Fitness reduction for uncooperative fig wasps through reduced offspring size: a third component of host sanctions

Mutually beneficial interactions between two species-mutualisms-are ancient, diverse, and of fundamental ecological importance. Nonetheless, factors that prevent one partner from reaping the benefits of the interaction without paying the cost are still poorly understood. Fig trees and their unique pollinators, fig wasps, present a powerful model system for studying mutualism stability. Both partners depend completely on each other for reproduction, cooperation levels can be manipulated, and the resulting field-based fitness quantified. Previous work has shown that fig trees can impose two types of host sanctions that reduce the fitness of wasps that do not pollinate: (1) fig abortion, which kills all developing larvae, and (2) reduced number of wasp offspring in figs that are not aborted. Here we demonstrate a third component of host sanctions. Through manipulative field experiments, we show that for four of five studied species, offspring of pollen-free foundresses are only 50-90% the size of offspring of pollinating foundresses. We further show that in all four studied species, smaller wasps are less likely to reach and enter a flowering fig to become foundresses themselves. Therefore, the experimentally determined size reduction of offspring is estimated to cause an additional reduction of up to 80% in fitness for a pollen-free foundress. We determine that the size reduction of pollen-free offspring acts on the level of the entire fig fruit rather than on individual flowers. These results show that estimates of the fitness effect of host sanctions on uncooperative symbionts should consider not only offspring quantity but also offspring quality. We discuss implications beyond the fig tree-fig wasp mutualism.