The Molecular and Cellular Basis of Bitter Taste in Drosophila

Abstract

Summary The extent of diversity among bitter-sensing neurons is a fundamental issue in the field of taste. Data are limited and conflicting as to whether bitter neurons are broadly tuned and uniform, resulting in indiscriminate avoidance of bitter stimuli, or diverse, allowing a more discerning evaluation of food sources. We provide a systematic analysis of how bitter taste is encoded by the major taste organ of the Drosophila head, the labellum. Each of 16 bitter compounds is tested physiologically against all 31 bitter neurons, revealing responses that are diverse in magnitude and dynamics. Four functional classes of bitter neurons are defined. Four corresponding classes are defined through expression analysis of all 68 Gr taste receptors. A receptor-to-neuron-to-tastant map is constructed. Misexpression of one receptor confers bitter responses as predicted by the map. These results reveal a degree of complexity that greatly expands the capacity of the system to encode bitter taste.

A Systematic Analysis of Drosophila Gustatory Receptor Gene Expression in Abdominal Neurons which Project to the Central Nervous System

“…Gr-GAL4 expression patterns in the adult labellum correspond well with functional analysis (Weiss et al, 2011), validating this approach. Since expression driven by Gr-GAL4 transgenes can be variable depending on the independent insertion line, we initially selected and used representative lines for each Gr-GAL4 driver that were previously observed to be the most consistent in expression levels and pattern, with high penetrance (A.D., J.Y.K., L.W., F. L., J-H.P., and J.R.C., unpublished results) (Weiss et al, 2011). The 67 Gr-GAL4 drivers were systematically examined for expression in the abdominal ganglion, the abdominal wall, and the internal male and female reproductive organs.…”

“…In situ hybridization with Gr genes has been mostly unsuccessful (Clyne et al, 2000;Dahanukar et al, 2007;Dunipace et al, 2001;Moon et al, 2009;Scott et al, 2001), likely due to low expression levels, and thus the GAL4/UAS system has been more widely utilized to analyze Gr expression patterns (Brand and Perrimon, 1993;Chyb et al, 2003;Dunipace et al, 2001;Moon et al, 2009;Scott et al, 2001;Thorne and Amrein, 2008). Gr-GAL4 expression patterns in the adult labellum correspond well with functional analysis (Weiss et al, 2011), validating this approach. Since expression driven by Gr-GAL4 transgenes can be variable depending on the independent insertion line, we initially selected and used representative lines for each Gr-GAL4 driver that were previously observed to be the most consistent in expression levels and pattern, with high penetrance (A.D., J.Y.K., L.W., F. L., J-H.P., and J.R.C., unpublished results) (Weiss et al, 2011).…”

“…As negative controls, w 1118 and w 1118 ; UAS-mCD8-GFP; UAS-mCD8-GFP (a total of four copies of the UAS transgene) flies were stained as per the described protocol. In w 1118 , the background used to generate the Gr-GAL4 transgenic lines (Weiss et al, 2011), no GFP expression was observed. In the w 1118 ; UAS-mCD8-GFP; UASmCD8-GFP flies, background levels of GFP much lower than what we observed with the Gr-GAL4 drivers in this study were seen, with the exception of the male testes which show relatively strong non-specific GFP expression (Fig.…”

“…Many studies have focused on the expression and function of these Grs in taste organs that have access to external sensory cues including the labellum, the pharyngeal organs, and the tarsi (Isono and Morita, 2010;Thorne et al, 2004;Wang et al, 2004;Weiss et al, 2011). Gr5a, Gr64a, and Gr64f, which are members of a Gr subfamily of eight Grs, were found to act as sugar sensors through genetic approaches Jiao et al, 2008;Slone et al, 2007), and Gr33a, Gr66a, and Gr93a were found to be required for responses to caffeine and certain other bitter compounds Moon et al, 2006;.…”

“…genic lines used in this study were previously described (Weiss et al, 2011). 67 drivers were used to assess the expression of the 68 Gr proteins; two alternative transcripts of Gr23a share the same promoter.…”