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Gene reorder may drive evolution

By LIDIA WASOWICZ, UPI Senior Science Writer

Researchers from three British centers have put the natural course of events in reverse to reveal how a rearrangement of genetic matter could help drive evolution.

Working with baker's yeast, investigators from the University of Manchester, the University of Leicester and the Institute of Food Research in Norwich, England, found swapping large chunks of genetic material might spur the split of one species into two.

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By reshuffling similar chromosome regions so they align in two closely related but genetically diverse groups, the scientists were able to produce a hybrid life form capable of successful reproduction -- the hallmark of a new species. Chromosomes, threadlike structures in the cell center composed of a double strand of twisted DNA, or deoxyribonucleic acid, contain the archive of an organism's hereditary traits.

The findings -- that not only the content but also the layout of the draft of life can affect how different species may arise -- cast a shadow of doubt over how individual classes of organisms are currently defined, biologists said.

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In addition to revealing a possible evolutionary pathway, they told United Press International, the results carry implications for a number of endeavors, from genetic engineering of agricultural crops to understanding and managing some inherited diseases.

As reported in the March 6 issue of the British journal Nature, the team focused on baker's yeast, termed Saccharomyces cerevisiae by scientists, and its close relative, S. mikatae. Large regions of like chromosomes sit in different positions in the two species -- a variation Stephen Oliver, professor of genomics in the School of Biological Sciences at Manchester, and his team set out to undo.

With some repositioning of their genetic material, the two species took on such a similarity they were able to interbreed -- although with less than 100 percent success. The observations suggest chromosomal rearrangement, or translocation, might play an important role in the evolution of new species, geneticists said.

In contrast to traditional strategies that seek theoretical explanations of geographical or ecological influences on the birth of a species, the British sleuths "took a hands-on approach," said Ken Wolfe, professor of genetics at the University of Dublin in Ireland, who analyzed the findings in an accompanying commentary.

They cut to the most basic biological act: reproduction.

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Although the six species within the Saccharomyces family can mate, the offspring produced by such unions usually are as infertile as a mule, the misbegotten progeny of horse and donkey.

"All members of the Saccharomyces (family) can mate with one another and make viable -- even vigorous -- hybrids," Oliver told UPI. "However, these hybrids are sterile; they produce inviable spores."

Suspecting the cause relates to variations in the layout of the chromosomes, the investigators made some readjustments in baker's yeast to duplicate the S. mikatae design before pairing the two species. The match made in the laboratory produced healthy hybrids up to 30 times more likely to be fertile than the offspring of parents not molded in each other's genetic likeness.

"Some hybrid lines had levels of fertility as high as 20 percent or 30 percent, compared to less than 2 percent before their chromosomes were realigned," Oliver said.

The finding points to a pivotal role of chromosome arrangements in the splitting of one species into two, scientists said. At some point in the evolutionary gamble, nature shook the Saccharomyces family tree, creating six different species with a toss of the chromosomes, they noted.

"Our engineered baker's yeast calls into question how we define species," said Oliver, who led the study. "We have shown that chromosome arrangement plays a key role in differentiating between species, so by simply changing the arrangement of the chromosomes are we effectively creating a new species?"

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The answer to that question could have wide-ranging ramifications, from agriculture to medicine.

"One might deliberately introduce translocations into the genomes of genetically engineered crop plants to prevent them outbreeding with related plants in the environment," Oliver speculated. "This would be particularly important for brassicas (pasture and forage crops), such as rape and canola." In disorders spawned of chromosomal abnormalities, he added, the result "suggests a route to mimicking disease-associated translocations from humans in model organisms, e.g. mice."

The notion that reproductive barriers between species are not absolute should lead to some rethinking of certain key modern-day concepts, scientists said.

"This has implications for our thinking on genetically modified organisms," Oliver said.

"We are not just the sum of our genes; how the genes are organized is important as well," he added. "This has implications for our view of ourselves in relation to closely related animal species, e.g. chimps. It also means that it is important that scientists obtain complete genome sequences, not just collections of sequenced genes."

Likening research in evolution to forensic detective work on a time scale that spans millions of years, Wolfe pointed out the importance of the new work in offering clues to a puzzle that has mystified biologists since 19th century English naturalist Charles Darwin proposed his daring view of the origin of species.

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"Biological species are defined by their inability to mate successfully with other species. Usually, we can only speculate about how the barriers to successful mating might have arisen," Wolfe said. "But (the authors) describe how they took a hands-on approach, engineering genomes in an attempt to reverse speciation and turn two yeast species into one."

Their studies showed chromosome rearrangements had a significant effect on fertility, Oliver said.

"It is a very simple, elegant experiment that at the moment is only possible to do in yeast, because the genetic engineering technology for yeast is so well developed," Wolfe told UPI. "It's the first time anyone has been able to directly measure the contribution of chromosome rearrangements to a species barrier. We're all interested in what defines a species, and this study goes some way towards pinning that down."

In additional experiments, the researchers found the progeny of the hybrids often had extra copies of many chromosomes. Scientists view such genetic redundancy as critical to the evolution of new species, they pointed out.

"How species arise has been a subject that has fascinated biologists since Darwin's time," Oliver noted.

"Necessarily, the studies of evolutionary biologists are retrospective in nature. By examining the relationships between present-day species, they hope to infer how they arose over geological time," he said.

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"We have been able to take a more interventionist approach," he added, "and engineer the chromosomes of the humble baker's yeast in order to dissect out different contributors to the evolutionary process."

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