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Researchers: Dr. Maryse Vanderplanck, Dr. Denis Michez, PhD Pierre-Laurent Zerck

Through pollen collection, bees act concurrently as effective pollinators and herbivores. While the majority of bee species are pollen specialist, polylecty (i.e. pollen generalist) is widespread among social bees. Regardless of floral specialization, they display numerous adaptations as well as sensorial capabilities, which allow them to discriminate among plant species and to enhance floral rewards foraging. In response to excessive pollen harvesting, flowering plants have drawn up complex systems of chemical and morphological defenses that could minimize this potential excessive foraging. Among chemical defenses, constraints could act through toxicity or nutrient availability.

Nutritional constraint

It has been shown that nutritional quality of pollen is highly variable among plant species including sterols, essential compounds in bee physiology and that are exclusively exogenous. Our research suggest that sterolic composition of pollen could shape bee-flower interactions as well as their evolution. Involvement of these sterolic compounds in wasps-bees transition will be investigated through chemical analyses (sterols and ecdysteroids).
Moreover, the sterolic physiologies of various bee species will then be studied in details by using chemical analyses of pollen and insect both. Shotgun metagenomic analyses coupled with quantitative PCR (QPCR) will characterize the digestive bacterial communities and accurately determine their role in the metabolism of the host insect.

Toxic constraint

Plants synthesise an array of chemicals to defend themselves against attack by herbivorous insects, and some of these defense compounds also occur in nectar and pollen that constitute resources for pollinators. Such floral chemicals can be detrimental or beneficial to bee pollinators as they might protect bees against disease and pathogens but also cause toxicity. How these toxins can impact on pollinator behavior is not fully understood. Chemical constraint due to pollen toxicity will be considered through analyses of alkaloids and saponins, which are known for their negative effect on bees. Bees will be fed on pollen supplemented or not with these metabolites to determine whether rates naturally present in certain pollens are lethal to bees. In parallel, QPCR analysis will be conducted to highlight the presence or induction of potential detoxification pathways as bee adaptation (e.g. cytochrome P450).

A specific case : Asteraceae paradox

Recent findings support that Asteraceae pollen possesses unfavourable or protective properties and suggests that bees need physiological adaptations to successfully utilize it. Although Asteraceae are ubiquitous in most terrestrial habitats and produce considerable amounts of pollen, this pollen plays only a marginal role in the diets of the pollen generalists by striking contrast to specialists. Despite numerous hypotheses, this paradox remains unsolved. To investigate the unfavourable pollen properties of the Asteraceae for generalist bees, bee rearing are performed on diverse pollen diets. The larval development are compared between the diets. Moreover, digestibility as well as toxicity and nutrient content of Asteraceae pollen are examined by quantifications of nitrogen, alkaloids and total amino acids.

According to the floral species, pollen grains are higlhy diverse in their color, their microscopical shape but also in their chemical composition which impacts on the digestibility, the toxicity and the nutrient supply for bees .

Nests of Osmia cornuta closed with septa of soil in a hollow stem. Generalist females selected floral pollen to ensure their larvae development.

Bombus gerstaeckeri queen foraging on Aconitum napellus, its exclusive and toxic host-plant. This peculiar floral specialization is studied in Swiss Alps.

Polypeptide analyses of pollen are performed using colorimetric assays after a specific procedure of extraction .