Microbiota interactions and dynamics in megachilid bee nests

Abstract

Insect microbiota plays an essential role on the hosts’ health and fitness, regulating their development, nutrition and immunity. The natural microbiota of bees, in particular, has been given much attention, largely because of the globally reported bee population declines. However, although the worker honey bee has been associated with distinctive and specialized microbiota, the microbiota of solitary bees has not been examined in detail, despite their enormous ecological importance. The main objectives of the present thesis were a) the bacterial community description for various solitary bee species, b) the association of the solitary bee microbiota with ecological factors such as landscape type, c) the relation of the bee foraging preferences with their nest bacterial microbiota, d) the examination of the nest building material contribution to the nest microbiota, e) the isolation of bacterial strains with beneficial or harmful properties for the solitary bee larvae and f) the pathological investigation of bacteria found in deceased solitary bee larvae. The findings of the present study revealed a high bacterial biodiversity in the solitary bee nests. At the same time, the bacterial communities were different for each bee host species. Furthermore, it was shown that the pollen bacterial communities underwent compositional shifts reflecting a reduction in floral bacteria with progressing larval development, while a clear landscape effect was absent. The examination of the nest pollen provisions showed different foraging preferences for each included bee species. Both the pollen composition and the host species identity had a strong effect on the pollen bacteria, indicating that the pollen bacterial communities are the result of a combinatory process. The introduced environmental material also contributed to the nest natural microbiome. However, although the larval microbiota was significantly influenced by the pollen microbiota, it was not much associated with that of the nest material. Two Paenibacillus strains isolated from O. bicornis nests showed strong antifungal activities, while several isolated strains were able to metabolize various oligosaccharides which are common in pollen and nectar. Screening for potential pathogenic bacteria in the nests of O. bicornis unveiled bacterial taxa, which dominated the bacterial community in deceased larvae, while at the same time they were undetectable in the healthy individuals. vi Finally, larvae which were raised in vitro developed distinct bacterial microbiomes according to their diet, while their life span was affected. The present thesis described aspects of the microbiota dynamics in the nests of seven megachilid solitary bee nests, by suggesting which transmission pathways shape the established bacterial communities and how these are altered with larval development. Furthermore, specific bacterial taxa were associated with possible services they might provide to the larvae, while others were related with possible harmful effects. Future studies should integrate microbiota examination of different bee generations and parallel investigation of the microbiota of the nests and their surrounding environment (plant community, soil) to elucidate the bacterial transmission paths which establish the nest microbiota of solitary bees. Functional assays will also allow future studies to characterize specific nest bacteria as beneficial or harmful and describe how they assist the development of healthy bees and the fitness of bee populations.