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Old 03-15-2013, 03:22 AM   #1
Delaney
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Default Yeast evolution, mitosis, and meiosis???

I'm trying to figure out how yeast, and perhaps other single celled organisms evolve. Meiosis involves the production of at least 4 gametes per parent, which are cells themselves. It therefore seems impossible for single celled organisms to reproduce sexually.

If sexual reproduction is indeed impossible in single celled organisms, how do they evolve?

If sexual reproduction is indeed impossible for yeast, would this imply that "generations" of a yeast culture in a given fermentation are infact simply clones of the parent strain, produced via mitosis? If so, why do recycled yeast cultures yield different flavors from one batch to the next?

Thank you for the insight.

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Old 03-16-2013, 04:05 AM   #2
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Even with mitosis there can be evolutionary change. Maybe even more so than in sexually reproducing organisms. Being single-celled, and thus in possession of only a single copy of DNA, any mutations that occur to the parent cell's DNA will be passed on to the daughter cells. Whereas, in a sexually reproducing organism, only mutations in the germ cell DNA will be passed on to the next generation. Additionally, when preparing for cell division, the parent cell's DNA is replicated, which is not a perfect process. If the DNA is not copied 100% accurately, one of the daughter cells will have different DNA. How this different DNA impacts the yeast metabolism will depend on which genes were affected.

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Old 03-16-2013, 04:22 AM   #3
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Quote:
Originally Posted by Delaney View Post
If sexual reproduction is indeed impossible for yeast....
It isn't but I'm not qualified to say much more than that.
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Old 03-16-2013, 06:47 PM   #4
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Sexual reproduction, aka meiosis, is possible for yeast. In fact, there are hundreds of labs that force yeast into sexual reproduction every day. The difference is that they are using special environments (high-stress) not found in wort. The yeast "generations" we use are indeed clones of the original culture, and the difference in "flavour" after several generations is due to the overgrowth of one variant in the yeast population of the original starter culture. In any culture, there are always slight differences between the yeast cells, and after time one will accumulate enough differences to outcompete the other variants. These will be slight differences, but noticable if you pay enough attention.

Wikipedia on Mating of Yeast, note difference between a-MAT strains, and alpha-MAT strains.

This mitotic process is practically the same as in bacteria, which do not undergo meiosis, and have used mitosis exclusively to evolve into the numerous species found today, which have evolved into organisms that can tolerate all kinds of environments. The Lanski Lab has been running a famous experiment in bacterial evolution for the last 25 years.

http://en.wikipedia.org/wiki/E._coli...ion_experiment

If you are looking for more information, I would suggest a college level biology book (Campbell and Reece is very good) or something like Carl Zimmer's Evolution: Making Sense of Life.

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