Pollen sterols are highly diverse but phylogenetically conserved
Sterols regulate cell membrane fluidity and are precursors for hormone and secondary metabolite production in plants, but plant sterols also have a critical role as nutrients in herbivores. Here we describe the distribution of 78 different sterols from pollen of 295 UK wildflower taxa and use this data to develop an evolutionary rationale for the diversity of sterols in pollen compared to vegetative tissues. The sterolome was a function of plant lineage and conserved in groups as high as subfamily. Insect herbivores are auxotrophic for sterols, and notably bees can not modify them therefore rely on dietary sources, primarily pollen, to meet their metabolic needs. Most pollen in the present study contained high proportions of {Delta}5 sterols including {beta}-sitosterol, 24-methylenecholesterol and isofucosterol, which are important sterols for bees. The sterols recorded in honey bees occurred in the pollen of only 68% of plant taxa, however, none matched these proportions exactly suggesting they must forage pollen from multiple plant taxa to satisfy their sterol requirements. We conclude that there is evidence for pollen sterol composition being the result of diverse driving forces including plant lineage and pollinator nutritional requirements.