Keeping yeast mutation in check

My SWMBO, who is a PhD student of Biology at Northwestern in a yeast lab, told me how they keep their yeast from mutating too much. She says that the first trick is to re-plate the yeast often (their lab does it every day), because when the yeast deplete most of the material they normally feed on, they will start suppressing certain genes in order to keep going. Second, she says that when they propagate yeast from a plate, they choose a single well-formed colony. Those that look funky or are "clumped" are not propagated. She also said that I could probably keep some yeast frozen with glycerin in the freezer (there are tutorials around here for that) and grab some of the frozen yeast from time to time (without thawing) and put it on an agar plate to check for viability and to select a good-looking colony to propagate into a starter. I think this would keep mutation to a minimum. Anyway, just passing on the info.

Sooooooo, basically what you are saying is that you have unlimited access to non-mutated yeast and propogation supplies.

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Interesting stuff. Pls encourage her to share anything else she might think helpful to us amateur yeast wranglers.

But, by replating everyday and picking for colonies that aren't clump they're selecting for yeast that are suppressing the genes for flocculation, such as flo1. This is standard practice for yeast labs and is also why the majority of lab strains of yeast don't flocculate nearly as well as the yeast strains used by brewers. There was a good article in the journal "Cell" last month about it for any super science nerds (like myself) interested.

FLO1 is a variable green beard gene that drives bi...[Cell. 2008] - PubMed Result

kubilusaurus said:

But, by replating everyday and picking for colonies that aren't clump they're selecting for yeast that are suppressing the genes for flocculation, such as flo1. This is standard practice for yeast labs and is also why the majority of lab strains of yeast don't flocculate nearly as well as the yeast strains used by brewers. There was a good article in the journal "Cell" last month about it for any super science nerds (like myself) interested.

FLO1 is a variable green beard gene that drives bi...[Cell. 2008] - PubMed Result

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I think you just misunderstood what I meant by "clumpy." I only meant misshapen, and she suggested that this meant that there was some infection present in the colony. The article you pointed out says that lab yeast were specifically bred out of feral strains and selected to be non-flocculent. It is not due just to the method of plating/replating (although non-flocculent yeast reproduce much more quickly than flocculent yeast). Also, the article says that environmental factors that select for the FLO genes are mostly chemical (since flocs of yeast protect the inner cells from coming into contact with the chemicals). Ethanol stress selects them 2 to 1, and peroxide/amphotericin B selects them 100 to 1. Unless you are adding ethanol or peroxide/amphotericin B to your growth medium or you replate before the medium is saturated, you are not selecting either for or against FLO expression (meaning the flocculation rates will remain stable).

On an interesting note, it looks like there are 2 very easy was to manipulate flocculation in a particular strain. You could subject the strain to 10% ethanol (the article doesn't say for how long) which would select for flocculation, or you could subject your culture to 50C for 1 hour to select for non-flocculation (check out figure B on page 733). I would like to try this at home when I get a good yeast bank going.

good info....one way to preserve yeast strains that might be more practical for homebrewers (avoiding replating every day) is too freeze down multiple glycerin stocks of the yeast strain as soon as you get it. That way you can go back to a fresh, non-mutated batch if somwthing goes wrong.

Ahh, now I get it. I guess it makes sense that yeasties on agar with lots of resources and minimal stress aren't really that concerned with flocculating. Would/could they even flocculate on agar?

kubilusaurus said:

Ahh, now I get it. I guess it makes sense that yeasties on agar with lots of resources and minimal stress aren't really that concerned with flocculating. Would/could they even flocculate on agar?

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Absolutely they could. "Flocculate" really means "to aggregate". The reason flocculant yeast fall out of solution is because they aggregate together (because of the FLO surface proteins). Although, I'm not sure how you would be able to tell (without a microscope) whether agar-grown yeast were showing flocculation or not.