榕-蜂共生系统防御蚂蚁捕食的机制及其生态学效应

The relationship between fig tree (Ficus) and their pollinating fig wasps (Agaonidae) represents a remarkable example of mutualism and coevolution, the mutualistic partner needing each other for reproduction. In tropical and subtropical area, fig wasps form a stable food for ants, and are frequently captured by ants during their egg laying period and during they emerge from the figs of male phrase. It has been found that ants can locate the fig-fig wasp by identifying the volatiles, fig-fig wasps and ants have formed a symbiotic system. However, on a species of fig tree (Ficus altissima) that grow in the Xishuangbanna region of Yunnan, we found significantly less ant activities. In the fig species, we also observed and reported a unique phenomenon that pollinating fig wasps will arrive at the tree before female phase, fig of Ficus altissima has persistent tight-fitting bud covers that prevent its pollinator from entering its figs for several days, pollinators cannot enter the figs until the surrounding bracts split apart, pollinating fig wasps forming large aggregations on the underside of the leaves, while fig wasps predation by ants have never been observed. Moreover, this particular fig species is parasitized by other 25 non-pollinating fig wasp species, which is the most abundant non-pollinating fig wasp species parasitied in one host fig species in Asian tropical area. This unusual finding has led to an initial investigation and research. We have sampled ants in Ficus altissima and used Y-tube to test whether a main predator ant (Oecophylla smaragdina) were responded to odours of figs or wasps. The initial results showed that the ants were repelled by receptive figs as well as young fig leaves, which suggested a possible development of a defense mechanism against ants. The proposed study aims to compare the symbiotic systems of fig-fig wasp and ant between Ficus altissima and several other fig species using field observations, chemecology, and electrophysiology techniques etc., to define the characteristics of this defense, as well as the ecological effect of the defence, the results will reveal a novel mechanism of fig-fig wasps against predators attack.

榕-蜂系统构成了自然界互利共生和协同进化的经典例子。作为蚂蚁稳定的食物来源，榕小蜂在产卵期和出蜂期均会遭受蚂蚁的捕食，蚂蚁与榕-蜂系统已进化出了稳定共存的关系。但分布于西双版纳地区的高榕上蚂蚁种类很少，且大量积聚在叶片上等待榕果发育的小蜂也未遭受到蚂蚁捕食，此外高榕上还寄生着25种停歇在果壁产卵的非传粉小蜂，是亚洲热带地区非传粉种类最丰富的榕树，这些现象反映了高榕的特殊性。前期工作中，我们进行了蚂蚁抽样并采用Y型嗅觉仪测定了黄猄蚁对榕树叶果的行为反应，结果发现：高榕上没有西双版纳地区常见的黄猄蚁，且嫩叶片及雌花期榕果对黄猄蚁有趋避性，说明高榕-小蜂系统可能已演化出了防御蚂蚁捕食的能力。本项目将采用野外观察、化学生态学、电生理学等方法，通过对比高榕与其他榕树与蚂蚁的关系，探明高榕及其小蜂对蚂蚁的防御特点及机制，以及防御产生的生态学效应，其结果有望揭示一种榕-蜂互利共生系统防御捕食者的新机制。

榕树及其传粉昆虫榕小蜂构成了自然界互利共生和协同进化的经典例子。作为蚂蚁稳定的食物来源，榕小蜂在产卵期和出蜂期均会遭受蚂蚁的捕食，蚂蚁与榕-蜂系统已进化出了稳定共存的关系。在前期工作中，我们发现分布于西双版纳地区的高榕上有一种及其特殊的现象：高榕的传粉小蜂及其欺骗者会在榕果进入到雌花期之前约半个月就来到高榕叶片上等待，且大量积聚在叶片上的小蜂也未遭受到蚂蚁捕食，故我们推测高榕的榕果或叶片对蚂蚁有一定的趋避作用。该研究采用野外观察与室内试验相结合的方法，研究了高榕对蚂蚁的趋避及其生态学效应。结果发现：在西双版纳地区的4种常见榕树：高榕、聚果榕、钝叶榕和垂叶榕上，聚果榕和垂叶榕上的优势种蚂蚁为黄猄蚁，而高榕的优势种为长足光捷蚁。进一步利用Y型嗅觉仪测试黄猄蚁对高榕不同时期的叶果及榕小蜂的反应发现：高榕雌花期榕果及叶片对黄猄蚁具有趋避作有，而其他时期的榕果以及榕小蜂对黄猄蚁无趋避作用。进一步利用GC-MS对不同时期叶果的气味进行分析发现：高榕雌花期榕果以及叶片的挥发性化合物数量多于其他时期，且二者具有一定相似性。在自然条件下，黄猄蚁对包括高榕传粉小蜂在内的5种榕小蜂的捕食并无显著差异，说明高榕小蜂并未进化出趋避黄猄蚁的功能，这种趋避作用主要是来自于榕树。另外我们对高榕上的优势种长足光捷蚁和黄猄蚁的竞争关系进行了研究，结果表明：长足光捷蚁相较于黄猄蚁，二者的竞争强度并无显著强弱，说明这种趋避作用也不是来自高榕上的优势种蚂蚁。此外我们还验证了黄猄蚁对榕树上传粉小蜂以及小蜂后代的影响，结果表明黄猄蚁的存在可以显著减少传粉小蜂的进蜂量以及小蜂后代数量。我们的研究结果揭示了榕-蜂互惠共生系统对榕小蜂的主要捕食者蚂蚁的防御现象，及这种防御产生的机制和生态学效应。其结果揭示了一种特殊的榕-蜂防御系统捕食者的机制。为进一步认识榕-蜂互惠共生关系提供了新的视角，也为相似系统的研究提供借鉴。