chromosomal rearrangement, yeast
To Test Evolution, Press the 'Undo' Button
By CAROL KAESUK YOON - New York Times, March 18, 2003 - original
Scientists working with yeast have in effect reversed the process of evolution in the laboratory and, in so doing, found support for a long-debated theory on the way new species can evolve.
Researchers have long known that changes in the DNA sequences of genes can cause a population to evolve into a new and separate species. But decades ago, theorists also proposed that a new species could evolve without any such changes, but instead simply as a result of large DNA strands' moving from one chromosome to another within a genome, a change known as a chromosomal rearrangement.
While the theory sounded promising, since such rearrangements can be quite common, it eventually waned in popularity, in part because scientists had no way of testing it.
Now in a slick feat of molecular maneuvering, a team of researchers has reorganized huge portions of one yeast species' chromosomes, rendering its chromosomal map identical to that of a closely related species, just as it was once, in the distant past.
What they have found is that when chromosomal rearrangements wrought by evolution are undone, once distinct species — so defined because they could not interbreed — can suddenly produce fertile offspring.
Evolutionary biologists say the study shows, at last, how important rearrangements can be in the evolution of new species.
"People were always saying, `Wouldn't it be great if we could undo this and see what happens,' and here they did it and saw an effect," said Dr. Mohamed Noor, an evolutionary biologist at Louisiana State University.
Dr. Michael Travisano, an evolutionary biologist at the University of Houston, said: "It's really amazing stuff. I don't know anyone who's done wholesale genomic rearrangement," at least not with anything more complex than a bacterium.
The findings were reported March 6 in the journal Nature.
The researchers did their tinkering with Saccharomyces cerevisiae, the species most people know as baker's yeast. Though yeasts are one-celled creatures, they are part of a vast group known as the eukaryotes, one of the three so-called domains into which all living organisms are divided. The eukaryotes — which also include all animals, plants and fungi — all possess compartments, like the nuclei, inside their cells and share similarities in their genetics.
Because of that affinity, researchers predict that chromosomal rearrangements are likely to be important in the evolution of other organisms as well.
Dr. Stephen G. Oliver, who is a molecular biologist at the University of Manchester and an author of the new study, said the discovery that researchers could so precisely reorganize yeast chromosomes was entirely accidental.
The researchers were trying to remove a number of specific yeast genes from a number of different chromosomes by excising small chunks of DNA. In the process, they cut in two each linear chromosome that contained a gene. But when researchers let the pieces of chromosome patch themselves back together, they did not always reattach to the correct partner. Sometimes they attached elsewhere, creating recombinations, just as evolution does.
"We realized, wow, this is not an irritation," Dr. Oliver said. "This is a tremendous opportunity."
But when the researchers undid rearrangements, not all species pairs regained the ability to interbreed. And even with those that did, their hybrid offspring were still impaired, unable to produce a full complement of healthy offspring.
Scientists said this was probably because the species also had important differences in DNA sequences of specific genes, differences that would not have been affected by the large-scale rearrangement of those genes on chromosomes.
But with all of this lab work, another question has arisen: how are these species evolving in the wild? And that raises an even more difficult question: where are the wild yeast species found?
Yeasts can sometimes be found living on grapes and figs (often not terribly wild species themselves) and in the sap of oak trees, Dr. Oliver said. But, researchers noted, with yeasts it is sometimes hard to know how wild one really is.
Dr. Travisano said: "They may be closer to being like feral cats, since there are domesticated yeasts in all these vineyards and breweries and in baking. We really don't have a clue about what's happening with them in the wild."
Researchers also pointed out that the interest in chromosomal rearrangements in yeasts is not limited to the laboratory. These are issues of concern to everyone, at least everyone who likes beer.
In brewing, Dr. Oliver said, disaster can strike, with all the yeast in a vat suddenly calling it quits and dropping uselessly to the bottom of the fermenter. Researchers are learning, he said, that such fermentation failures appear to be due to evolution in the vat — the arising of yeasts with new chromosomal reconfigurations that make them fall down on the job.
But while scientists can now explain the glitch, brewers have not needed such wizardry to fix it.
"The solution isn't too dramatic," Dr. Oliver said. The brewers simply throw out the uncooperative mutants, go back and get more yeast and "start all over again."