Speeding Up Yeast's Evolution And What It Says About Cancer
DAVID GREENE, HOST:
We typically think of evolution as a process that occurs over millions, or even hundreds of millions, of years. But some evolutionary changes happen much faster than that. Scientists in California are seeing that as they study a rapidly reproducing organism. NPR's science correspondent Joe Palca says their work could have implications for human cancer.
JOE PALCA, BYLINE: When Gavin Sherlock was growing up in London, his parents used to take him to the Natural History Museum. Young Sherlock loved to look at displays of creatures that had gone extinct, like dinosaurs, and the organisms that had come along and replaced them
GAVIN SHERLOCK: The idea that organisms can change over time had fascinated me, so I decided I really wanted to study evolution.
PALCA: So that's what he did. When he got to graduate school, he decided to pick a single organism to study that would reveal the workings of evolution. He could have picked a dinosaur, a cuddly mammal, a bird or even a fish - but no.
SHERLOCK: To be honest, I fell in love with the organism yeast.
PALCA: Sherlock says there are a lot of good reasons to study yeast. They're simple critters, easy to grow in the lab, a lot is already known about their DNA and you don't have to wait years or weeks or even days for them to reproduce.
SHERLOCK: Yeast has a very short generation time, you know, in the lab - maybe 90 minutes to a couple of hours.
PALCA: So you can study evolution in yeast in a matter of months, not centuries. Now, for organisms to evolve, there have to be changes in their DNA over time. These changes are called mutations. Some mutations are bad and can be lethal to the organism. Others can be good and provide a survival advantage. Sherlock wanted to track those good changes in successive generations of yeast, generations sort of like a family over time. And he wanted to track a lot of different families so he could find out whether good mutations were common or rare.
SHERLOCK: What we did was we came up with a way, essentially, to tag half a million different yeast families, or lineages.
PALCA: You can think of these tags as family surnames. So one yeast family could be the Palcas, and another, the Sherlocks, the Smiths, the Jones - well, you get the idea - a half a million different surnames.
SHERLOCK: We then take our population of yeast carrying these kind of family markers, propagate them in the lab for a couple of hundred generations.
PALCA: Now remember, good mutations makes one family survive better than another.
SHERLOCK: And then we look to see which ones increase in frequency greater than we would expect by chance.
PALCA: You look to see if there are more Palcas than Sherlocks or more Joneses than Smiths.
PALCA: As they reported in the journal Nature, Sherlock and his colleagues found the yeast were producing a surprisingly large number of favorable mutations. Now, this is interesting news for people who are passionate about yeast. But there are implications here even for people who, shall we say, are agnostic about yeast because what Sherlock has found out about yeast can also be used to understand cancer. Cancer cells also reproduce over time, and they, too, can have good mutations. But a good mutation for a cancer cell is bad for us because that means it's harder to kill a cancer tumor.
SHERLOCK: Which is often why, you know, after somebody has a round of treatment, they go into remission, but then the tumor comes back. When it comes back, it's then resistant to the drugs that they were being given previously.
PALCA: Knowing more about the rate of mutation could help doctors design better treatment strategies, like sometimes using a combination of drugs, making it harder for cancer cells to mutate a way around the treatment.
So that's the end of the yeast story, but I've got to keep going because there's some amazing history behind how Sherlock figured out how to study his yeast families. OK, here's the story. Back in 1875, a scientist named Francis Galton and a colleague published a paper titled "On The Probability Of The Extinction Of Families," where they presented some mathematical methods for determining why certain family names were disappearing.
SHERLOCK: And it's exactly the mathematical approach that they took that we were able to use to underpin the analyses that we carried out.
PALCA: And it turns out Francis Galton's grandfather was Erasmus Darwin, who was also the grandfather of Charles Darwin. Remember him? He's the guy that proposed the theory of evolution by natural selection - small world. Joe Palca, NPR News.