Parasitic wasps increase fig tree production by placing limits on the mutualism between figs and fig wasps.

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Mutualism is cooperation between species that helps each of them survive. One species provides something for the other and in exchange, receives something good in return. One example is the relationship between fig trees and fig wasps that has evolved for longer than 60 million years.

In any mutual relationship, there needs to be some way to maintain balance. Otherwise, one partner might take advantage of the other. In Australia, there is a species of fig tree that is pollinated by a specific wasp, known as a fig wasp.

The figs of the tree are sweet fruit full of small seeds. The flowers are tiny and found inside the fig, attached to the inside wall (see photo and illustration). As with any flowering plant, a fig can’t produce seeds without being pollinated. The tiny fig wasp pollinates the plant as it enters the fig. It spreads pollen around as it lays its eggs inside the flowers’ ovules. The eggs eventually grow into adult wasps and leave to pollinate other trees. However, the fig wasp doesn’t lay its eggs in all of the ovules, leaving some to grow into the seeds that crunch when you eat the fig.

What keeps the fig wasp from laying eggs in all of the ovules? Another partner in this three-way mutualism is a parasitic wasp. It uses its long egg-laying tube to pierce the outside of the fig and lay its own eggs into the ovules with the fig wasps’ eggs (the red ovals in illustration). The parasitic wasps’ young then eat the young of the fig wasp. However, the egg-laying tube isn’t long enough to reach all of the ovules, just the ones closest to the surface. As a result, to escape the parasite, the fig wasp tends to lay most of its eggs in the “enemy-free zone” deepest inside the fig (the blue ovals in the illustration). This leaves the outer ones (yellow ovals) free to develop into seeds and create the next generation of trees.

What can we learn from the mutualistic relationship between the fig tree, wasp and parasite? Sometimes, we need to rethink something we might automatically think of as “bad”, like parasitism. The parasitic wasp is an important  member of this relationship. By limiting where the fig wasp lays its eggs, the parasite allows the fig tree to produce seeds to make the next generation of fig trees. This in turn benefits the fig wasp, as it has another generation of trees to pollinate and lay eggs for its young.

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References

“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.” (Dunn et al. 2008:0491)

Journal article
Dunn DW, Segar ST, Ridley J, Chan R, Crozier RH, Yu DW, Cook JM. 2008. A role for parasites in stabilising the fig-pollinator mutualism. PLoS Biol 6(3) e59:0490-0496. PLoS BiologyDunn DW, Segar ST, Ridley J, Chan R, Crozier RH, Yu DW, Cook JM

Journal article
Dunn DW. 2020. Stability in fig tree-fig wasp mutualisms: how to be a cooperative fig wasp. Biol J Linn Soc XX:1-7 Biological Journal of the Linnean SocietyDunn DW

Web page
PLoS Biology. 2008. Cooperation between figs, wasps and parasites proves three is not always a crowd. ScienceDaily, 13 March 2008. ScienceDailyMarch 13, 2020
PLoS Biology

Journal article
Al-Beidh S, Dunn DW, Power SA, Cook JM. 2012. Parasites and mutualism function: measuring enemy-free space in a fig-pollinator symbiosis. Oikos 121:1833-1839. OikosAl-Beidh S, Dunn DW, Power SA, Cook JM

Journal article
Wang R, Chen X-Y, Chen Y, Wang G, Dunn DW, Quinnell RJ, Compton SG. 2019. Loss of top-down biotic interactions changes the relative benefits for obligate mutualists. Proc. R. Soc. B 286: 20182501.Proc. R. Soc. BWang R, Chen X-Y, Chen Y, Wang G, Dunn DW, Quinnell RJ, Compton SG

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