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Study Links Origin of Sexual Reproduction With High Mutation Rates

Robert Tindol
Caltech
July 10, 2001
 
PASADENA, California—Biologists have long known the advantages of sexual reproduction to the evolution and survival of species. Sex helps a fledgling creature pass on its good mutations and respond better to environmental stresses that would leave its asexual neighbors floundering in the shallow end of the gene pool.



But a puzzling question is, how did sex begin in the first place?

Researchers from the California Institute of Technology (Caltech) and the Jet Propulsion Laboratory (JPL) propose an answer, based on studies of RNA material and computer-simulated activity of bacteria.

Claus Wilke of Caltech and Chris Adami, of both Caltech and JPL, have concluded that asexual bacteria can be nudged to evolve into sexual reproduction if they are subject to high levels of mutation induced by environmental stress—from, say, radiation exposure or a catastrophic meteor.

Many primitive single-celled organisms do just fine with asexual reproduction. But mathematical models have demonstrated that a sexual mutant in an asexual population isn't likely to compete successfully and pass on its genes. The researchers say their work shows that higher rates of mutations enable an asexual population to adapt sufficiently to give mutant individuals a greater advantage if those mutants reproduce sexually.

"The reason the origin of sexual reproduction has been such a big mystery is that we look at the world as it is now. But the early world was a much more stressful place, sometimes changing very rapidly," said Adami, who is co-publishing a report on the work in the July 22 issue of Proceedings: Biological Sciences B, a scientific journal of the Royal Society.

"We can't say how or when sexual reproduction came to take a hold in nature," he added, "but we can now say that high mutation rates can, under the right conditions, force an asexual organism to become sexual."

Computer-Based Research

The work was based in part on studies of "digital organisms," or self-replicating computer programs designed to closely resemble the life cycles of living bacteria. Such an approach is necessary because it's hard to obtain accurate data on how living organisms respond over time to multiple mutations and gradually lose their fitness.

"It is difficult to construct constantly changing environments in a petri dish," Adami said. Computer-based simulations, on the other hand, make it possible to study many generations in a short period of time.

Although the study did not involve living organisms, the researchers say their findings have significant implications for understanding the origin of sexual reproduction in the early world.

Past collaboration by Adami and experts in bacteria has indicated that the computer simulations are realistic. So Adami thinks a living colony of asexual bacteria would probably respond much like the computer-generated organisms when subjected to comparable kinds of actual stress.

Lingering Mystery

The study builds on earlier work by Adami and others showing that digital organisms can adapt to become more robust, or less vulnerable to harm by environmental changes and other factors.

"What we showed in the other paper," said Adami, "is that if you transfer a fragile organism that evolved with a small mutation rate into a high-mutation-rate environment, it will adapt to this environment by becoming more robust."

The origin of sexual reproduction has been a mystery in part because of an effect known as "mutation accumulation." This means organisms tend to adapt in ways that reduce the effects of mutations, thus making the organisms less vulnerable.

But this kind of robustness is actually deleterious, the researchers contend, because harmful mutations would accumulate in the organism through sexual recombination, leading to a gradual loss of genes. This handicap of sexual creatures would be enough to guarantee their extinction when competing against asexual ones.

This could be avoided if mutations were unable to accumulate. Wilke and Adami propose that through a "conservation law," an organism may be able to withstand the harmful effects of a few mutations but incapable of surviving a large number of mutations.

As organisms become increasingly resistant to the effects of single mutations, they are unable to tolerate multiple mutations. This removes organisms with multiple harmful mutations from the population; sexual recombination allows the organisms to reap the rewards from sharing beneficial mutations.

Stressful environments that cause high mutation rates make organisms more robust—or less vulnerable—to the harmful effects of single mutations. The researchers say their proposed conservation law ensures that this evolutionary pressure puts asexual organisms on to the road toward sexual recombination.

The published article is available online at: www.pubs.royalsoc.ac.uk/proc_bio/proc_bio.html
 

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