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Zusammenfassung:
Strongyloides spp. are intestinal parasites of vertebrates including man. Strongyloides spp. and its facultative parasitic sister genus Parastrongyloides are being developed as models for basic and translational research. Strongyloides spp. alternate between parthenogenetic parasitic and free-living sexual generations. The latter produce only female parasitic progeny. Although the free-living Strongyloides spp. adults superficially resemble the model nematode C. elegans, there are dramatic differences between them, in particular with respect to the organization of the germline. Combining light and electron microscopy, immunohistochemistry and quantitative DNA and RNA sequencing we characterized the S. ratti germ line and compared it with the germ lines of other nematodes of various phylogenetic distance. We particularly focused on two features, which have puzzled investigators for several generations. First we characterized a population of non-dividing giant nuclei with a very high DNA content in the distal gonad. In C. elegans this regions is populated by mitotically dividing germline stem cells and early meiotic cells. The chromatin of these giant nuclei is rich in histone modifications normally associated with high transcriptional activity, like H3K4me3. We showed that in these nuclei, autosomes are present in higher copy numbers than X chromosomes. Consistently, autosomal genes are expressed at higher levels than X chromosomal ones. This suggests that these worms use differential chromatin amplification for controlling gene expression. Second, we addressed the lack of males in the progeny of the free-living generation. We found that male determining (nullo-X) sperm are present in P. trichosuri, a species known to produce male progeny and absent in S. papillosus, which does not. Surprisingly, nullo-X sperm and very young embryos with a male karyotype appear to be present in S. ratti, even though this species does not produce any surviving male progeny. Interestingly, also the patterns of some histone modifications in spermatogenic cells differ between these three species. This suggests that different species of Strongyloides employ various strategies to prevent the formation of males in the all-female and parasitic progeny of the free-living generation.
