Pure yeast strains? (fwd)

Robert Preston rapr at MED.PITT.EDU
Mon May 17 22:04:45 EST 1993

re: storage and retrieval of yeast, rich buckholz wrote:
> Since we are all aware that every generation introduces some small number of
> mutations into every cell, that must mean there is no such thing as isogenic
> strains, or even isogenic colonies on a plate.  Thus, when one goes to store a
> new strain (say, one that has recently arrived via slant from a generous fellow
> yeast person), what is the careful researcher to do?  Should I streak for
> singles and store one, as I learned in Micro Lab?  Or should I streak for
> singles to check for contamination, then store a gob or mixture of singles?
> Then, when reviving a strain to do experiments, or starting from a plate in the
> fridge, do I pick a single colony, knowing full well that it is slightly
> different from the one I chose for last weeks experiment?  Or do I grab a gob,
> and live with the knowledge that it may be contaminated?

I say, freeze down both from the head of the isolation streak-plate, and
also from a single colony, for strains received from other labs (and, by
all means, check the markers to hedge against strain mix-ups).  The
descendants of the single colony would then provide the most reproducible
cultures subsequently.  Frozen stocks can be sampled without thawing by
scraping some ice-cell mix from the freezer vial for restreaking.  Unless
a mutation of interest causes poor growth, the random changes in genetic
background that occur during limited numbers of generations after the
freezer are highly unlikely to be relevant to any experiment.  In any
case, reasonably careful workers assay the phenotype of interest in back-
cross segregants (the number to be analyzed being a compromise that
depends on the difficulty of the analysis) to test whether or not the
genetic background variations are relevant to the phenotype.  In most
cases, the variations have no detectable effect from one segregant to
the next, meaning "this weeks" colony is practically identical to "last
weeks".  But, hey, check it out by backcross if you want to be sure.

Rob Preston
UPMC Dept. Pathology
rapr at med.pitt.edu

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