PLoS Biology: parasites may help to stabilize the mutualism between figs and fig wasps.
In nature, mutualistic relationships usually conceal a tug-of-war between interacting species. This is especially clear in the case of pollinating seed parasites, like yucca moths (my favorite) and fig wasps. Both these insects pollinate their eponymous host plants, then lay eggs in the fertilized flowers so their larvae can eat some of the seeds produced. Natural selection should push each interactor to overexploit this deal: the pollinator "wants" to lay lots of eggs, but the plant "wants" to get as many seeds as possible. Yuccas keep yucca moths in check by killing off flowers with too many eggs inside [subscription], but there hasn't been a similar mechanism found in figs.
Until now. In the new paper, Dunn et al. show that the figs might benefit from parasites that attack the pollinating wasps. Fig flowers grow in "synconia," hollow globes like the one in the photo above, which are lined inside with tiny flowers. Fig wasps climb inside the synconia to pollinate and lay their eggs in the flowers. Another wasp species parasitizes the pollinators by laying its eggs near pollinator eggs, so their larvae can eat the pollinator larvae when they hatch. Turns out, the parasites lay their eggs from the outside of the synconium, and the flowers inside the synconium vary in how close they are to the outer wall. Any pollinator eggs laid too close to the outer wall of the synconium are nailed by parasites - so the pollinators have an incentive to only lay eggs in the innermost flowers. Neato!
Dunn, D.W., S.T. Segar, J. Ridley, R. Chan, R.H. Crozier, D.W. Yu, and J.M. Cook. 2008. A Role for Parasites in Stabilising the Fig-Pollinator Mutualism. PLoS Biol 6(3): e59.