Evolution and cognate ligands of mammalian taste receptors - T1Rs and T2Rs – have been studied in some detail, whereas comparatively little is known about their counterparts in earlier-diverging vertebrates. Here we report phylogenetic studies to evaluate the evolutionary dynamics of these families in fish. Both families are absent in lamprey, but T1Rs can already be found in cartilaginous fish. In bony fish a clear distinction between ray-finned and lobefinned fish is seen for the evolution of T2Rs. In ray-finned fish four ancestral T2R genes exhibit little evolutionary dynamics, with few exceptions such as the Mexican cavefish that shows a moderate degree of gene duplications in one of the four ancestral genes1. In contrast, a lobe-finned fish, coelacanth, exhibits a very large T2R gene repertoire equaling that of amphibians and clearly surpassing mammalian T2R repertoires2. Remarkably, the ligand profile of the most basal coelacanth receptor, T2R01, is identical to that of its ortholog in zebrafish3, consistent with functional conservation over 400 million years of separate evolution.
In mammals taste is an intraoral sense which mainly serves to evaluate the nutritious content and potential toxicity of food taken in. In teleosts taste buds can also be found on the body surface such as head skin and lips, consistent with a possible additional function of taste as a distant sense similar to olfaction. Here we show that the nostrils of zebrafish possess a high density of T1R and T2R-expressing cells surpassing that of the oral cavity and of head skin by far. Thus, the incoming stream of odors first is sampled by sentinel taste cells on the nostrils, before it reaches the olfactory epithelium, opening up the possibility of cross-talk between these two sensory modalities.

1 Shiriagin and Korsching, Chem Senses 44, 23–32 (2019).
2 Syed and Korsching, BMC Genomics 15, 650 (2014).
3 Behrens et al., Genome Biology and Evolution 13, evaa264 (2021).