Relaxation of selective constraint on the sweet-taste receptor gene TAS1R2 in lorisiform primates

Recent research has revealed considerable evolutionary diversity in umami-taste (amino acids and nucleotides) and sweet-taste receptor TAS1R genes across vertebrate species. To contribute to a growing understanding of how diet shapes taste evolution, we studied TAS1R genes in non-anthropoid primates with highly diverse diets, including members of the Strepsirrhini, comprised of Lorisiformes (lorises) and Lemuriformes (lemurs), as well as members of the Tarsiiformes (tarsiers). We employed a targeted capture (TC) approach specifically probing all the three mammalian TAS1R genes, i.e., TAS1R1 (for sensing umami), TAS1R2 (sweet) and TAS1R3 (required for forming a heterodimer), followed by short-read massive-parallel sequencing for three lorisiform, four lemuriform, and one tarsiiform species. Analyzing together with publicly available whole-genome assemblies (WGAs) of non-anthropoids, we found that TAS1R1 and TAS1R2 of some lorisiform species were disrupted. The relative evolutionary rates in introns and synonymous sites of all the three TAS1R genes, as well as non-genic genome regions, of lorisiforms were higher than those of lemuriforms, a finding consistent with the higher genome-wide mutation rate of the lorisiforms. We found a similar pattern in the amino acid sequences and nonsynonymous sites of the sweet receptor TAS1R2 in lorisiforms. Evolutionary rates of amino acid sequences and nonsynonymous sites in TAS1R1 and TAS1R3 of lorisiforms were as slow as those of lemuriforms. This suggests that functional constraint on sweet sensing has been relaxed in lorisiform primates since their common ancestor. These results shed a new light on understanding evolutionary diversification of umami and sweet sensing in a diverse group of mammals.