February 03, 2016
Back before trains, planes and automobiles, people didn’t get around as much. And for the people of medieval Europe, this could be a real problem genetically.
At this time there were a lot of small, isolated villages scattered across Europe. If people in these villages stayed put, inbreeding might have gotten as bad as the poor Spanish Hapsburgs. Their last king, Charles II, was infertile, riddled with genetic diseases and his royal line died out with him.
One reason (among many) that this didn’t happen to people all over Europe was market towns. These were centrally located places where villagers came to sell goods. And where they also found partners to bring home to freshen up the gene pool.
Turns out that out in the wild, our friend yeast is in an even worse predicament than medieval Europeans. Because they are all clones of each other, they exist in isolated colonies with almost no genetic diversity.
Yeast are also way less mobile than people. They do have spores but these don’t tend to travel very far without help.
And yet, looking at yeast DNA shows that yeast definitely get around. There are all sorts of signs of various DNA mixing over time. So where are all these yeast hooking up?
This makes sense in a lot of ways. First off, as we described in an earlier blog, there is good evidence that yeast winter in wasp guts.
So there are definitely a variety of yeast hanging around for months, waiting for warmer weather. The gut is also the kind of harsh place where spore dissolution, the first step in yeast mating, can happen.
When the authors looked at the yeast isolates from a wasp’s gut they saw a lot more outbreeding compared to other sources. This suggests that a lot of mating is indeed going on there.
The next step was to directly test how much mating can actually happen in a wasp gut. Stefanini and coworkers tested this by having the wasps eat five different yeast strains and then analyzing the isolates genetically over time. They compared the results from this experiment to the amount of mating that happens in wine must and under ideal lab conditions.
What they found was a whole lot of mating going on.
After two months, around 1/3 of the yeast in the wasp’s gut were outcrossed. This is OK but pretty comparable to what is found in wine must.
It was a different story after four months. Now 90% of the yeast were outcrossed. This is an even better result than scientists typically get in the lab. Clearly the wasp gut is a great place for a yeast to find a partner.
The authors also found that the S. paradoxus strain had to mate to survive in the gut. The only time they found this strain in yeast isolates was in hybrids with S. cerevisiae.
The next steps will be to see if this kind of mating actually has a big effect on yeast diversity in the wild. And of course what, if anything, the wasp gets out of hosting these cavorting yeast.
A market town was great for both the town and the visitors. People met up, sold goods, found partners and the towns prospered from all of this traffic. I can’t wait to find out if the wasp/yeast situation is so mutually beneficial as well.
Jerry Lee Lewis has a whole lot of shakin’ going on, just like a wasp’s gut has a whole lot of matin’ going on.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight