Mutualisms play fundamentally important roles in ecosystems but are relationships where selfish individuals benefit and thus are potentially unstable. Mechanisms that punish such selfish individuals, or sanctions, is thought to stablize mutualisms. However, apparent lack of sanction in many mutualisms suggests that there exist other mechanisms that maintain mutualisms.

The obligate pollination mutualism between Phyllanthaceae plants and Epicephala moths, in which the moths actively pollinate and the larvae feed on the seeds, is a model to understand the stability of mutualism. We tested whether the stalk-like elongation of fruit base in Breynia vitis-idaea (Phyllanthaceae) is an adaptation to limit seed consumption by pollinator larvae. We found that the stalk length is likely determined genetically, and that this trait has an effect to keep the number of larvae in the fruit low. Therefore, the stalk is a defense trait that prevents all seeds in the fruit to be eaten up by Epicephala moths.

While sanction is a mechanism where the host decreases the fitness of selfish individuals in a selective manner, the defense trait we found here decreases the fitness of symbionts in a non-selective manner. Mechanisms other than sanction may be more important in stabilizing mutualisms than commonly thought.