Pollen and sterol content differentially affect solitary bee development

Pollen sterols are essential micronutrients for bees, with roles as membrane components, hormone precursors and for regulating gene expression. It is historically accepted that bees are unable to produce sterols de novo or modify them and use them as they occur in pollen. This may require adaptation by bees to specific plants because pollen sterols vary dramatically across taxa. Here we investigated the effects of pollen with different sterol composition or supplementation on development of solitary red mason bees, Osmia bicornis, comparing bee provisioned pollen, with Castanea sativa pollen containing campesterol a key Osmia sterol, a sterol supplemented C. sativa pollen, a polyfloral pollen from which the key campesterol and cholesterol sterols were almost absent, and a combination of polyfloral and C. sativa pollen. DNA barcoding was used to characterise the Osmia pollen, which comprised at least 38 taxa but was mostly Quercus spp. Pollen diet significantly affected the larval final weight, larval development time and development over time. Larvae fed chestnut pollen supplemented with sterols developed significantly faster over time compared to those from the other treatments, including the chestnut pollen with no sterol supplementation. Despite sterol composition being distinctive among pollen diets, sterol profiles were more similar among bees from different pollen foods (i.e., different treatments) than between bees and their pollen foods. Moreover, bees fed on polyfloral pollen had much higher relative concentrations of campesterol (~8.5 %) and cholesterol (~26 %) than the pollen itself (~3 % and ~ 0.7 %, respectively), addressing the possibility that Osmia bees are highly efficient in sterol selective accumulation or possible transformation of sterols in Osmia bees. Our results suggest greater flexibility in sterol handling in bees than previously assumed.