New thoughts on evolution arise from UH yeast study

November 28, 2002

HOUSTON, Nov. 28 - The sex life of yeast has University of Houston biologists fermenting new ideas about evolution and beer.

Researchers studying yeast reproductive habits have for the first time observed a rapid method for the creation of new species, shedding light on the way organisms evolve and suggesting possible ways to improve yeast biotechnology and fermentation processes used in beer and wine-making.

"Most models of speciation require gradual change over a very long period of time, and geographic or ecological isolation for a new species to arise," says University of Houston biologist Michael Travisano. "Our study suggests that mating two separate species to produce hybrids can result in a new species readily and relatively quickly, at least in yeast, but possibly in other organisms as well."

Travisano, an assistant professor in the UH Department of Biology and Biochemistry, says the findings extend the range of known mechanisms that cause reproductive isolation. The study appears in the Nov. 29 issue of the journal Science.

Duncan Greig, a postdoctoral researcher in Travisano's lab, conducted experiments that put two different species of yeast together, Saccharomyces cerevisiae and Saccharomyces paradoxus. One way that yeast, a one-celled organism, can replicate is by producing spores. When spores from these two species joined, they produced hybrid offspring, similar to crossing a female horse with a male donkey and getting a mule.

Unlike mules, which are sterile, a few of the yeast hybrids were fertile. Those hybrids produced viable offspring when they were allowed to "autofertilize," which means an individual's spores fertilized themselves to produce an offspring without involving another yeast cell.

However, the hybrids did not produce viable offspring when mated back to their parent species. "Other labs have generated hybrids such as these before, but we went a step further and crossed the fertile ones back with their parents," Travisano says. While there are various definitions of a species, Travisano says individuals that are fertile with themselves and isolated from their parents certainly qualify as a new species. He estimates the experiment took about a month to generate the new yeast species.

Understanding why some hybrids are fertile and others are not is a key question, according to Greig and Travisano, and may have implications for the evolution of species besides yeast. "What are the genetic or molecular mechanisms that make some hybrids sterile and others fertile and able to propagate as a new species? While our work was done with yeast, presumably the interactions that prevent or encourage speciation occur in other organisms as well," Travisano says.

The method by which the hybrids replicated and formed a new species is called homoploid hybrid speciation, in which the new hybrid species contain the same total amount of genetic material as the parental species. It is not found in any animal species and only very rarely among plants, Travisano says.

"We think it may be happening in nature, but this is the first time this mode of speciation has been observed in a microorganism such as yeast," he says. "In terms of how we typically think of speciation, this method is pretty rare, which makes it kind of a surprise how easy it was to get it to work." This method is in contrast with polyploid hybrid speciation, which occurs readily in plants and involves an increase of two or more times the genetic material in the new hybrid species than in the parental species, Travisano says.

He adds that the yeast's ability to speciate so quickly in the lab is due in part to its ability to autofertilize.

"Autofertilization is thought to be relatively common in wild yeast, but the natural history of yeast is not very well understood," he says.

One application of the research may be to benefit industries that utilize yeast in fermentation.

"If we put these hybrid individuals in various environments, we'd like to see whether they do better in some environments than their parental species," Travisano says. For example, one parent species thrives in cold temperatures and the other parent does well in the heat - what kind of environment might the hybrid prefer?

"Presumably you might be able to optimize wine or beer-making by genetically engineering a yeast species specific to your needs," Travisano says. "If you're interested in yeast biotechnology, studies such as this could tell you something about the nature of your yeast and how to engineer it."

Travisano and Greig's research was funded by the Wellcome Trust and was done in collaboration with Edward J. Louis and Rhona H Borts at the University of Leicester.
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