Dietary Specialization in Mammals: Constraints and Costs of Detoxification of Plant Secondary Compounds

食用植物的哺乳动物有被植物产生的自然毒素中毒的风险。关于哺乳动物处理植物毒素的机制，或者像考拉这样的特殊物种如何能够食用对其他物种有毒的植物，几乎没有实验工作。本研究将结合生态学、化学、生理学和药理学来研究食草动物解毒植物毒素的机制以及解毒的成本。PI将调查专门食用一种有毒植物的植食木鼠与食用多种植物的通才木鼠在解毒生理上的差异。木鼠是解决这个问题的典型系统，因为木鼠的专家和通才的多样性是任何其他哺乳食草动物属所无法比拟的。到目前为止，PI已经发现，专业食草动物比通才能够耐受更高剂量的毒素。与传统观点相反，与通才相比，专家似乎没有独特的排毒途径，而是具有更大能力的系统。初步结果表明，专家能够处理高毒素负荷，因为他们可以比通才更快地排除毒素。这些研究将进一步调查专家和通才在解毒能力上的差异，并将解决饮食专业化的成本和限制问题。例如，专家在处理毒素时是否比通才更有效率？专业食草动物是否会牺牲大量毒素的解毒能力，以换取对一部分毒素的强化处理？在本世纪，食草哺乳动物将面临严重的解毒挑战。新的毒素（如二恶英、多氯联苯）正以前所未有的速度被添加到环境中。此外，在未来50年，由于化石燃料的大量燃烧，大气中的二氧化碳水平预计将翻一番。在二氧化碳浓度升高的情况下生长的许多植物的营养质量明显较差，因为它们的蛋白质含量较低，所含的毒素浓度是在当前二氧化碳浓度下生长的植物的两倍。食草哺乳动物是否会因为食物营养质量的急剧变化，尤其是毒素的增加而受到不利影响？这项关于野生食草动物解毒过程的研究结果将为食草动物如何受到食物营养质量变化的影响提供重要的见解。

Mammals that consume plants run the risk of being poisoned by naturally occurring toxins produced by plants. Little experimental work exists on the mechanisms mammals employ to deal with plant toxins, or how specialized species such as the koala are capable of consuming plants that are toxic to other species. The proposed research interfaces with ecology, chemistry, physiology and pharmacology to address mechanisms by which herbivores detoxify plant toxins as well as the costs of detoxification. The PI will investigate how woodrat herbivores that specialize on one species of toxic plant differ in detoxification physiology from generalist woodrats that consume a variety of plant species. Woodrats are a model system for this problem because the diversity of specialists and generalists woodrats is unparalleled by any other genus of mammalian herbivores. Thus far, the PI has found that specialist herbivores are capable of tolerating higher doses of toxins than generalists. Contrary to conventional wisdom, specialists do not appear to have unique detoxification pathways compared to generalists but rather systems with a greater capacity. Preliminary results suggest that specialists are able to deal with high toxin loads because they can eliminate toxins faster than generalists. These studies will further investigate how specialists and generalists differ in detoxification abilities and will address issues on costs and constraints of dietary specialization. For example, are specialists more efficient than generalists in processing toxins? Do specialist herbivores trade-off the ability to detoxify a wide range of toxins in exchange for enhanced processing of a subset of toxins? In this century, mammalian herbivores will confront profound detoxification challenges. Novel toxins (eg dioxins, PCBs) are being added to the environment at unprecedented rates. Moreover, in the next 50 years, levels of CO2 in the atmosphere are predicted to double due to extensive burning of fossil fuels. Many species of plants grown under elevated levels of CO2 are substantially inferior in nutritional quality in that they are lower in protein and contain up to twice the toxin concentration as plants grown under current CO2 concentrations. Will mammalian herbivores be aversely impacted by such a radical change in the nutritional quality of their food, particularly the increase in toxins? The results of this research on detoxification processes of wild herbivores will provide vital insights on how herbivores will be affected by this change in nutritional quality of food.