The impact of pollen diet and microbiome on honey bee health

In the last decade, drought has been identified as one of the most relevant climate change factors affecting ecosystem integrity across countries, affecting plant growth in agroecosystems, and leading to changes in the trophic potential of nectar and pollen. As a cascade effect, a deficit in the nutritional composition and availability of pollen can weaken pollinators, triggering additional threats to ecosystem stability. In this scenario, understanding the impact of trophic sources on honey bee health remains a significant gap that needs to be addressed. Our research aims to correlate pollen of different botanical and geographical origins, and therefore of different trophic potential, with selected honey bee markers: the abundance of core gut microbial taxa and proteins involved in the immune response detectable in the haemolymph. A holistic proteomic approach based on MALDI BeeTyping® and SDS-PAGE profiles, together with qPCR for the quantification of target microorganisms, was used to elucidate these interactions in bees fed with pollen of different botanical families. Briefly, newly-emerged caged honeybees were daily fed with a fresh suspension of pollen and sugar syrup (1:1 w/v) for a total of 40 monofloral and 13 polyfloral pollens belonging to 8 botanical families (Cistaceae, Asteraceae, Cornaceae, Boraginaceae, Fagaceae, Leguminosae, Rosaceae and Salicaceae) collected in 5 Italian regions. After 5 days, haemolymph and guts were extracted for the MALDI-BeeTyping, SDS-PAGE, and qPCR analysis after DNA extraction. Our results show that different pollens significantly affect the core gut microbiome composition but do not significantly affect the concentration and the total amount of small and large haemolymph proteins. Furthermore, the effect of different diets on the microbiome suggests an indirect effect on the immune system response by modulating and influencing the synthesis of some immune-related peptides. This research confirms the importance of gut symbionts in honey bee health also in a changing diet scenario. Finally, our results represent the first step in the understanding on the honeybees response to climate changes.