A Preliminary Study On Function And Evolution Of Umami Taste Receptors Genes In Grass Carp(Ctenopharyngodon Idellus)

Fish have different food habits that are regarded as a characteristic common for fish to adapt to the environment and change with ontogeny and various environment. This plasticity of food habit is result of adapting to environment and different diets in the permanent evolution. Taste perception provides the final sensory evaluation of fish in the feeding process. Umami regarded as a basic taste is associated with animals’food habits and generally reflects the identification of nutrients. Animals can perceive umami by umami taste receptor pathway that located in taste buds. What is the relationship between umami taste receptor pathway and food habits of fish? To gain insight into the molecular regulation mechanism of umami taste receptor pathway in patterning of food habits of fish, we take grass carp as research object. Grass carp larvae at the’start feed’stage were fed with zooplankton. After ontogenetic diet shifts, the fish were divided into two groups and fed with zooplankton and duckweed, respectively. The transcriptomes of esophagus from three treatment groups were examined using Hiseq2000by mapping on to the grass carp genome. Then the gene expression patterns of umami taste receptor pathway in three treatment groups were analysed by depicting different gene expression from transcriptomes, including KEGG, and also we caculated the number of taste buds before and after ontogenetic diet shifts by scanning electron microscope. On the other hand, we gained the DNA sequence of T1R1、T1R3genes and fish T1R2genes from the Genbank and Ensembl, and then constructed phylogenetic tree and analysed the evolution by MEGA and PAML to understand the dynamic evolution of T1R in fish and the relationship between variation of taste perception and feeding ecology. The results suggested that:1. After patterning of food habits, the number of taste buds on the bottom of the oropharyngeal cavity increase sharply and this change is adapted to patterning of food habits in grass carp. T1R1and Gy3genes in umami taste receptor pathway are down-regulated compared with expression before patterning of food habits. T1R3and Gγ13genes in umami taste receptor pathway of fish fed with duckweed are significantly down-regulated compared with treatment group fed with zooplankton. This indicates that the down-regulated expression of T1R1and T1R3genes is in accord with taste perception changes to umami and different patterns of gene expression in umami taste receptor pathway are existed between patterning of food habits and fed with different bait. It reveals that there may be different genetic expression of molecular mechanisms between patterning of food habits and responsing to ecological food source.2. There is no correlation between pseudogenization of umami taste receptor genes and particular food habits in vertebrates. The evolution of umami taste receptor genes is very complicated: it is not only associated with the food preferences of species, but also associated with feeding behaviors. Phylogenetic analysis indicates that the divergence of T1R in fish is earlier than the divergence of fish. The evolution of T1R gene family is different between fish and other vertebrates: T1R expansion is due to duplication of T1R2in fish while pseudogenization of TlR1and T1R3shape repertoires of T1Rs in other different vertebrates. Evolutionary analyses revealed that all these T1Rs in fish are subjected to purifying selection, suggesting that the gene function of T1R1, T1R2and T1R3are conserved.