Ecology and evolution of sterols in pollen and wild bees

Thesis

Bees are reliant on flowers for their dietary intake of protein and fats. This includes sterols which act as essential membrane components and hormone precursors in insects. The sterol composition of pollen has been shown to vary widely between plant species and bees are unable to develop with insufficient or unsuitable sterols. However, we know little about the sterol requirements of most bee species. My research addresses the current deficit of information on pollen sterol resources and wild bee sterol requirements by analysing a taxonomically and ecologically wide range of species using a novel and highly sensitive LCMS method developed at RBG Kew.

My analysis discovered constraints on the diversity and quantity of pollen sterols, reflected in the phylogenetically conserved profile shown by related species. Flowers belonging to Asteraceae showed a lower proportion of sterols with a double bond at C-5 of the B-ring in comparison to other plant species, a result mirrored in the tissue profiles of bee species specialising on Asteraceae pollen. All Bombus species, including cuckoos (subgenus Psithyrus), were dominated by 24-methylenecholesterol, isofucosterol and β-sitosterol. This trend was mirrored in solitary bee species; however there was greater inter-species variation and significant differences between generalists and specialist species.

In conclusion, bees acquire a specific suite of sterols from pollen and achieve this by foraging across a wide range of plant species. The sterol profile variation among bee species may be due to differing physiological requirements or foraging preferences. Pollen was in general more diverse in sterol composition and expressed high proportions of sterols that were not abundant in bees. This suggests bees do not passively uptake all sterols found in pollen and instead selectively acquire sterols. It also indicates that plants are not tailoring pollen nutritional content to attract bees.

Authors: Baker, EC

• DOI: 10.5287/ora-9ojr1g52o
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: pollen; sterols; bee nutrition; nutritional ecology
• Subjects: Pollen; Pollination; Sterols; Nutrition; Bees