Evolutionary conflict between Trollius europaeus and its seed-parasite pollinators Chiastocheta flies

Mutualisms are characterized by balanced reciprocal exploitation. This crea tes an evolutionary conflict in that selection will favour individuals that increase their fitness at the cost of the mutualist partner. To counter th is evolutionary instability, each partner must be able to prevent over-expl oitation by the other. In plant/seed-parasite pollinator mutualisms like th at involving the globeflower Trollius europaeus and the globeflower fly (Ch iastocheta spp.), ovipositing females can have a more or less mutualistic/a ntagonistic effect on plant seed output, depending on the amount of pollina tion achieved during oviposition, the number of eggs laid and seed predatio n per larva. We found that flowers with no Chiastocheta egg had a high seed set and there was no significant increase in seed set before predation wit h increasing egg load, suggesting that most pollination is achieved by non- ovipositing visitors (males and/or non-ovipositing females). Hence, additio nal eggs do not lead to higher pollination, oviposition is a non-mutualisti c behaviour and, therefore, there is a conflict between T. europaeus and Ch iastocheta flies for the number of eggs laid. Egg load increases throughout flower lifespan. No mechanism seems to have evolved to regulate the number of eggs laid on T. europaeus. For example, controlled pollination experime nts showed that T europaeus cannot limit Chiastocheta oviposition by trigge ring flower senescence as soon as full pollination has been achieved. In th is context, the high average number of eggs per flower observed in alpine p opulations is not surprising. Finally, the decrease in net seed production with increasing egg load was weak. We discuss the other factors involved in the regulation of the conflict between T europaeus and Chiastocheta flies.