New Thoughts On Evolution Arise From UH Yeast Study

<editor's note>
From the University of Houston web site, 
http://www.uh.edu/admin/media/nr/2002/112002/yeast11272002.html
</editor's note>

Novel Method of Creating New Species Observed in Laboratory Yeast 

HOUSTON, Dec. 2- 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's 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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