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Abstract

The nematode Pristionchus paciVcus has been established as a model system for modern evolutionary studies. Evolutionary reconstruction of the natural history of organisms requires knowledge about the development, ecology, and phylogeny of species. Mutations are the source of natural variation, hence studies of mutational processes improve the understanding of the natural history of an organism. Mutation accumulation (MA) lines experiments facilitate the study of spontaneous mutation rates over many generations and offer the possibility of inferring calibrated phylogenies for taxa that lack fossil records. In this study, we used the MA lines setup to calculate mutation rates at the level of the mitochondrial genome and microsatellite loci of the nuclear genome of P. paciVcus. Mitochondrial DNA (mtDNA) evolves rapidly in populations, is usually transmitted maternally without intermolecular recombination and has therefore been intensively used for phylogeographic studies. The P. paciVcus mtDNA is 15, 955 bp in length and contains all the known mitochondrial genes. In the 142nd generation of the 82 MA lines, we found an overall mutation rate of 7.6 × 10−8 per site per generation. The unusual presence of a suppressor transfer RNA for the codon UAA has, most likely, inWuenced the spectrum of observable mutations, in that the lines containing such a premature STOP codon were actually able to survive until the end of the experiment. Using the mutation rate estimate in a comparison of the mitochondrial genome of nine P. paciVcus isolates, we calculated the minimum time to the most recent common ancestor at 105–106 generations. Microsatellites are widely used in evolutionary biology, but still little is known about the true nature of their evolution and the factors that affect their frequency, distribution, and mutation. Around 0.59 − 3.83% of the P. paciVcus genome is composed of di- to hexanucleotides simple tandem repeats. In the MA lines, we analysed a set of 41 randomly chosen microsatellites, in order to evaluate the mutation rates at various loci in the nuclear genome. A total of 31 mutations yielded a mean mutation rate of 7.4 × 10−5 per allele per generation. We observed a strong correlation between allele size and mutation rate, a tendency towards lengthening, and little evidence of multistep mutations. The mutation rates obtained for individual markers provide a powerful tool for divergence time estimates in future studies. These studies provide Vrst insights into mutation processes that shape the genome architecture of P. paciVcus and help further evolutionary studies of this organism.