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Due to the tight mutualism between pollinators and flowers, this system is also highly interesting as a model to investigate and understand microbial transmissions between hosts, their tri-partite coevolution and potential arising conflicts. Microbes can contribute positively or negatively to host health, development and fecundity. Detrimental effects on pollinators and plants are caused by pathogens and competitors, while beneficial symbionts enhance nutrition, detoxification, spoilage inhibition and pathogen defense. Insights into the occurrence, evolution, and implications of these associations strongly contribute to our understanding of the current risk factors threatening pollinator and plant populations. In DFG funded projects, we found that pollinator microbiomes can exhibit a wide range from strongly conserved and stable associations to being rather environmentally driven. Life-history strategies like e.g. sociality can promote stability in bees, as well as diverse developmental factors, nest materials used and phylogenetic predisposition. We found that landscape, environment and agriculture can strongly impact wild bee microbiomes, particularly for solitary bee species. We also found that such environmentally driven microbiomes are susceptible to pathogens and microbiome shifts due to anthropogenic changes. Further, works included genomic study of relevant microbes and their consequences on bee health. For plants, we found that floral microbiome composition is strongly co-evolutionary driven by host phylogeny and their metabolome profiles, but yet dependent of the local microbial species pool and highly relevant for floral functions. We found that land-use intensification results in reduced diversity and homogenization of floral microbiomes due to reduced plant diversity and interactions with pollinators.
With increasing knowledge on hosts-microbiome interactions in the pollination system, the field now undergoes a paradigm shift: Microbe-host associations in pollination systems are not isolated, but highly intertwined with each other and with the pollination network itself. For example, we found that flower microbes affect pollinator visitation by changing attractiveness for specific pollinators. On the other hand pollination network structure influences which microbes are dispersed between pollinators via flowers. Investigating the resiliences of the three parties and spill-over of pathogens and beneficial microbes between species with respect to anthropogenic land-use changes is particularly intruiguing. We are currently at the very beginning of understanding these reciprocal effects in these multipartite networks, how land-use intensification modifies this complex system and the consequences of changes on host health.
