Specialization and adaptation in pollen sterol use by wild bees

Sterols are stabilising components of membranes and hormone precursors in eukaryotes. Honeybees incorporate a subset of pollen-derived sterols into their tissues, rather than converting phytosterols to cholesterol as occurs in most insect herbivores. To establish whether this approach to sterol acquisition was typical of all bees, we measured the sterolome of 56 bee species representing all major bee families to identify and quantify which pollen sterols they used. The Δ5 sterols, 24-methylenecholesterol, isofucosterol and β-sitosterol, which are common components of pollen sterolomes were also the main sterols in most bee species, but this was not so in a minority of species suggesting they had different sterol requirements. Generalists contained more 24-methlyenecholesterol than their specialist congeners, suggesting an adaptation to use pollen sterols that occur widely in plants whereas Asteraceae specialists use pollen sterols that are not used by generalists, which may explain the Asteraceae paradox. Overall, our data suggests an ecological rather than phylogenetic driver of bee sterol composition.