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Project 15

Microdiversity and transmission of nephridial symbionts in earthworms

PhD position. Supervisors: Andreas Schramm (UniAarhus), Siv Andersson (UniUppsala), Michael Lalk (UniGreifswald). Host: Aarhus University. Secondment internship: University of Greifswald; University of Uppsala


Almost all species of lumbricid earthworms harbor extracellular symbiotic bacteria (genus Verminephrobacter) in their excretion organs (nephridia). The symbiosis increases host fitness, is host species-specific with indications for a 100 million year-old co-speciation, and the symbionts are vertically transmitted. Still, the partners can be separated in the lab, and stable cultures of several symbiont strains and symbiont-free earthworm species have been established, which makes this symbiosis an ideal model system to study the molecular mechanisms of symbiont transmission, host specificity, and other host-symbiont interactions.


(i) To assess the (micro)diversity within geographically distinct populations of the specific symbionts of three earthworm species; and (ii) to determine the role of this variation for the vertical transmission, species-specificity, and evolution of the symbiosis.

Key methods

Cross-infection experiments with selected host and symbiont species/varieties will be coupled with a combination of sequence analyses (e.g. of the intergenic spacer (ITS) region and fast-evolving genes from the symbiont genomes), in situ imaging techniques (FISH, lectins, electron microscopy), and laser microdissection. These analyses will be supplemented by NMR and mass spectrometry analyses of signal molecules (e.g. lipo-oligosaccharides) potentially mediating host specificity (Michael Lalk) and by bioinformatic analyses (Siv Andersson).