Unraveling solitary bee microbiota: ecology, importance during development and function

• Applicant: Professor Dr. Alexander Keller
• Subject Area: Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions ; Microbial Ecology and Applied Microbiology
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 279735434

• Final Report Year: 2019

Final Report Abstract

• Microbiomes of solitary bees were mostly understudied, yet our results from this project indicate intriguing and important associations of bees with microbes. These results set a foundation for future research. • Bacterial microbiota in the nesting environment were mostly homogeneous within species and not significantly affected by the landscape. For each bee species, we identified bacterial taxa that showed consistent occurrence in the larvae and stored pollen. • For the pollen provision, we also described a community shift with progressing larval development, suggesting a reduction of imported floral bacteria. • We found a weak correlation between bacterial and pollen alpha-diversity and significant associations between the composition of pollen and that of the nest microbiota. Since solitary bees cannot establish bacterial transmission routes through eusociality, this transitional link between floral and bee microbiomes could be essential for obtaining bacterial symbionts for this group of pollinators. • We were able to identify three bacterial taxa indicative for bacterial communities of the deceased larvae of Osmia bicornis. Furthermore, in an experiment we inoculated the pollen provisions of healthy larvae with such strain. This inoculation resulted in a drastic change on the larval microbiome and acute mortality, proving the pathogenic effect of this bacterium on Osmia bicornis larvae. • We found further candidates with potential pathogenic capabilities of fungi and protozoans, including microsporidians, trypanosomes and alveolates. Beside these potential pathogens, we also found protozoans feeding on dead plant materials, in our case likely the nest materials and pollens, as well as various bacterivores. • We further isolated a strain of Paenibacillus from solitary bee nests, which was a common, but irregularly occurring part of wild bee microbiomes, present on adult body surfaces, guts and within nests especially in Megachilids. This strain is likely a useful associate to avoid fungal penetration of the bee cuticula and a beneficial inhabitant of nests to repress fungal threats in humid and nutrient rich environments of wild bee nests. • During the final phase of the project, we synthesized and reviewed insights from several recent studies on diversity, function and drivers of solitary bee microbiota and compared these to social bee microbiota. Despite basic similarities, the social bee model with host-specific core microbiota and social transmission is not representative of the vast majority of bee species. The higher environmental exposure and the lack of active brood care of the offspring in the nests are factors leading to microbiota which are diverse and environmentally driven.

Publications

• (2018) Wild bees and their nests host Paenibacillus bacteria with functional potential of avail. Microbiome 6, 229
  Keller A., Brandel A., Becker M.C., Balles R., Abdelmohlsen U.R., Ankenbrand M.J., Sickel W.
  
• (2019) Bacterial community structure and succession in nests of two megachilid bee genera. FEMS Microbiology Ecology 95, fiy218
  Voulgari-Kokota A., Grimmer G., Steffan-Dewenter I., Keller A.
  
• (2019) Drivers, dynamics and functions of solitary bee microbiota. Trends in Microbiology
  Voulgari-Kokota A., McFrederick Q.S., Steffan-Dewenter I., Keller A.
  
• (2019) Linking pollen foraging of megachilid bees to their nest bacterial microbiota. Ecology and Evolution
  Voulgari-Kokota A., Ankenbrand M., Grimmer G., Steffan-Dewenter I., Keller A.