the structure of the bee's Gr genes, we predict it has less than 20 functional Grs. For this reason, it would be possible to identify the chemical ligands for the Grs produced by these genes with the aim of using the bee as a model to understand the principles of gustatory coding. Having few Gr genes makes the bee a tractable model system in contrast to Drosophila with its 60 genes. Ligands have been determined for only 13 Drosophila Grs.This proposal describes a project that will use two approaches to identify the ligands for the receptors associated with the honeybee's Gr genes. Using a 'gain-of-function' approach, we will employ a newly-developed transgenic fruit fly line in which all of the putative genes for sugar receptors have been knocked out. Each of the bee's Gr genes will be expressed in this line. Flies from each bee Gr line will be assayed using calcium imaging of their tarsal gustatory neurons. By stimulating with a series of ligands, we will be able to identify whether functional receptors are produced by the expression of single Gr genes and to identify their Grs' ligands. Based on what we know about fruit fly sugar receptors and their bee homologues, we will also test whether expression of multiple Gr genes that encode sugar receptors is necessary to form functional Grs. We do not know if several Gr genes must be expressed to form functional receptors for the detection of compounds other than sugars. For this reason, we must also use a 'loss-of-function' approach in which we knock down expression of each Gr gene in vivo in the bee using small-interfering RNA (siRNA). Using this method, we will knock down expression of each Gr gene and assay the bee's taste neurons using electrophysiology and behaviour. We will test a suite of nutrients and toxic compounds that includes common pesticides encountered by bees in flowering crops. In spite of the fact that bees have only 10 Gr genes, they are still able to detect some toxins and to regulate their intake of nutrients like sugars and amino acids that are detected by Grs. The experiments proposed here will reveal how the bee's few Gr genes translates into the spectrum of what it can taste and will lead to future work that identifies how populations of Gr neurons encode information about the chemical nature and complexity of food.