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Research

Endosymbioses

Our research is focussed on host-symbiont interactions between ciliates and their endosymbionts. As the limelight of our work we understand the symbiosis induced changes in the host's phenotype, its molecular causes as well as its ecological and evolutionary consequences. With ciliates and especially the different Paramecium species intracellular symbionts are a common phenomenon. Such a symbiotic relationship usually leads to phenotypic adaptations. We aim to understand the reasons for such changes.
Therefore we conduct comparative analyses of infected and aposymbiotic (symbiont-free), genetically identical cells. The observable interaction's spectrum ranges from being of mutualistic nature to purely parasitic. For the symbionts of Paramecium it usually is context-dependend. To characterize those interactions we implement functional experiments, microscopy as well as genome and genexpression analyses.

 

Biodiversity

We investigate the biodiversity of ciliates and their endosymbionts using morphological, molecular, phylogenetic and phylogenomic analyses. Our most recent papers were published in Scientific Reports (Lanzoni et al., 2019), FEMS Microbiology Ecology (Potekhin et al., 2018) and Systematic and Applied Microbiology (Schrallhammer et al., 2018).

 

Positive fitness effect without the provision of nutrients?

Most commonly, infected organisms exhibit variations compared to genetically identical but aposymbiotic ones. Those deviations can manifest in different growth rates or maximal possible cell densities. Infected cells have to sustain and nurish their symbionts in which case the cell number's reduction of infected cells is plausible. On the contrasting site, it is mostly unknown how symbionts not producing essential nutrients for their host lead to an increased growth rate or even increase its host's capacity limit (Bella et al. 2016, Grosser et al. 2018, Schu & Schrallhammer 2018).

In the case of Caedibacter the mechanism's deciphering was successful (Grosser et al., 2018)!

 

The role of endosymbionts with regards to predation and competition

A fascinating example for symbiont induced changes in the host's phenotype is the so-called killer trait. Caedibacter-harboring paramecia are toxic for symbiont-free Paramecium spp. (Schrallhammer and Schweikert, 2009). The essentiel component of this lethal effect is the R-body, a particular protein complex produced by the endosymbionts (Schrallhammer et al., 2012). Therefore, the killer trait provides a selective advantage for infected paramecia against their aposymbiotic competitors.

Just recently, we were able to show the limited ecological relevance of the killer traits for other ciliates, among those Paramecium-predators Dileptus and Climacostomum (Koehler et al., 2019). Hence, the symbiosis with Caedibacter does not confer protection against predation (Koehler et al., 2019).

 

In the spotlight - swimming behaviour under the symbiont's influence

Endosymbionts influence their host's biology with respect to many different aspects, such as the behaviour. P. bursaria hosts hundreds of microalgae (in the further course referred to as Chlorella). In this mutualistic interaction the algae provide their host with oxygen and carbohydrates (photosynthesis). P. bursaria offers among others an increased motility: they swim into areas which provide optimal light conditions for their symbiont and stay there (photoaccumulation). We adress the question to what extend the swimming behaviour of P. bursaria is directly influenced by the algae.

Furthermore, in the framework of an interdisciplinary project with colleagues of the Physical Institut of the University of Freiburg we model the swimming behaviour of Paramecium.

 

Genome and transcriptome analyses of symbionts

Together with different cooperation partners we rise to the challenge to sequence the genome and transcriptome of those endosymbionts which so far were not culturable outside their host (Zaburannyi et al., 2018). Momentarily, we work on an enhanced version and more detailed analysis of the Caedibacter genome.