Specialization and adaptation in pollen sterol use by wild bees

Sterols are essential metabolites in eukaryots, acting as stabilizing components of membranes and steroid hormone precursors. Unlike other animals, insects cannot biosynthesise sterols de novo so must acquire them from diet. Honeybees selectively uptake a subset of pollen sterols for use in their own tissues, rather than converting b-sitosterol or stigmasterol to cholesterol, as occurs in most insect herbivores because they do not have the chemical architecture to modify sterols like other insects. We do not know if this strategy is also employed by other bee species.

Here, we measured the sterol profiles of 56 species representing all families of UK bees to identify whether the same pollen sterols are used by solitary and eusocial bee species.

The Δ5 sterols 24-methylenecholesterol, isofucosterol and β-sitosterol which are common components of pollen sterolomes were also the main sterols found in all bee species. Campesterol and cholesterol were recorded almost universally in bees but in proportionately small amounts, despite their low abundance in pollen.

Our data suggest an ecological rather than phylogenetic driver of bee sterols because related species differed as a function of pollen diet. Generalist species contained more 24-methlyenecholesterol than their specialist congeners suggesting a trade-off between competition for food and desirable sterols. Asteraceae specialists have adapted to use pollen sterols that are not used by generalists, potentially gaining an advantage over these species. Despite the uptake of less common sterols by specialists, they still relied on similar sterols to other bee species. The dietary requirements of specialists could be fulfilled by pollen from non-host species, suggesting that while essential, sterols are not the primary driving force of pollen foraging choices.