高原鼢鼠食物选择的生态学和遗传学机制

Animals select various food resources for daily energy and nutritional requirements. In order to avoid the intake of toxic chemicals, and thus to reach their maximum fitness, they generally identify different food items by taste. Previous studies on foraging ecology, gustatory physiology, and related molecular mechanisms showed that, both external factors such as the value (contents of energy, nutrition, and toxic chemicals) and availability of food, and internal factors such as age (foraging experience), sex, physiological status, genetic variation of gustatory genes and detoxifying enzymes of animals could influence and finally determine their food selection. However, the respective contributions of each of these factors on an animal's food selection were not well studied; especially little was known on the relationships between genetic variation and expression of gustatory genes and detoxifying related genes and food selection; i.e. the ecological and genetic mechanisms of food selection are still far from being well clarified. This study will focus on the food selection and related mechanisms of plateau zokor (Eospalax baileyi), a unique herbivorous subterranean rodent endemic to the Qinghai-Tibetan Plateau. We will use foraging ecology methods to show the food selection of zokor individuals with different ages and sexes in different habitats; use analytical chemistry methods to measure the main chemical contents of their foods; use molecular techniques to identify the genetic variations and expression of taste receptor genes as well as P450 genes. Based on these results, we aim to systematically analyze the effects of biological characteristics, habitat types, food chemical contents, and genetic variations and expression of taste receptor genes and P450 genes on the food selection of zokors, and try to completely elucidate the ecological and genetic mechanisms of these animals.

动物通过选择性觅食满足其生命活动所需能量和营养，并主要经味觉辨识避免摄入有毒化合物，进而使其适合度最大化。觅食生态学、味觉生理及其分子机制的研究表明，外在因素如食物自身价值（营养和有毒成分含量）与食物在环境中的可利用性以及内在因素如动物年龄、性别、生理状态、味觉遗传变异等共同影响并决定动物的食物选择，但这些因素对食物选择结果的贡献尚未得到充分认识，特别是味觉受体和解毒酶基因遗传变异和表达特征与食物选择的关系至今仍无研究，因此，有关食物选择的生态和遗传机制并不清楚。本研究以青藏高原唯一的土著植食性地下鼠高原鼢鼠为对象，应用觅食生态学方法确定其在不同栖息地条件下的食物选择，用分子生物学技术确定其味觉受体和解毒酶基因的遗传变异和表达特征，综合分析高原鼢鼠年龄、性别、栖息地类型、食物化学成分、味觉受体和解毒酶基因型与表达量等因素对其食物选择的影响，阐明高原鼢鼠食物选择的生态和遗传机制。

本项目按计划对高原鼢鼠食物选择的生态学和遗传学机制进行研究，取得如下几个主要结论：（1）根据越冬粮仓与样方中的植物组成比较，高原鼢鼠食谱较宽，可以取食生境中大部分植物。然而，高原鼢鼠具有明显的食物选择偏好，在纳入统计的48种植物中，正选择的有12种，负选择的有16种，偏好不显著的有20种。（2）高原鼢鼠的年龄、植物的味道、栖息地类型对高原鼢鼠食物选择有显著影响，而高原鼢鼠性别对其食物选择无明显影响。影响高原鼢鼠的生态因素重要性排序为：植物味道>年龄与栖息地类型交互作用>栖息地类型>年龄。（3）植物的粗蛋白、水溶性糖和粗纤维含量都显著影响高原鼢鼠的食物选择，而粗灰分和单宁酸的影响则都不显著。总体来看，高原鼢鼠喜食蛋白和水溶性糖含量高的食物，而不喜食粗纤维含量高的食物。（4）根据高原鼢鼠基因组和转录组数据，我们对高原鼢鼠苦味受体基因进行测序和鉴定，结果显示，高原鼢鼠至少有26个苦味受体基因。许多苦味受体基因在小肠、肝脏和舌头中都有一定的表达，且舌头和肝脏中的苦味基因表达谱显著相关。（5）对高原鼢鼠苦味受体基因种群内变异进行测序分析，同时探讨其与苦味食物选择的关系。结果显示，苦味受体基因在个体之间有丰富的变异，其中有5个基因（zT2r115、zT2r119、zT2r126、zT2r134、zT2r136）显示出与苦味植物的选择有显著关联，首次在种群水平上证明了动物食物选择的分子机制。（6）高原鼢鼠zT2r134基因所受选择压力在种群之间存在差异，与纬度和年均温成正比，而与海拔和降水成反比。我们推测，随着纬度增加/海拔降低，降水减少/气温升高，导致其喜食的杂类草减少，高原鼢鼠苦味受体基因放松选择，以便取食更多的苦味食物。（7）在同等饲喂条件下，高原鼢鼠蛋白和纤维素的消化率显著高于大鼠，消化道器官相对重量显著高于大鼠，而一些与纤维素和蛋白降解相关的肠道微生物也多于大鼠。这些因素导致高原鼢鼠食物选择上有较高的耐受性，在食物匮乏时可利用质量较差的植物种类。