Centre for Ecological and Environmental Sciences, Northwest Polytechnical University, Xi'an, 710072, China.
College of Life Sciences, Northwest University, Xi'an, Shaanxi, 710069, China.
Ecology. 2019 Mar;100(3):e02597. doi: 10.1002/ecy.2597.
In some insect nursery pollination mutualisms, plant hosts impose net costs to uncooperative "cheater" symbionts. These "sanctions" promote mutualism stability but their precise adaptive nature remains unclear. In fig-wasp mutualisms host trees (Ficus spp.) are only pollinated by female agaonid wasps whose larvae only use galled fig flowers as food. In actively pollinated systems, if wasps fail to pollinate, sanctions can result via fig abortion, killing all wasp offspring, or by increased offspring mortality within un-aborted figs. These sanctions result from selective investment to pollinated inflorescences, a mechanism present in almost all angiosperms. To more fully understand how selective investment functions as sanctions requires the measurement of variation in their costs and benefits to both hosts and symbionts. Gynodioecious fig-tree-fig-wasp mutualisms are particularly suitable for this because pollen and wasps are produced only in the figs of "male" trees and seeds only in the figs of "female" trees. Male and female trees thus incur different net costs of pollen absence, and costs of sanctions to pollen-free "cheater" wasps only occur in male trees. We used the actively pollinated host tree Ficus hispida and introduced into male and female figs either 1, 3, 5, 7, or 9 all pollen-laden "cooperative" (P+) or all pollen-free "cheater" (P-) wasps. Abortion in both male and female trees was highest in P- figs, with P- fig abortion higher in females (90%) than in males (40%). Fig abortion was negatively associated with foundress number mainly in P+ figs; in P- figs abortion was only weakly associated with the number of "cheater" wasps, especially in female figs. In un-aborted male figs, wasp offspring mortality was higher in P- figs than in P+ figs, and in P- figs correlated positively with foundress (cheater) number. Increased offspring mortality was biased against female wasp offspring and likely resulted from reduced larval nutrition in unpollinated flowers. Variation in selective investment to P- figs thus reflects costs and benefits of pollen absence/presence to hosts, variation that translates directly to net costs to cheater wasps.
在一些昆虫育雏授粉互惠关系中，植物宿主会对不合作的“骗子”共生体施加净成本。这些“制裁”促进了互惠关系的稳定性，但它们的确切适应性本质尚不清楚。在榕果蜂互惠关系中，宿主树（榕属植物）仅由雌性榕果蜂授粉，其幼虫仅以瘿花榕果为食。在主动授粉系统中，如果蜂未能授粉，制裁可能会通过榕果流产而产生，导致所有蜂后代死亡，或者在未流产的榕果中增加后代死亡率。这些制裁是由于对授粉花序的选择性投资而产生的，这种机制几乎存在于所有被子植物中。要更全面地了解选择性投资如何作为制裁措施发挥作用，需要衡量其对宿主和共生体的成本和收益的变化。雌雄异株榕果蜂榕果互惠关系特别适合这种情况，因为花粉和蜂仅在“雄性”树上的榕果中产生，种子仅在“雌性”树上的榕果中产生。因此，雄性和雌性树承担着不同的无花粉成本，并且只有在雄性树上才会对无花粉的“骗子”蜂产生制裁成本。我们使用主动授粉的宿主树榕属植物并将其引入雄性和雌性榕果中，要么引入所有携带花粉的“合作”（P+）蜂，要么引入所有无花粉的“骗子”（P-）蜂，数量分别为 1、3、5、7 或 9。在 P-榕果中，雄性和雌性榕果的流产率最高，雌性榕果的 P-榕果流产率（约 90%）高于雄性榕果（约 40%）。榕果流产与创始蜂数量呈负相关，主要在 P+榕果中；在 P-榕果中，流产与“骗子”蜂数量的相关性较弱，尤其是在雌性榕果中。在未流产的雄性榕果中，P-榕果中的蜂后代死亡率高于 P+榕果，并且与创始蜂（骗子）数量呈正相关。后代死亡率的增加对雌性蜂后代不利，可能是由于未授粉花朵中幼虫营养减少所致。因此，对 P-榕果的选择性投资的变化反映了宿主中无花粉/有花粉的成本和收益，这种变化直接转化为骗子蜂的净成本。
