Abstract :
[en] Represented by more than 16 000 described species, bees are the major pollinators of angiosperms in most ecosystems. Since early Cretaceous, they share a long and intimate evolutionary history with flowering plants.
Bees forage on pollen and nectar as the exclusive food source. Interactions between flowering plants and wild bees are highly diverse. Different pollen foraging strategies have been described among bees. Some taxa display floral specificity, restricting their flower visits to closely related plant taxa (pollen specialists) while other bee species are more opportunistic, exploiting a wide range of different flowers (pollen generalists). To characterize different degrees of host-plant associations, different terms were progressively introduced by several authors until better reflect the reality of a continuum in bee host breadth, from extreme specialisation to extreme generalisation.
Ancestral host-plant and specialist behavior seem highly inherited both. However some rare 'host-plant shifts' occured during evolution inside clades. The origin and the mechanism of these host-plant shifts remain misunderstood. They can be based on morphological or phylogenetical similarity with ancestral host-plant but host switches to unrelated plant families are also common. The constraints of these switches are not well known. Because the proportions of nutrients can vary widely among pollens of di?erent plant species, ?oral specialisations in bees could be constrained by the chemical composition of the pollen, reducing the range of suitable alternative hosts. In particular, sterol metabolism could constitute a dietetic constraint of host plant specialisation in bee evolution (Hymenoptera, Apoidea). Indeed, sterolic compounds are involved in some key metabolism pathways like syntheses of ecdysteroid hormones. As insects do not produce endogenous sterols, they must assimilate them from their food. The understanding of sterol requirements and metabolism in insects would shed light on how insect sterol physiology interfaces with ecology.
Preliminary results in Colletid bees show that in a group of closely related species, sterolic profles of host plant pollen are similar, corroborating the hypothesis that host plant specialisation and evolutionnary shifts could partly be explained by chemical composition of pollen.
