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Abstract

Nematodes of the genus Strongyloides are common small intestinal parasites of vertebrates. They have a complex life cycle, which in addition to parthenogenetic parasitic adults also contains a facultative free-living adult generation, with males and females. The presence of sexually reproducing adults outside of the host offers opportunities for genetic research, which are quite unique for an endo-parasitic organism. Accordingly, Strongyloides spp. is developing into a model system for parasitological, basic biological and evolutionary studies. In the first part of my thesis, I examined the Small Ribosomal Subunit rDNA (SSU) sequences from S. stercoralis larvae isolated from human patients in Cambodian highly S. stercoralis prevalent areas. Three polymorphic positions and three different haplotypes were identified within a region of the SSU normally considered to be essentially invariable within a nematode species. Interestingly, no hybrid individuals were found. These results suggested a low frequency of interbreeding between the different haplotypes in this area, either because S. stercoralis in this region reproduces only asexually or because crossing happens only within rather than between haplotypes. Many research tools and techniques routinely used in model organisms like Caenorhabditis elegans are not yet available for Strongyloides spp. One of these methods is mutagenesis using chemical mutagens. In the second part of my thesis I devised a protocol to mutagenize S. ratti with the chemical mutagen Ethyl Methanosulfonate (EMS). Using this protocol, I generated S. ratti mutants with a higher proportion of animals developing into the parasitic form. As a control, I also attempted to obtain the same effect by selection only, in absence of EMS. Next I evaluate the possibility of identifying the mutated genes by whole genome sequencing of multiple mutagenized and selected strains. While this approach appeared promising, I also found that the currently used laboratory strain is not sufficiently isogenic such that the number of resulting candidate mutations, which need to be tested is rather high. The strong population bottlenecks associatiated with the mutagenesis and selection procedures reduced the genetic complexity of the populations significantly. This demonstrated that generating a more isogenic S. ratti strain for genetic work is possible. In the third part of my thesis I isolated and characterized Strongyloides mariner-like transposons (SMARTs). Contrary to S. ratti, in S. papillosus several copies of SMART appeared potentially active. These transposons have the potential to be used as genetic tools as it has been demonstrated for the related Tc1 transposons in C. elegans.