Functional Decline of a Bitter Receptor Gene in New World Vultures

The bitter taste perception, crucial for avoiding harmful foods, is mediated by Tas2r taste receptors in vertebrates. Vultures are obligate scavengers of considerable conservation concern, consisting of Old World and New World vultures. While vultures primarily subsist on carrion, which contains various bitter secondary metabolites produced by microbes, their ability to sense bitterness remains unclear. In this work, we identified all Tas2r genes from the genomes of 6 vultures and 22 other Accipitriformes birds. Our analysis revealed that every species, except the osprey, possessed intact Tas2r1 and Tas2r2 genes. We observed the lack of genetic divergence in Tas2r1 among all species and relaxation of functional constraint in Tas2r2 in New World vultures. Molecular docking simulations revealed reduced binding affinity of Tas2r2 in New World vultures after testing 843 bitter compounds. Additionally, we conducted cell-based functional assays for Tas2r2 to assess its responsiveness to 24 natural bitter compounds with diverse chemical structures, and confirmed lower responsiveness in New World vultures compared to other birds. These findings suggest a functional decline of bitter taste perception in New World vultures, not observed in Old World vultures, aligning with functional relaxation and reduced binding affinity of Tas2r2 predicted in New World vultures. The functional decline of bitter taste may compromise their natural defense against synthetic bitter pesticides or veterinary drugs, highlighting the potential risks faced by New World vultures in contemporary environments.