Aranda-Rickert Adriana, Rothen Carolina, Diez Patricia, González Ana María, Marazzi Brigitte
Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja (CRILAR), Provincia de La Rioja, UNLaR, SEGEMAR, UNCa, CONICET, Entre Ríos y Mendoza s/n, CP 5301, Anillaco, La Rioja, Argentina.
Centro de Investigaciones y Transferencia de Catamarca (CITCA), CONICET, UNCa, Prado 366, CP 4700, SFV de Catamarca, Catamarca, Argentina.
Ann Bot. 2017 Nov 10;120(5):765-774. doi: 10.1093/aob/mcx075.
The most widespread form of protective mutualisms is represented by plants bearing extrafloral nectaries (EFNs) that attract ants and other arthropods for indirect defence. Another, but less common, form of sugary secretion for indirect defence occurs in galls induced by cynipid wasps. Until now, such galls have been reported only for cynipid wasps that infest oak trees in the northern hemisphere. This study provides the first evidence of galls that exude sugary secretions in the southern hemisphere and asks whether they can be considered as analogues of plants' EFNs.
The ecology and anatomy of galls and the chemical composition of the secretion were investigated in north-western Argentina, in natural populations of the host trees Prosopis chilensis and P. flexuosa . To examine whether ants protect the galls from natural enemies, ant exclusion experiments were conducted in the field.
The galls produce large amounts of sucrose-rich, nectar-like secretions. No typical nectary and sub-nectary parenchymatic tissues or secretory trichomes can be observed; instead there is a dense vascularization with phloem elements reaching the gall periphery. At least six species of ants, but also vespid wasps, Diptera and Coleoptera, consumed the gall secretions. The ant exclusion experiment showed that when ants tended galls, no differences were found in the rate of successful emergence of gall wasps or in the rate of parasitism and inquiline infestation compared with ant-excluded galls.
The gall sugary secretion is not analogous to extrafloral nectar because no nectar-producing structure is associated with it, but is functionally equivalent to arthropod honeydew because it provides indirect defence to the plant parasite. As in other facultative mutualisms mediated by sugary secretions, the gall secretion triggers a complex multispecies interaction, in which the outcome of individual pair-wise interactions depends on the ecological context in which they take place.
最广泛存在的保护性互利共生形式是带有花外蜜腺（EFNs）的植物，这些蜜腺吸引蚂蚁和其他节肢动物以进行间接防御。另一种用于间接防御的含糖分泌物形式较少见，出现在瘿蜂诱导形成的虫瘿中。到目前为止，此类虫瘿仅在北半球侵害橡树的瘿蜂中被报道过。本研究首次提供了南半球分泌含糖分泌物的虫瘿的证据，并探讨它们是否可被视为植物花外蜜腺的类似物。
在阿根廷西北部，对寄主植物智利牧豆树和弯曲牧豆树的自然种群中的虫瘿生态学、解剖学以及分泌物的化学成分进行了研究。为检验蚂蚁是否能保护虫瘿免受天敌侵害，在野外进行了蚂蚁排除实验。
虫瘿产生大量富含蔗糖的、类似花蜜的分泌物。未观察到典型的蜜腺和蜜腺下薄壁组织或分泌毛；相反，有密集的维管束，韧皮部元素延伸至虫瘿周边。至少六种蚂蚁，还有胡蜂、双翅目和鞘翅目昆虫会取食虫瘿分泌物。蚂蚁排除实验表明，与排除蚂蚁的虫瘿相比，当蚂蚁照料虫瘿时，瘿蜂成功羽化率、寄生率和寄居昆虫侵染率均无差异。
虫瘿的含糖分泌物与花外花蜜不同，因为其不伴有产生花蜜的结构，但在功能上等同于节肢动物的蜜露，因为它为植物寄生虫提供间接防御。与其他由含糖分泌物介导的兼性互利共生一样，虫瘿分泌物引发了复杂的多物种相互作用，其中个体间两两相互作用的结果取决于它们发生的生态背景。
