猴科味觉受体Tas1rs基因家族适应性进化与食性相关性研究

In mammals, sweet and umami taste perception is conferred by taste receptor cells through the use of G protein-coupled receptors (GPCRs) TAS1Rs which were encoded by gene family of Tas1rs genes. The TAS1R1 protein forms a heterodimer with TAS1R3 to form a two-part umami taste receptor, meanwhile the TAS1R2+TAS1R3 heterodimer functions as the sweetness receptor. TAS1R proteins are characteristic of seven domains spanning the plasma membrane. The TAS1R proteins consist of about 850 amino acids and have a large extracellular N-terminus. The gene structure of tas1rs family is believed to be conserved among species.In recent year, analyses on umami and sweet taste receptors have been conducted in number of species. Tas1r1 is found to be pseudogenized in the giant panda (family Ursidae), which is herbivorous. Additionally, tas1r2 were revealed to be inactivated in the cat family Felidae, vampire bats, horse, chicken, zebra finch and western clawed frog. However, the ecological causes of pseudogenization of tas1r genes are unclear.We choose to study the evolution of the sweet and umami taste receptor gene tas1rs in Cercopithecidae species because they exhibit a wide range of diets. Within the family Cercopithecidae, subfamily Colobinae is also known as leaf-monkeys and feed predominantly on relatively low-energy leaves, while another subfamily Cercopithecinae feed predominantly on relatively high-energy foods such as fruits, seeds, insects, and other vertebrates.The difference on food type between close-related subfamilies means significant variation of the selective press on taste genes. In this study, we plan to get the sequences of tas1rs gene of species in Cercopitheciade and functionally expressed these taste receptor variants. We plan to analyze their cellular response using an in vitro cellular assay.The main goal are to: (1)calculate the nonsynonymous/synonymous substitution rate ratio (ω = dN/dS) which measures the selective pressure at the protein level,(2)find the fuctionanl varition among taste receptors in Cercopithecidae species with different food favor,(3)measure behavioral preference to sweet and umami liquid in different species .

哺乳动物对鲜味和甜味的感觉，是通过味觉受体TAS1Rs蛋白家族来实现的。TAS1R1与TAS1R3组成的二聚体感受鲜味物质，TAS1R2与TAS1R3二聚体感受甜味物质。前期研究表明编码TAS1Rs蛋白基因外显子的异义突变会导致蛋白序列的变化，导致感受刺激物范围的变化，或者是味觉灵敏度的差异。猴科(Cercopithecidae)内部食性差异非常显著，猴亚科主食肉和果，而疣猴亚科主食叶，自然选择对其甜味和鲜味受体蛋白的选择压力可能会出现明显变化,编码基因的突变也可能导致受体蛋白敏感度或感受刺激物范围的变化。本项目计划克隆猴科各个属代表物种的TAS1R1、TAS1R2、TAS1R3的编码区序列，构建表达载体，在HEK细胞系中进行表达，检测不同刺激物浓度下敏感度差异，并通过行为学测试验证不同物种对鲜味和鲜味剂的偏好性。从而揭示猴亚科和疣猴亚科味觉受体蛋白的适应性进化过程及与食性的相关性。

味觉（Taste）是动物的基本功能之一，在一定程度上决定了动物对食物的选择。甜味和鲜味与碳水化合物、蛋白质等营养物质的摄入相关，而苦味味觉则能够防止动物摄入对机体不利的物质。因而，面对复杂的自然环境和食物来源，味觉在动物的进化历程中必然扮演着重要角色。到目前为止，有关甜味、鲜味和苦味基因的研究已经在多个物种中得以开展。然而，大多数甜味和鲜味基因（Tas1rs）的研究主要着眼于食性特化物种中假基因的发现；苦味基因（TAS2Rs）的研究则主要关注于食性对物种间苦味基因数量差异的影响。本论文中，我们首次在以植物为主要食物的狭鼻灵长类（Catarrhine primates）中开展了全面的甜味和鲜味基因适应性进化分析，探讨了食性差异对基因进化的影响。基于此，我们提出了动物食性进化的“进化漩涡假说”：：食性特化会导致相关味觉受体选择压力放松，进而出现敏感度降低，这样会导致动物取食范围的缩小，取食其它食物的风险增大。这种趋势又会近一步缩小取食范围，再次导致味觉受体编码基因选择压力降低，有可能出现假基因化，彻底丧失功能。这个漩涡会逐渐导致动物食性越来越特化。