Obligate mutualism between Trollius europaeus and its seed-parasite pollinators Chiastocheta flies in the Alps

International audienceTrollius europaeus (Ranunculaceae) is involved in a mutualistic interaction with several species of Chiastocheta flies (Anthomyiidae) that are both seed predators and pollinators. In the present work, the pollination biology of T. europaeus and its association with Chiastocheta were studied for 3 years in six populations along an elevational gradient in the northern French Alps. We showed that T. europaeus is mainly xenogamous and Chiastocheta represented 99 % of all visitors. We suggest that almost all seed production was achieved by these obligate seed-parasites. Despite high variability in Chiastocheta activity, the pollination service they provided was high and reliable in all populations during this 3-year study and the seed set was not pollen-limited. The ability of T. europaeus to extend flower longevity during rainy periods and a long stigmatic receptivity in the absence of pollinators may help to explain the high female reproductive success observed at all elevations. (C) Academie des sciences/Elsevier, Paris