Moselio Schaechter

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February 09, 2012

Talmudic Question #84

Within a group of well-studied DNA-based microbes (phages, bacteria, and eukaryotes), genome sizes are found to vary by almost four orders of magnitude. Their rates of mutation per base pair also vary by about that much, so the net result is that their mutation rates per genome vary less than 10-fold. Why do you think this is?


probably a little late to add a comment, but just in case ...

CHIMERAS posted about research showing that the nucleosomes of eukaryotes serve to reduce mutation rate. Here's an example of "mechanisms for reducing or repairing mutations" in nuclear DNA that I suggested in my earlier comment.

Perhaps larger genomes are more complex, integrated -- and therefore less tolerant of mutation (I’m ignoring giants formed via polyploidy etc.). Perhaps in larger genomes there are mechanisms for reducing or repairing mutations. CHIMERAS put up a very interesting post last week about “gene migration” from captured organelles to the host genome. Once there, the rate of mutation drops and so these genes are a bit like “molecular fossils” compared with organelle DNA, which has continued to mutate at the higher organelle rate. See post for more details and paper cited.

do we even understand why genome size varies so much?

If I understand the car analogy, then I think agree with Francisco. As a virologist, I tend to think it is mutation rate that dictates the genome size - things with a higher mutation rate cannot have larger genomes, because the odds of suffering a lethal hit are higher (but meanwhile the higher mutation rate ensures diversity). This is tied to evolutionary history: if the smallest genomes came first (the pre-cellular self-replicators and eventually simple polymerases) then perhaps the emergence of error correction systems and "better" polymerases allowed for increases in genome size.

Genome size is always a vexing subject; I believe that lily plants have enormous genome sizes. Kangaroo rats have gigantic centromeric regions. I keep thinking of the genome itself as an "ecological niche," perhaps for Richard Dawkins style "selfish DNA" entities.

But in the final analysis, the basal unit of selection must be the organism itself. Perhaps that helps limit the overall rate of mutation, in terms of an analogue to "genetic load"?

Easier to be a mechanic in an older simpler car than in a more modern complex one. Trial and error on the first one is not able to make it much worst and can make it better; random hacks in a complex engine usually result in an expensive and fatal situation.
Or, more points of failure make it more affordable/needed to have failure avoidance systems.

And if it is that 'mathematical' then failure avoidance systems must be supported by diversity - or have a 2.0 character as it would be fashionable to say some years ago.

PS: To state what's obvious around here: Great job with this blog! It's much appreciated. Thank you!

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