Throughout evolution, plants and insects have exploited each other in mutually beneficial relationships where two or more parties benefit from essential ecological interactions. For example, insects act as plant pollinators while plants act as hosts providing food or housing for insect offspring. Such partnerships often require a delicate balance of give and take. Recently, a group from Imperial College of Science, Technology and Medicine (London) led by James M. Cook questioned the intrinsic risk of such cooperative relationships. They investigated the relationship between fig trees and pollinator wasps, with the intention of finding what keeps one partner from taking advantage of the other.

Conditions favoring one partner over the other constantly threaten to erode the balance in mutually beneficial relationships. For example, when a female pollinator wasp pushes her way into a fig blossom, pollinating the tree while depositing her eggs, the tree loses one seed. The sacrifice is offset for the tree by the fact that each egg laid produces the offspring that will emerge and eventually disperse that tree's pollen. Still, what keeps the wasp from taking advantage of the host and destroying the tree's entire crop?

As is often the case, there are more than two players in this relationship. The fig tree also plays host to small parasitic wasps that attack pollinator wasp larvae in the fruit. Cook and his colleagues wondered whether the parasitic wasp, previously thought to be harmful to both parties, may contribute to the stability of the fig tree and pollinator wasps' relationship by keeping the pollinator larvae in check.

Suspecting that that the pollinator wasp is under selective pressure to deposit her eggs in the flower's innermost real estate where her larvae are less likely to be killed, the team collected fig flowers from six different sites in Queensland, Australia, brought them back to the lab and sliced them open. They measured the distance from the wall to the inner cavity of the fig flower, taking note of the precise position where the pollinator eggs were deposited inside the flower. They also assessed the offspring survivorship and found that the larval eggs deposited in the flower's innermost folds were more likely to survive compared with eggs deposited near the flower's outer layers. The researchers determined that parasitic wasps are able to attack the pollinator offspring only if the pollinator eggs are deposited in the outer layer of the fig flower ovule (the `small egg' within the flower that will develop into a seed) and that larvae buried deep in the flower bud were almost parasite free.

The team also noticed that that pollinator larvae, parasites and seeds are relegated to different neighborhoods over the entire tree, which appear to be determined by flower length. Cook's research shows that longer flowers provide the pollinators' larvae with plenty of `enemy-free space' at the fruit's centre because the parasitic wasps prefer to invade shorter flowers alone. So the female pollinating fig wasp actively selects regions of the tree with long flowers, relegating the parasitic wasps to tree zones covered in shorter blooms.

Whether the fig tree controls its own fate by producing variable length flowers for the wasp to `choose' from when depositing her eggs, or the parasitic wasp determines the territorial layout for the tree, the end result is the same. A portion of the fig tree seeds will come to fruition. This led Cook and his colleagues to suggest that the parasitic wasp, previously designated a harmful opportunist, helps to ensure that a fraction of the tree will bear seeds and thus may contribute to the stability of the pollinator–fig beneficial relationship by keeping the pollinator wasp in check.

Dunn, D. W., Segar, S. T., Ridley, J., Chan, R., Crozier, R. H.,Yu, D. W. and Cook, J. M. (
). A role for parasites in stabilizing the fig–pollinator mutualism.
PLoS Biol.
, 490-494.