'Genetic Hitchhikers' Documented in Yeast Evolution

Scientists studying evolution of yeast strains discovered that the evolutionary process is not driven by one single, beneficial genetic mutation, but rather by multiple mutating genetic groups, including ones known as "genetic hitchhikers" that just appear to be along for the ride.

These genetic hitchhikers do not seem to play a contributing role in the organism's evolution, but may prove to play an important role down the road, researchers from Princeton University report.

In a study of 1,000 generations of adaptations in 40 yeast populations, the researchers found that five to seven mutations -- rather than just one -- were needed for the organism to succeed.

The researchers report that knowledge of how genetic mutations drive evolution can provide a better understanding of how tumors resist chemotherapeutics and how bacteria evolve resistance to antibiotics.

Evolution occurs when an individual organism experiences spontaneous beneficial mutations in its genome that improve its ability to adapt to its environment. Prior understanding of genetic evolution was that a single mutation could boost the survival of an individual, which would then reproduce and pass the genetic mutation to its offspring.

The research team, which included members from Harvard University, Rowan University and Washington University School of Medicine, found that instead of just one mutation causing enhanced survival, as many as seven mutations are required. The researchers termed the extra mutations genetic hitchhikers because they don't appear to contribute to the enhanced fitness of the organism.

"Our study indicates that evolution is more of a group effort," said Gregory Lang, first author on the paper and an associate research scholar at Princeton.

"The finding goes against the traditional view of evolution being determined by individual mutations that provide a large fitness advantage by themselves," Michael Desai, an assistant professor at Harvard, said. "We found that small groups -- which we call cohorts -- of mutations were associated with increased survival. No single mutation is driving adaptation. The whole group, which includes hitchhikers, drives adaptation together."

While the research was conducted on strains of yeast, Princeton genomics professor David Botstein said that understanding the basic evolutionary process in yeast translates to other organisms by expanding scientific understanding of how evolution occurs. More research is needed before the role of the hitchhiker groups and their function alongside the so-called "driver" group of genes is fully understood.

But the research marks the latest progress in the science behind understanding genetic mutations and the evolutionary process.

"By following genomic changes across cell populations over time, this study allows a rare glimpse of evolution in progress," said Stefan Maas of the National Institute of Health's National Institute of General Medical Sciences, which partially funded the research. "This systems biology approach yields insights that could help us understand how mutations spread through other evolving systems, such as cancer cells or disease-causing microbes."

The research is published in the journal Nature.