Pollination by bees: a case study of the leafcutting bees and their hosts

The relationships between bees and flowers are complex and not merely mutualistic, but are better described as a reciprocal exploitation. Obviously, the primary interest of the bees is not to pollinate the flowers, but to efficiently collect pollen and nectar, their main larval food source. While nectar is chiefly produced as a reward for pollinators, pollen is a valuable resource, rich in protein and thus expensive for the plant, and essential for its reproduction. Therefore, many flowers have developed well-documented morphological adaptations to avoid excessive pollen harvesting by bees. It remains largely unclear, however, whether plants protect their pollen through secondary chemistry, as they do for other tissues. In fact, the secondary chemical composition of pollen is highly complex and probably plays a dual role in attracting visitors and protecting the pollen. The goal of this project is to examine how pollen chemical composition influences, and is influenced by, interactions with pollinators. We will focus on several plant families presenting strikingly different pollination modes: the Fabaceae, which are typically dependent on bees and morphologically adapted to them; the Asteraceae, which have a more generalized pollination syndrome; and lastly, the Boraginaceae, which show a mixture of different strategies. Pollen chemical composition will be studied by LC-MS and GC-MS analyses; the biological functions of selected, key secondary compounds will be examined in bioassays in a cage rearing of one species of solitary bee, the alfalfa leafcutting bee (Megachile rotundata). Lastly, phylogenetic methods will be used to trace the evolution of floral specialization in solitary bees of the genus Megachile. This genus is the second largest bee genus in the world, including over 1500 species. Worldwide, its members are associated with two main families of plant, the Asteraceae and the Fabaceae.