Dunn Derek W, Segar Simon T, Ridley Jo, Chan Ruth, Crozier Ross H, Yu Douglas W, Cook James M
Division of Biology, Imperial College London, Ascot, United Kingdom.
PLoS Biol. 2008 Mar 11;6(3):e59. doi: 10.1371/journal.pbio.0060059.
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.
互利共生是一种种间相互作用，其中双方都能受益。解释它们的维持存在问题，因为作弊者应该会在竞争中胜过合作的同种个体，导致互利共生不稳定。雌雄同株的榕树（榕属）由宿主特异性的黄蜂（榕小蜂科）授粉，其幼虫在榕树的“果实”（隐头花序）中使胚珠形成虫瘿。雌性授粉黄蜂从可接受的隐头花序内部直接将卵产入榕属植物的胚珠中。在整个榕属物种中，传粉者的虫瘿集中在内部胚珠，而种子则分布在外部胚珠，这一现象有广泛的记录。这种模式表明黄蜂会避开或被阻止在外部胚珠中产卵，这就导致了互利共生的稳定性。然而，阻止黄蜂利用外部胚珠的机制仍然未知。我们报告，在锈叶榕中，外部胚珠中的后代容易受到从隐头花序外部产卵的寄生蜂攻击。向隐头花序中心方向，寄生风险降低，在那里内部胚珠为传粉者的后代提供了无天敌空间。我们认为，由此产生的后代生存能力梯度可能有助于选择传粉者避开外部胚珠，并且通过迫使黄蜂将注意力集中在一部分胚珠上，降低了它们形成虫瘿的比率。因此，这种先前未被识别的机制可能有助于互利共生的持续存在，而与其他因素无关，这些因素包括植物对传粉者产卵的防御，或者传粉者在所有可用胚珠中产卵时受到的生理限制。
