Study Of Convergent Evolution Of TAS2R16in Mammals

There are tens of thousands of bitter substances in nature, and most of them are thought to be poisonous and harmful. Bitter taste plays an important role in the process of animal survival defense. Sensitivity to bitter substances between different species or different individuals among the same species varies a great deal, reflecting the evolution of animal dietary changes. During the long-term evolution, different species living in the same or similar environments, may have the same feeding habits, that is, they are likely to meet same bitter substances when foraging. Thus, it gradually exhibits convergent evolution on sensitivity to bitter substances, also the performance of adaptive evolution.Bitter taste receptors (TAS2Rs) are typical of G protein-coupled receptors. There are many family members. The number of mammalian bitter taste receptor varies greatly. Different bitter taste receptors recognise different bitter substances, some maybe the same. Different species having the same receptor by amino acid substitutions show different sensitivity. Thus, bitter taste is of quite complexity. TAS2R16is the first identified bitter taste receptor recognising beta-glucopyranoside. About10%plants have this bitter substance. We put forward hypothesis that convergent evolution of TAS2R16function may occur in mammals, during the process of evolution. In order to verify the hypothesis, we used both evolution analysis and functional experiment to further study. The results are as following:1) TAS2R16sequence alignment showed that only white-faced saki and human have the same amino acid asparagine (N) at172site; The site was not random changes but parallel evolution by building common ancestral sequences and biometric calculation; Natural selection test showed that there was positive selection on172site of white-faced sakis and human TAS2R16, suggesting this site important for adaptation to the environment in evolution. In addition, it was found that the amino acid substitution rates were increasing, from common ancestor of apes, Old World monkeys, New World monkeys, to the primitive monkey (Prosimians). Common ancestor branch of Hominoidea was significantly detected positive selection signals.15positive selection sites were also found. Thus it gave theoretical guidance for further functional experiments.2) Experiment of point directed mutation showed that EC50values of cells responding to bitter ligands (salicin and helicin) increased, meaning less sensitive, when172site of white-faced saki TAS2R16mutated to lysine (K). The docking of modeling receptor and ligand also became less easier after mutation, with binding (docking) free energy value increasing. We also found172site located on extracellular and intracellular junction of the fifth transmembrane domain of TAS2R16, playing an important role on receptor function. In summary, white-faced saki and human TAS2R16showed convergent evolution in ability to detect bitter substances. This is the first time to find convegent evolution of bitter taste receoptor function in mammals. It will provide new vision for future study of convegent evolution of bitter receptors.