Aliphatic esters: The missing link in fruit-frugivore olfactory communication?

直到最近，人们才认识到水果气味是食果动物的重要信息来源，并且可能是植物和种子传播者之间相互作用的信号系统。大多数关于水果气味的研究都集中在成熟水果的整个气味束上，而没有考虑单个化学物质在向食果动物传递信息中的作用。乙醇和甲醇是一个例外，它们都是水果成熟过程中由于糖发酵或细胞壁降解而产生的副产品，并被建议用于动物。然而，行为测试未能证明它们作为食果动物引诱剂的功能。这可能是它们有害作用的结果。脂肪酯在水果气味中无处不在，是人们认为水果气味的主要原因。它们通常是由花合成的，以增加它们的气味特征并吸引传粉者。至关重要的是，它们是甲醇和乙醇等醇类的直接生化产物。许多水果成熟后会主动合成脂肪脂，而酒精实际上是酯生成的限制因素。因此，果实气味中的脂肪酯可能与甲醇和乙醇水平高度相关，从而与果实成熟度密切相关。该项目将测试以下假设：(a)水果成熟伴随着甲醇和乙醇含量的增加；(b)这些醇转化为脂肪酯，以水果气味的形式散发，因此气味中的酯水平是水果成熟的可靠信号；(c)以这些果实为食并散播种子的动物被脂肪酯吸引，并利用它们来选择果实。该项目将在乌干达基巴莱国家公园和Nagugabo研究站进行。它将结合野生水果的化学分析和猴子的行为实验，并首次对野生非洲象进行实验。该结果有望为水果气味，特别是脂肪脂酯的进化生态学提供一个高分辨率的前景，并开发出可应用于该领域许多未来研究的方法。

Fruit odor has only recently been recognized as an important source of information to frugivores and potentially as a signaling system mediating the mutualistic interaction between plants and seed-dispersers. Most studies on fruit odor have focused on entire odor bouquets of ripe fruits and did not consider the roles of individual chemicals in conveying information to frugivores. An exception to this are ethanol and methanol, which are both byproducts of fruit maturation due to sugar fermentation or cell-wall degradation and have been suggested to be used by animals. Yet behavioral tests failed to document their function as attractants of frugivores. This may be the results of their deleterious effects. Aliphatic esters are ubiquitous in fruit odor and are responsible for most of what people perceive as fruity odor. They are often synthesized by flowers to increase their odor profiles and attract pollinators. Crucially, they are a direct biochemical product of alcohols such as methanol and ethanol. Many fruits actively synthesize aliphatic esters upon ripeness and alcohols are in fact a limiting factor in ester production. As a result, aliphatic esters in fruit odor are likely to be highly correlated with methanol and ethanol level and thus with fruit ripeness. The project will test the hypothesis that (a) fruit maturation is accompanied with increased levels of methanol and ethanol; (b) these alcohols are converted to aliphatic esters which are emitted as fruit odor, thus making ester levels in the odor an honest signal for fruit ripeness; and (c) that animals feeding on these fruits and dispersing their seeds are attracted to aliphatic esters and use them for fruit selection. The project will be conducted in Kibale National Park and Nagugabo Research Station, Uganda. It will incorporate chemical analysis of wild fruits and behavioral experiments with monkeys and, for the first time, wild African elephants. The results are expected to provide a high-resolution outlook on the evolutionary ecology of fruit odor and particularly aliphatic esters, as well as to develop methods which can be applied in many future studies in the field.