Evolutionary Ecology Research, 2000, 2: 885–896 © 2000 Laurence Després Evolutionary conflict between Trollius europaeus and its seed-parasite pollinators Chiastocheta flies Nicolas Jaeger, Irène Till-Bottraud and Laurence Després* Laboratoire de Biologie des Populations d’Altitude, CNRS-UMR 5553, Université J. Fourier, BP 53, 38041 Grenoble Cedex, France ABSTRACT Mutualisms are characterized by balanced reciprocal exploitation. This creates an evolutionary conflict in that selection will favour individuals that increase their fitness at the cost of the mutualist partner. To counter this evolutionary instability, each partner must be able to prevent over-exploitation by the other. In plant/seed–parasite pollinator mutualisms like that involving the globeflower Trollius europaeus and the globeflower fly (Chiastocheta spp.), ovipositing females can have a more or less mutualistic/antagonistic effect on plant seed output, depending on the amount of pollination achieved during oviposition, the number of eggs laid and seed predation 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 with increasing egg load, suggesting that most pollination is achieved by non-ovipositing visitors (males and/or non- ovipositing females). Hence, additional eggs do not lead to higher pollination, oviposition is a non-mutualistic behaviour and, therefore, there is a conflict between T. europaeus and Chiastocheta 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 experiments showed that T. europaeus cannot limit Chiastocheta oviposition by triggering flower senescence as soon as full pollination has been achieved. In this context, the high average number of eggs per flower observed in alpine populations 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. Keywords: Chiastocheta, conflict, flower senescence, mutualism, oviposition, seed predation, Trollius europaeus INTRODUCTION In obligate plant/seed–parasite pollinator systems, such as fig–fig wasp, yucca–yucca moth and globeflower–globeflower fly, the visit of ovipositing females benefits the plant through pollination but is costly because the resulting larvae consume a fraction of the seeds produced; therefore, there is a potential conflict of interest between the two partners of the * Author to whom all correspondence should be addressed. e-mail: laurence.despres@ujf-grenoble.fr Consult the copyright statement on the inside front cover for non-commercial copying policies.