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Zusammenfassung:
Strongyloides sp. alternate between parthenogenetic parasitic and sexual free-living generations. Some species (e.g. S. ratti and S. stercoralis) employ an XX/XO sex determining system. In S. papillosus and S. venzuelensis the X chromosome and one autosome are fused and males are formed by sex specific chromatin diminution, which eliminates the X chromosome derived portion (=diminished region) from one of the two fusion chromosomes. Coincidentally, we had noticed that there appeared to be no meiotic recombination within the diminished region even during female meiosis. If true, the sex chromatin in S. papillosus, different from the autosomal regions in the very same genome, undergoes essentially non- sexual reproduction. This would be expected to have profound consequences for the evolution of this region and might lead to the accumulation of "junk" DNA. Fittingly, the genome of S. papillosus had been more difficult to assemble, compared with other Strongyloides genomes, because it appears to contain long repetitive regions. First, we used a combination of PacBio and Illumina sequencing to improve the S. papillosus genome assembly. Second, by analyzing single male - female crosses, we found no indication for crossing over events between loci located within the diminished region but separated by considerable physical distances. Finally, we whole-genome-sequenced individual S. papillosus isolated from the wild and performed linkage analysis. While the autosomal regions appeared to recombine freely, we observed a very strong linkage disequilibrium between all loci in the diminished region, confirming at the population level that there is very little, if any, recombination in this region. In contrast, we readily observed meiotic recombination on the X chromosome in S. ratti and we found no indication for linkage disequilibrium over the entire X chromosome, in S. stercoralis, indicating, that in this species recombination does occur on their free X chromosomes.
