Complex evolutionary history of the vertebrate sweet/umami taste receptor genes

Adaptive evolution plays a role in the functional divergence and specialization of taste receptors and the sense of taste is thought to be closely related to feeding ecology. To examine whether feeding ecology has shaped the evolution of taste receptor genes in vertebrates, we here focus on Tas1r gene family that encodes umami (Tas1r1 and Tas1r3 heterodimer) and sweet (Tas1r2 and Tas1r3 heterodimer) taste receptors. By searching currently available genome sequences in 48 vertebrates that contain 38 mammals, 1 reptile, 3 birds, 1 frog, and 5 fishes, we found all three members of Tas1rs are intact in most species, suggesting umami and sweet tastes are maintained in most vertebrates. Interestingly, the absence and pseudogenization of Tas1rs were also discovered in a number of species with diverse feeding preferences and distinct phylogenetic positions, indicating widespread losses of umami and/or sweet tastes in these animals, irrespective of their diet. Together with previous findings showing losses of tastes in other vertebrates, we failed to identify common dietary factors that could result in the taste losses. Our results report here suggest the evolution of Tas1rs is more complex than we previously appreciated and highlight the caveat of analyzing sequences predicted from draft genome sequences. Future work for a better understanding of taste receptor function would help uncover what ecological factors have driven the evolution history of Tas1rs in vertebrates.