Mutually exclusive expression of Gαia and Gα14 reveals diversification of taste receptor cells in zebrafish.

A comprehensive reevaluation of the G protein alpha subunit genes specifically expressed in taste buds in the tongue epithelium of rodents revealed that Gq and G14 of the Gq class and Gi2 and Ggust (Gt3, also known as gustducin) of the Gi class are expressed in mammalian taste buds. Meanwhile, a database search of fish genomes revealed the absence of a gene encoding an ortholog of the mammalian Ggust gene, which mediates sweet, umami, and bitter taste signals in mammalian taste receptor cells (TRCs). Histochemical screening identified two G protein alpha subunit genes, zfGia and zfG14, expressed in subsets of TRCs in zebrafish. The expression patterns of zfGia and zfG14 in taste buds were mutually exclusive, and the expression of known T1R and T2R genes in zebrafish was restricted to a subset of zfGia-expressing TRCs. These findings highlight the existence of a novel subset of TRCs in zebrafish that is absent in mammals and suggest that unidentified G protein-coupled receptors are expressed in zfG14-expressing TRCs and in zfGia-expressing TRCs where known T1R and T2R genes were not expressed in zebrafish. The existence of not only generalized but also specialized subsets of TRCs may imply a strong connection between the evolution of the peripheral gustatory system and the evolution of particular species.