Exploring the ecological role of sterols in bee-plant interactions

It is known that bees collect pollen to satisfy their nutritional and physiological requirements, and those of their offspring. Pollen is a chemically complex mixture that contains both central metabolites, which are vital for plant survival, and specialized metabolites, which play a key role in interaction of the plant with the environment such as insect attraction or deterrence. Among pollen metabolites, sterols are indispensable for bees since they are involved in hormone production, gene expression, and cell membrane stability. As bees cannot synthesize sterols de novo, these compounds represent an important constraint in shaping bee-plant interactions. Sterolic composition is indeed highly variable among plant species so that bees face a high degree of variation in their availability. Although some bee species appear able to modify the sterolic composition of foraged pollen during its transport , such strategy remains limited and is not sufficient to make all pollen types suitable. Especially, some sterols appear to be toxic to unspecialized bee species. This implies that bees cannot forage randomly on all available resources, even generalist species, but have to display selective foraging to increase their individual health and reproductive success. Sterolic composition of pollen is therefore clearly involved in the selection and use of resources, even in an evolutionary framework. Ensuring access to an appropriate sterol supply through managed floral communities is then a critical aspect for maintaining bee populations. The purpose of this communication is to illustrate these various concepts from experiments and observations carried out within analytical and ecological perspectives.