PT Unknown AU Fraune, S TI Towards understanding a holobiont: host-microbe interactions in Hydra PY 2009 PU Christian-Albrechts-Universität zu Kiel WP https://macau.uni-kiel.de/receive/diss_mods_00003420 LA en DE cnidaria; host-microbe interactions; antimicrobial peptides; epithelial defense; innate immunity; Wirt-Mikroben Interaktionen; Antimikrobielle Peptide; Epitheliale Abwehr; Angeborenes Immunsystem AB Epithelia in all animals, which are exposed to the environment, are colonized by more or less diverse communities of microbes. Since Hydra is an early-branching metazoan and has preserved much of the genetic complexity of the common metazoan ancestor it promises to be highly informative to discover evolutionary conserved mechanisms controlling epithelial host-microbe interactions. To obtain a better understanding of the microbiota associated with phylogenetically ancient epithelia, I identified the epibiotic and endosymbiotic bacteria of two species of the cnidarian Hydra on the basis of rRNA gene comparisons. I discovered that individuals from both species differ greatly in their bacterial microbiota. Hydra oligactis appears to be associated with only a limited number of microbes, while Hydra vulgaris polyps have a more diverse microbiota. Surprisingly, the microbial composition showed similar characteristics comparing polyps from culture maintained in the laboratory for more than 30 years with polyps from the wild. The significant differences in the microbial communities between the two species and the maintenance of specific microbial communities over long periods of time strongly indicate distinct selective pressures imposed on and within the epithelium. To understand the factors involved in the maintenance of the specific association I first analysed how epithelial homeostasis affects the associated microbiota. I could show that eliminating derivates of the interstitial stem cell lineage from the epithelium causes significant changes in hydra´s microbial community. Due to the loss of the interstitial cell lineage, bacteria of the β-Proteobacteria group decreased and bacteria of the Bacteroidetes group increased in relative abundance. These results indicate a previously unrecognized link between cellular tissue composition and microbiota. To elucidate the role of the innate immunity, especially the antimicrobial peptides (AMP), in shaping the commensal microbiota, I made use of transgenic Hydra overexpressing the AMP periculin1a. I could show that overexpression of an endogenous AMP not only reduces the bacterial burden but also leads to drastic changes in the biodiversity of the colonizing microbiota. These findings indicate that AMPs as an evolutionary old ‘invention’ may play a crucial role in shaping the composition of the commensal microbiota. Antimicrobial peptides, until now known as gene–encoded key elements of innate immunity, therefore, appear also to have an essential role in host-microbe co-evolution. PI Kiel ER