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

直到最近，水果气味才被认为是食果性动物的重要信息来源，并可能作为一种信号系统来调节植物和种子传播者之间的相互作用。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气味上，而没有考虑到个别化学物质在向食果动物传达信息方面的作用。乙醇和甲醇是一个例外，这两种物质都是由于糖发酵或细胞壁降解而导致水果成熟的副产品，已被建议用于动物。然而，行为测试未能证明它们作为果食者的引诱剂的功能。这可能是它们有害影响的结果。脂肪酸酯在水果气味中无处不在，是人们所认为的水果气味的主要原因。它们通常是由花合成的，以增加它们的气味并吸引传粉者。至关重要的是，它们是甲醇和乙醇等酒精的直接生化产物。许多水果在成熟后会主动合成脂肪酸酯，而酒精实际上是酯生产的限制因素。因此，水果气味中的脂肪酸酯可能与甲醇和乙醇水平高度相关，从而与水果成熟度有关。该项目将测试假设：(A)水果成熟伴随着甲醇和乙醇水平的增加；(B)这些醇类转化为脂肪酸酯，并作为水果气味散发出来，从而使气味中的酯水平成为水果成熟的真实信号；以及(C)以这些水果为食并散布种子的动物会被脂肪酸酯吸引，并将其用于水果选择。该项目将在乌干达的Kibale国家公园和Nagugabo研究站进行。它将包括对野生水果的化学分析，以及对猴子和野生非洲象的行为实验，这是第一次。这些结果有望为水果气味特别是脂肪酸酯的进化生态学提供高分辨率的展望，并开发出可应用于该领域未来许多研究的方法。

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.