Spontaneous mutation accumulation and reproductive strategies in free-living ciliates

Knowledge of the frequency and fitness effects of mutations is essential for understanding a diversity of issues in evolution; thus, many efforts have gone into elucidating the mutational process. However, our understanding of mutation is far from complete, largely due to the fact that mutations are rare and frequently eliminated by natural selection. This renders studies of the mutational process inherently difficult. The ciliate Tetrahymena thermophila provides a unique opportunity to overcome these difficulties by allowing the accumulation of mutations over many generations in the absence of selection on the germline genome. Estimates of the rate and fitness effect of mutations, the first from the eukaryotic supergroup Chromalveolata, are within the range of those of previously studied eukaryotes. Mutations are partially recessive on average and the rate of lethal mutations is substantially lower than the deleterious mutation rate. Germline mutation accumulation in T. thermophila used the reproductive idiosyncrasy that the germline genome only gets expressed during sexual reproductions; thus, germline mutations could be hidden from selection during asexual transfers. There might be other useful reproduction strategies for evolutionary genetics, so as a start, another marine free-living ciliate Glauconema trihymene was explored and shown to have diverse asexual reproductive strategies. Selfing was also observed with peculiar macronuclear events in G. trihymene.