A role for parasites in stabilising the fig-pollinator mutualism.
A role for parasites in stabilising the fig-pollinator mutualism.
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寄生虫在稳定无花果 - 授粉互助中的作用。
DOI：
10.1371/journal.pbio.0060059
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发表时间：
2008-03-11
期刊：
影响因子：
9.8
通讯作者：
Cook, James M.
中科院分区：
文献类型：
--
作者：
Dunn, Derek W.;Segar, Simon T.;Ridley, Jo;Chan, Ruth;Crozier, Ross H.;Yu, Douglas W.;Cook, James M.
互惠是种间的相互作用，双方都受益。解释他们的维护是有问题的，因为作弊者应该胜过合作的同种，导致互惠不稳定。雌雄同株的无花果（Ficus）由寄主特异性的小蜂（Agaonidae）授粉，其幼虫在它们的“果实”（syconia）中虫瘿胚珠。雌蜂直接将卵从榕果的承珠座内产卵到榕果胚珠中。在榕属物种中，有一个广泛记录的分离传粉者虫瘿在内部胚珠和种子在外部胚珠。这种模式表明，黄蜂避免，或被阻止产卵到，外胚珠，这导致互利共生稳定。然而，阻止黄蜂利用外部胚珠的机制仍然未知。我们报告说，在无花果rubiginosa，在外部胚珠的后代很容易受到攻击的寄生蜂，产卵从外面的隐孢子。寄生的风险降低对中心的隐孢子，在那里内胚珠提供无天敌空间传粉后代。我们认为，由此产生的梯度在后代的生存能力可能有助于选择传粉者，以避免外部胚珠，并通过迫使黄蜂专注于一个子集的胚珠，降低他们的擦伤率。因此，这种以前未鉴定的机制可能有助于互利共生的持续性，而与引发植物防御传粉者产卵的其他因素或对传粉者的生理限制（阻止所有可用胚珠产卵）无关。许多生物多样性最终依赖于不同物种之间的合作，这种相互作用被称为互利主义。一个伙伴通过从另一个伙伴获得资源而获得利益，这就产生了一个问题：是什么阻止一个伙伴以不可持续的方式剥削另一个伙伴？无花果树是由小黄蜂授粉的，小黄蜂只能通过摧毁一朵雌性无花果花来成功地发展自己，否则这些花就会成为种子。黄蜂往往出现在靠近果实中心的长花中，种子在外壁附近发育。因此，雌性黄蜂喜欢长花的后代，而留下短花发育成种子。为了理解为什么黄蜂会持续地利用无花果树，我们需要解释为什么这种偏好会进化。无花果传粉者互惠利用小寄生蜂攻击传粉者从外面的水果。在三种澳大拉西亚无花果中，我们发现花外层的传粉者后代比花内层的传粉者后代更容易被寄生。因此，我们的数据表明，长花在果实中心为传粉者后代提供了无天敌的空间。我们建议，提供可变长度的花，无花果树可能有助于互利稳定间接涉及第三方：寄生蜂，以前被认为是有害的互利。传粉小蜂幼虫在无花果内花中是安全的。寄生虫可能有助于无花果传粉者互惠共生的稳定性，因为传粉者避免外部花往往会发育成种子。
Mutualisms are interspecific interactions in which both players benefit. Explaining their maintenance is problematic, because cheaters should outcompete cooperative conspecifics, leading to mutualism instability. Monoecious figs (Ficus) are pollinated by host-specific wasps (Agaonidae), whose larvae gall ovules in their “fruits” (syconia). Female pollinating wasps oviposit directly into Ficus ovules from inside the receptive syconium. Across Ficus species, there is a widely documented segregation of pollinator galls in inner ovules and seeds in outer ovules. This pattern suggests that wasps avoid, or are prevented from ovipositing into, outer ovules, and this results in mutualism stability. However, the mechanisms preventing wasps from exploiting outer ovules remain unknown. We report that in Ficus rubiginosa, offspring in outer ovules are vulnerable to attack by parasitic wasps that oviposit from outside the syconium. Parasitism risk decreases towards the centre of the syconium, where inner ovules provide enemy-free space for pollinator offspring. We suggest that the resulting gradient in offspring viability is likely to contribute to selection on pollinators to avoid outer ovules, and by forcing wasps to focus on a subset of ovules, reduces their galling rates. This previously unidentified mechanism may therefore contribute to mutualism persistence independent of additional factors that invoke plant defences against pollinator oviposition, or physiological constraints on pollinators that prevent oviposition in all available ovules. Much biodiversity ultimately relies on cooperation between different species, interactions called mutualisms. Benefits to one partner are gained by obtaining resources from the other, presenting a problem: what prevents one partner from exploiting the other at an unsustainable level? Fig trees are pollinated by tiny wasps that only develop successfully themselves by each destroying a single female fig flower that would otherwise become a seed. Wasps tend to occur in long flowers near the fruit's centre, with seeds developing near the outer wall. Female wasps therefore favour long flowers for their offspring, and leave short flowers to develop into seeds. To understand why wasps exploit fig trees sustainably, we need to explain why this preference has evolved. The fig-pollinator mutualism is exploited by small parasitic wasps that attack pollinators from outside the fruit. In three Australasian fig species, we found that pollinator offspring in the outer layer of flowers were more likely to be parasitized than those in the inner layer. Our data thus indicate that long flowers provide enemy-free space for pollinator offspring at the fruit's centre. We suggest that the provision of variable length flowers by fig trees may contribute to mutualism stability by indirectly involving a third party: parasitic wasps, previously regarded as detrimental to both mutualists. The larvae of pollinating wasps in the inner flowers of figs are safe from parasitic wasps. Parasites may contribute to stability in the fig-pollinator mutualism because outer flowers avoided by pollinators tend to develop into seeds.
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