OBLIGATE MUTUALISM BETWEEN TROLLIUS-EUROPAEUS AND ITS SEED-PARASITE POLLINATORS CHIASTOCHETA FLIES IN THE ALPS

Trollius europaeus (Ranunculaceae) is involved in a mutualistic intera ction with several species of Chiastocheta flies (Anthomyiidae) that a re both seed predators and pollinators. In the present work, the polli nation biology of T. europaeus and its association with Chiastocheta w ere studied for 3 years in six populations along an elevational gradie nt in the northern French Alps. We showed that T. europaeus is mainly xenogamous and Chiastocheta represented 99 % of all visitors. We sugge st that almost all seed production was achieved by these obligate seed -parasites. Despite high variability in Chiastocheta activity, the pol lination service they provided was high and reliable in all population s during this 3-year study and the seed set was not pollen-limited. Th e ability of T. europaeus to extend flower longevity during rainy peri ods 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.