Research

(selection)

Research on the fig tree - fig wasp mutualism

To date, our main study system has been the mutualism between fig trees and their pollinating fig wasps. This is a very attractive model system for basic research because cooperative behaviour can be manipulated, and lifetime reproductive success (=fitness) can be quantified in the field. Further, each wasp species is typically specific to a particular host species, and there are over 700 species of figs across the tropics, enabling comparative studies across species.

In addition, the fig tree - fig wasp mutualism is a keystone resource for tropical forests, because fig trees produce fruit year round, also at times of year when other trees do not. Up to 70% of mammals in tropical forests eat figs! (see figure below). The fig mutualism can therefore give us valuable insights both as a model system for basic science, and as an important part of the tropical forest ecosystem.

1. Sanctions and cheating in the fig tree – fig wasp mutualism

Fig wasps develop inside fig fruits, and transfer pollen from the fruit in which they are born to a different, flowering, fig tree. Most fig wasp species actively collect pollen with their front legs, carry it in special pollen pockets on their thorax, and then again use their front legs to actively deposit the pollen onto fig flowers. Some wasp individuals fail to carry pollen, and can therefore not perform the pollination service for the tree. The vast majority of wasps, however, do pick up and deposit pollen. Why do they do this?

Through field experiments we have demonstrated that fig trees (probably by allocating more resources to better-pollinated figs) reduce the fitness of wasps that do not pollinate: unpollinated figs are more likely to abort, and in unpollinated figs that mature, wasp larvae have a higher mortality. These host sanctions vary in strength across fig species. The proportion of fig wasps that are uncooperative also vary across species: pollen-free wasps are least common in the species with strong sanctions, where costs of not cooperating are high. In fig species with weaker sanctions, pollen-free wasps are more common. Because sanctions act on the level of the entire fig rather than on individual flowers within the fig, uncooperative wasps that do not pollinate can free-ride on the efforts of pollinating wasps.

2. Actors external to the mutualism

Ants are everywhere in tropical forests and frequently make their nests in fig trees. Although ants sometimes prey upon the pollinator wasps, they have easier access to the herbivores and parasitic wasps that lay eggs from the outside of the fig. In fact, having ants present on the tree increases the fitness of the tree and the pollinators dramatically. Ants therefore function as indirect mutualists to the fig tree – fig wasp mutualism.

We also look at the effect of other actors, such as herbivores, parasitic wasps, endosymbiotic bacteria, symbiotic yeasts etc, on the mutualism.

3. Conflict within species: Fatal fighting among female fig wasps

Female fig wasps lay their eggs in the flowers that line the inside of the fig. Once a female wasp has entered a fig she can typically not leave it to lay eggs in another fig. Each wasp larva consumes one flower. This creates a competitive situation among the egg-laying females over the limited egg-laying sites. Female wasps aggressively fight, sometimes maiming or killing each other!