UF Study: Similar Species Can Show Different Rates Of Genetic Mutation

April 6, 2005

GAINESVILLE, Fla. — Even closely related species may have different rates of natural genetic mutation, according to a new University of Florida study.

The study’s finding that mutation rates between similar organisms may still be significantly different creates a new wrinkle for scientists pondering a classic evolutionary question: What causes spontaneous mutations – generally considered either bad or, at best, neutral – in those organisms?

Understanding the variability in the baseline mutation rate for closely related species is also an essential first step toward answering the larger question of how external conditions, such as climate change, can affect mutation rates, said UF zoology professor Charles Baer, the paper’s lead author.

The study, co-authored with scientists from Indiana University and the University of Minnesota and funded by UF and the National Institutes of Health, appears in this week’s Proceedings of the National Academy of Sciences.

“Mutation is the ultimate source of genetic variation and is therefore fundamental to all aspects of biology,” Baer said. Although there is increasing evidence that different organisms have evolved different baseline mutation rates, scientists are still trying to determine which ecological and evolutionary factors are responsible.

The source of mutations is a hot topic for debate among evolutionary geneticists, Baer said. Understanding why mutations occur, and what role natural selection plays in the process, is fundamental to answering questions such as the evolutionary basis for sexual versus asexual reproduction, and how genetic variation is maintained within a species.

In the current paper, Baer’s team reports that one species of worm, Caenorhabditis briggsae, accumulated new mutations that affect its evolutionary fitness, or ability to survive and reproduce, about twice as fast as two related species, Caenorhabditis elegans and Oscheius myriophila.

In the experiment, the scientists allowed mutations to accumulate in two strains of each species of worm for 200 generations. Natural selection was unable to weed out the genetic variations resulting from those new mutations because the worm strains were highly inbred and the population sizes used in the study were sufficiently small. By comparing the fitness of the descendent populations with those of the ancestral control stock, the scientists found that the estimated mutation rate varied by as much as a factor of 10 between the strains.

“What this means is that generalizing about the mutational properties of life on Earth from one genotype or one species may not be justified,” Baer said. “If you just randomly pick worms, the mutation rate might vary tenfold. This (finding) obviously shrinks the level of generalizations you can do – certainly as far as extrapolating those rates from flies to humans,” he said.

Knowing that different organisms have different mutation rates can help scientists determine boundaries for those rates, said Wyatt Anderson, a professor of evolutionary genetics at the University of Georgia.

“This is an outstanding paper that addresses a topic of real importance,” Anderson said. “It’s a very large-scale study of mutation accumulation, and provides a fresh look at the genome-wide properties of mutation – because they’re looking at mutations across all of the chromosomes and genes of these organisms.”

Furthermore, the observed variability in rates creates an exciting possibility: that the mutation rate itself may have evolved, Anderson added.

Recent studies have suggested that mutation rate may be a function of body size and environmental temperature, based on rates of evolution among species living in different environments, Baer said.

“These sorts of experiments can allow us to directly test that,” he said. “If it’s true, you can consider, as the climate warms: What are the consequences for the baseline mutation rate of any organism? And we can test that idea in this context. It’s a powerful tool but a slow-moving one.”

Baer’s team is now investigating how those mutations depend on environmental factors such as temperature. If the environment influences the rate of new mutations in a species, that has important implications for the long-term effects of environmental change on these organisms, he said.

“There is reason to believe that man-made factors are making the environment increasingly mutagenic,” Baer said. Those factors may include, for example, the introduction of man-made chemical mutagens to the environment. However, the actual effect that environmental changes might have on an organism’s mutation rate is unclear, because the underlying evolutionary factors responsible for variation are not well known, he said. “We know quite a lot about the effects of broadly changing environments, stressful environments, on mutations in bacteria, but we don’t know so much about multicellular organisms,” he said.

One question Baer plans to study is whether the effects of environmental change on mutation rate are reversible. If the baseline mutation rate of an organism is fairly constant, then removing the mutagen from the environment may return the organism’s mutation rate to normal. However, if an increase in the rate is dependent on the number of mutations it has in its genome, then even a temporary increase in rate due to environmental factors could be irreversible in the long term.

“The ultimate goal of my research program is to understand the evolutionary factors responsible for genetic variation,” Baer said.