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要旨:
Evolution is the ultimate reason why planet earth is full of complex living systems. Evolutionary theory is thus at the heart of biology and necessary to understand our world. A complete picture of evolutionary processes must incorporate multiple levels of organisations and timescales. The nematode Pristionchus pacificus is a suitable model organism that allows such interdisciplinary studies with the long-term goal of reconstructing the evolutionary history of the species. In my work, I focused on two aspects of P. pacificus evolution: organismal interactions and genome evolution. Selection pressures in nature often derive from interactions between organisms. P. pacificus lives in a necromenic association with scarab beetles. We extracted nematodes from 114 Geotrupes beetles and found diverse communities. The only life stage of P. pacificus present was the Dauer stage. Beetle-associated bacteria are a source of food as well as a potential pathogen to P. pacificus. We isolated 23 bacterial strains from beetle-associated nematodes and studied their interaction in laboratory assays. Bacterial pathogenicity is species-specific and nematodes are able to sense and avoid harmful bacterial strains. Genome evolution is shaped both by natural selection and a multitude of stochastic processes. First, we studied natural variants in 104 strains of P. pacificus and observed strong linkage blocks. These blocks allow deleterious alleles to avoid natural selection and thus increase in frequency stronger than expected. Second, we analyzed the properties of 802 de novo mutations derived from 22 Mutation Accumulation Lines. Mutations occur randomly across the genome, but their spectrum is strongly biased towards creation of AT nucleotides. In natural variants, this ATbias is counterbalanced over time and the GC content of the P. pacificus genome is stable. Third, we could observe recombination in hybrid lines of P. pacificus and show that there is intraspecific variation in recombination rates. In addition, recombination is positively correlated to natural variants, probably because it promotes natural selection by reshuffling alleles. Finally, an analysis of transmission rate distortion revealed strong hybrid incompatibilities even between closely related strains of P. pacificus.
