Flycatcher genome sheds light on causes of mutations
A research team at Uppsala University has determined the complete genetic code of 11 members of a flycatcher pedigree. Doing this, they have for the first time been able to estimate the rate of new mutations in birds. When they combined the new results with mutation rate estimates from other organisms, a clear pattern emerged: The more common a species is, the lower its mutation rate.
The research team at the Department of Ecology and Genetics at Uppsala University has sequenced the whole genome of 11 members of a three-generation flycatcher family consisting of grandparents, parents and offspring. By carefully screening letter by letter in the 1.1 billion letter genetic code of each individual they searched for new genetic variants. These represent mutations arisen in the germ cells of parents.
'This is really like looking for a needle in the haystack. There were just on average eight mutations found per individual. Mutations were randomly distributed across the genome, hence there were not short-cut to their identification', says Hans Ellegren, professor in evolutionary biology and leader of the study,
Based on the occurrence of mutations in the family the team could estimate the rate at which new mutations arise to one per 100 million letter per generation. This is the first time someone has been able to estimate the rate of mutation in birds. Mutation rate estimates are only available for a limited number of other organisms. The comprehensive computer work was performed by bioinformatician Linnéa Smeds. Anna Qvarnström provided blood samples from the analysed birds.
One major goal of the study was to shed light on the long-standing question of why there are mutations. One would think that mutations occur to ensure that there will be evolution – without them new life forms cannot evolve and all species would eventually become extinct. However, natural selection operates on the level of individuals and there is no benefit for individuals to mutate just because it might be good for the species to have genetic variation in the future. On the contrary, there is typically a cost associated with mutations: most mutations that affect the fitness of offspring are more or less deleterious. Theory therefore suggests that parents should avoid as much as possible to provide their offspring with new mutations. Ideally, the rate of mutation should be zero.
'When we combined our results with estimate rates of mutation in other species, a clear pattern emerged. The more common a species, the lower its mutation rate', says Hans Ellegren.
It is known that natural selection is generally more efficient in large populations. Selection can therefore better improve the machinery that replicates DNA in germ cells in abundant species, to save their offspring from deleterious mutations. But even in the most common species selection is not efficient enough for a replication machinery with 100% accuracy to evolve.
The answer to the question of why there are mutations (despite their deleterious effects) is thus probably that they cannot be avoided.
Linnéa Smeds, Anna Qvarnström, Hans Ellegren (2016) Direct estimate of the rate of germline mutation in a bird, doi: 10.1101/gr.204669.116 Genome Res. 2016. 26: 1211-1218, http://genome.cshlp.org/content/26/9/1211