Co-operation between figs, wasps and parasites proves three is not always a crowd!
Professor James Cook, from the School of Biological Sciences, describes new research into the unusual relationship between figs, wasps and parasites.
Nature is full of puzzles, and one of the most intriguing is the relationship known as mutualism. This occurs when members of two different species co-operate with each other, exchanging useful resources so that each benefits. So why help a member of a different species? The species within these types of relationship are often very different, for example a plant and an animal, and good at doing different things. This means they can trade one type of resource, such as food, for another, such as protection. However, the stability of the relationship in the long-term can become threatened by individuals who take too much advantage of the relationship in the short-term. Our new research suggests that the ancient and stable relationship between tropical figs and pollinator wasps may be maintained partly by the actions of a surprising third party - parasitic wasps! These parasites were previously thought to have a negative effect on the relationship between figs and pollinator wasps.
The relationship between figs and their pollinating wasps is such that the wasps pollinate the trees, and the trees provide resources for developing wasp offspring. The female wasp enters a fig fruit, and then pollinates the many tiny flowers within the fruit. However, she also lays eggs into some of the flower ovules, which are where the trees seeds usually develop, and this causes a problem. Each flower ovule can support the development of either a seed or a wasp offspring, but not both. Therefore, each egg laid costs the tree one seed and in return, the wasp offspring are responsible for dispersing the tree's pollen once they leave the fig fruit. Trees need to produce both wasps and seeds for the mutualism to persist, but what prevents individual wasps from laying as many eggs as possible and "using up" the maximum number of fig ovules in the short-term?
The fig fruits contain hundreds of ovules that can be grouped into inner, middle or outer ovules depending on whether they are closer to the center of the fig fruit or to the outer fig wall. Most pollinator wasp eggs are found in the inner ovules, whereas most fig seeds develop in the outer ovules. The female pollinator wasps appear to avoid laying eggs in the outer ovules, and this helps to keep the relationship between wasp and fig stable, but why does it happen? We found that pollinator wasp offspring developing in the outer ovules are at very high risk of attack by parasitic wasps. These parasites lay their eggs directly into ovules from outside the fruit, and kill pollinator wasp offspring. The risk from parasitic wasps is greatly reduced towards the centre of the fruit, which is an area of 'enemy-free-space'. Thisfavours pollinator wasps that lay their eggs in the inner ovules, and therefore reduces the overall numbers of eggs they lay.
The parasitic wasps have previously been thought to have a negative impact on the mutualism — after all, they kill the pollinating wasps. What we have shown is that they actually contribute towards stabilising the relationship by placing pressure on the pollinating wasps to leave a subset of flower ovules free to develop as seeds. Mutualisms are almost always exploited by parasites, so we now have to ask how often these parasites play important roles in stabilising mutualisms in general. In mutualism, three may not always be a crowd and we need to think of these partnerships being part of — and sometimes reliant upon — a wider network of species interactions.
Female fig-pollinating wasps crawl around the cavity of a fig fruit that has been sliced open by a researcher. The wasps pollinate the many tiny flowers but also lay eggs into some of them, as the bottom right insect is doing here.
Flower use inside fig fruits. Blue ovules contain pollinator wasp offspring, red ones contain parasite wasp offspring, and black ovules contain seeds.
This research is published in:
Dunn DW, Segar ST, Ridley J, Chan R, Crozier RH, et al. (2008) A role for parasites in stabilising the fig-pollinator mutualism. PLoS Biol 6(3): e59. doi:10.1371/journal.pbio.0060059