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Abstract:
The two nematode sister-genera Strongyloides and Parastrongyloides form the Strongyloididae family of nematodes. This nematode family is of particular inter- est because it contains important parasites of medical and veterinary relevance. Species of this family have a complex biphasic life-cycle, which has the advantage of alternating easily accessible, sexually reproducing free-living generations with parasitic generations. Therefore, the Strongyloididae offer unique opportunies to investigate not only the evolution and basic biology of these intestinal parasites, but also to investigate antihelminthic treatments. In this thesis I am presenting the results of my work on three Strongyloididae species: S. ratti (infecting rats), S. papillosus (infecting sheep) and P. trichosuri (infecting Australian possums). I worked on two main aspects of the Strongyloididae biology: 1.) Together with colleagues, I investigated differential chromatin amplification in the gonads and compared the chromosomal content of sperm of three Strongyloi- didae species. We could show that autosomes are present in higher copy numbers than X chromosomes in a population of giant nuclei with a very high DNA con- tent, located in the distal parts of the gonads of free-living adults of all three species. Quantitative RNA sequencing showed that autosomal genes are higher expressed than X chromosomal ones in these cells, implying that differential chro- matin amplification serves as a mechanism for the regulation of gene expression. Further we showed that S. ratti, but not S. papillosus, produces genetically male determining (nullo-X) sperm, although the free-living generations of both species do not produce any surviving male progeny. 2.) I sequenced and analyzed the small RNA classes of two different developmen- tal stages of the three Strongyloididae. For comparison, I also re-sequenced the sRNAs of the two free-living model nematodes Caenorhabditis elegans and Pris- tionchus pacificus. Recent studies had shown considerable variation in the sRNAs among nematodes, but no Strongyloididae species or close relative had been in- cluded in these studies. Therefore, I established a sRNA sequencing protocol. I identified conserved and taxon specific micro RNAs of which many are differen- tially regulated between the stages. I found that Strongyloididae have lost the highly conserved class of piRNAs, but possess a novel class of ∼27 nucleotide long RNAs, starting with 5’G or A. These 27GA RNAs appear to have triphosphates at their 5’ ends and are therefore presumably synthesized by RNA dependent RNA polymerases without 5’ processing. The 27GA RNAs have the potential to target transposable elements and endogenous genes. Finally, I showed that no parasite- derived sRNAs circulate in the blood of S. ratti -infected rats at detectable levels.
