Abstract
Cats are obligate carnivores and under most circumstances eat only animal products. Owing to the pseudogenization of one of two subunits of the sweet receptor gene, they are indifferent to sweeteners, presumably having no need to detect plant-based sugars in their diet. Following this reasoning and a recent report of a positive correlation between the proportion of dietary plants and the number of Tas2r (bitter receptor) genes in vertebrate species, we tested the hypothesis that if bitter perception exists primarily to protect animals from poisonous plant compounds, the genome of the domestic cat (Felis catus) should have lost functional bitter receptors and they should also have reduced bitter receptor function. To test functionality of cat bitter receptors, we expressed cat Tas2R receptors in cell-based assays. We found that they have at least 7 functional receptors with distinct receptive ranges, showing many similarities, along with some differences, with human bitter receptors. To provide a comparative perspective, we compared the cat repertoire of intact receptors with those of a restricted number of members of the order Carnivora, with a range of dietary habits as reported in the literature. The numbers of functional bitter receptors in the terrestrial Carnivora we examined, including omnivorous and herbivorous species, were roughly comparable to that of cats thereby providing no strong support for the hypothesis that a strict meat diet influences bitter receptor number or function. Maintenance of bitter receptor function in terrestrial obligate carnivores may be due to the presence of bitter compounds in vertebrate and invertebrate prey, to the necessary role these receptors play in non-oral perception, or to other unknown factors. We also found that the two aquatic Carnivora species examined had fewer intact bitter receptors. Further comparative studies of factors driving numbers and functions of bitter taste receptors will aid in understanding the forces shaping their repertoire.
Highlights
The so-called basic tastes have traditionally been presumed to have evolved to ensure that animals consume an appropriate source of calories while avoiding toxic compounds [1, 2]
The goal of this study was to determine whether domestic cats, which are obligate carnivores, have multiple fully functional bitter receptors based on their activity in cell-based assays
The human TAS2R38 receptor responds to a few thiourea compounds [26], including PTC and 6-propyl-2-thiouracil (PROP)
Summary
The so-called basic tastes (e.g., sweet and bitter) have traditionally been presumed to have evolved to ensure that animals consume an appropriate source of calories while avoiding toxic compounds [1, 2]. Most mammals avidly consume sugars, obligate carnivores such. Domestic Cats (Felis catus) Have Functional Bitter Receptor Genes. National Institutes of Health (1P30DC011735 to RFM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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