September 24, 2012
Anyone reading this blog probably knows how important the yeast S. cerevisiae is. It makes our bread better, our beer and wine more spirited, and our genetics more understandable.
Because it is such an important beast, this yeast is also incredibly well characterized. It was the first non-bacterial organism whose genome was sequenced and is a key model organism for teasing apart how eukaryotes like us work. We may know more about the molecular biology and genetics of S. cerevisiae than about any other organism on the planet.
And yet we know surprisingly little about S. cerevisiae in the wild. We know that it isn’t on unripe fruits but suddenly appears once they ripen. We also know it doesn’t tolerate winter particularly well. So where does yeast hang out when there isn’t ripe fruit around and/or it gets chilly? A group of researchers in Italy thinks a key place is inside a hibernating wasp.
When Stefanini and coworkers looked, they found lots of yeast (including S. cerevisiae) in wasp intestines. They were also able to show that the S. cerevisiae remained viable in a hibernating queen over the winter and that that the queen transferred the yeast to new wasps in the spring by regurgitation. With this one study, these scientists managed to find at least one way that yeast can survive the winter and get to ripe fruit.
To figure this out, Stefanini and coworkers did experiments both in the field and in the lab. They first collected wasps and bees from around the Italian countryside and showed that wasps, but not bees, harbored yeast in their gut. In all they found 393 yeast strains in the 61 wasps they dissected, 17 of which turned out to be S. cerevisiae. By sequencing and comparing the genes URN1, EXO5, and IRC8, they were able to conclude that these yeast were related to wine, beer, bread, and laboratory strains of S. cerevisiae.
The researchers figured out that the yeast could survive for three months and be passed on to the next generation of wasps with a couple of controlled experiments they did in the lab. They fed queens GFP labeled yeast and then let them hibernate. After three months they dissected some of them and found lots of viable yeast in their intestines.
The rest of the queens were allowed to wake up and find new nests. Larvae were removed from the nests and were found to contain GFP yeast as well. The yeast not only lived through the winter but passed on to the next generations!
Of course this doesn’t mean that this is the only way that it can happen. But it is the first time anyone has managed to get such a detailed look at feral yeast. And this kind of work is important if we want to use S. cerevisiae as a way to study evolution.
To understand its evolution, we have to understand the natural forces that shaped S. cerevisiae into the organism it now is. Only then can we piece together why S. cerevisiae has evolved the way that it has and so learn fundamental lessons about the mechanisms of evolution.
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight