Surprising genetic mish-mash for Saccharomyces species
The yeast, which normally reproduce asexually, had partnered multiple times for hundreds, possibly thousands, of years, with interesting results
There are indications that sharing of genetic information is precisely what gives that hint of passion fruit to your favorite glass of Chardonnay, or that round, plummy flavor to your most-recent quaff of Zinfandel.
"The strain of yeast used in fermentation definitely affects the flavor of the wine," said Gavin Sherlock, PhD, an associate professor of genetics who personally taste-tested several vintages specially produced by his Gallo collaborators as part of the research, "and we thought that these strains would fall into a nice evolutionary-tree-like structure with a common ancestor."
Instead they found chaos. The yeast, which normally reproduce asexually, had partnered multiple times not just with one other -- mingling strains -- but even with members of different species of yeast.
"We were surprised to see how much mating has occurred," said senior research associate Barbara Dunn, PhD. "It makes it much harder to follow the trail backward through history."
It's not clear whether the crossbreeding the researchers observed was deliberately fostered by ambitious winemakers or bakers, or if it happened by chance.
Sherlock is the senior author of the research, published online Feb. 27 in Genome Research. Dunn is the first author of the study. The work was done in collaboration with, and funded by, the E. & J. Gallo Winery. A special wine-production line was used for the study to be sure that all other variables, such as grape type and temperature, were eliminated.
The researchers used a technique called array-comparative genomic hybridization to analyze the genomes of 83 strains of yeast. The technique identifies variations in copy number among different yeast samples and can be used to deduce evolutionary relationships as an alternative to sequencing the entire genome. Sixty-nine of the strains studied by the researchers are used in commercial wine making; the remaining 14 are used either in other industrial applications or occur naturally in the wild.
They found a surprising genetic mish-mash. Several strains harbored bits and pieces of DNA from other cerevisiae strains or even other Saccharomyces species -- proof positive of past co-mingling. And more than one strain sported the same bits of DNA as another, suggesting that a particular gene or genes offered an evolutionary leg up.
"It suggests that there might be something advantageous to winemakers about that DNA," said Sherlock. "One gene is known to reduce haziness in white wines, while another affects the amount of foam produced during fermentation."
The researchers plan to continue their experiments on how yeast strains affect wine flavor and other characteristics. But they also plan to conduct whole-genome sequencing of many of their samples to nail down once and for all who is related to whom, and to figure out exactly what's been going on in those wine barrels.
"Historically, winemakers were just happy to see that fermentation was taking place," said Sherlock. "But it seems that all the sloshing about that occurs during the process may have facilitated some very interesting couplings."
Barbara Dunn, Chandra Richter, Daniel J Kvitek, Tom Pugh, Gavin Sherlock. Analysis of the Saccharomyces cerevisiae pan-genome reveals a pool of copy number variants distributed in diverse yeast strains from differing industrial environments. Genome Research, 2012; DOI: 10.1101/gr.130310.111