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Abstract

Strongyloides spp. are intestinal parasites of vertebrates including man. They alternate between parthenogenetic parasitic and facultative freeLliving sexual generations. The latter produce only female parasitic progeny. Although the freeLliving!Strongyloides spp. superficially resemble the model nematode C.! elegans, there are dramatic differences between them. Combining light and electron microscopy, immunohistochemistry and quantitative DNA and RNA sequencing we characterized the germ line of freeLliving S.! ratti 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 nonLdividing 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. We found that the chromatin of these giant nuclei is rich in histone modifications normally associated with high transcriptional activity and 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 freeLliving generation. We found that male determining (nulloLX) sperm are present in the sister taxon P.! trichosuri, which produces male progeny and absent in S.! papillosus, which does not. Surprisingly, nulloLX 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. These findings suggest that different species of Strongyloides employ various strategies to prevent the formation of males in the progeny of the freeLliving generation.