Moselio Schaechter

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April 01, 2010

Talmudic Question #60

Are there more different viral "species" or different plasmid genomes on Earth?


My bet is different plasmid genomes. Once I worked with collection of clinical Enterobacteriaceae, all independent isolates (~100) contained plasmids, ranging 3-10.

Merry comments: I'd like to say you won your bet, but who knows?

I think we have to go with viruses, even while admitting that we aren't at all sure. This is simply because there are so many more virions (probably) than anything else on earth. Even if every bacteria contained a unique plasmid, would there be as many? I think when you add in the potentially diversifying replication of RNA phage compared to DNA phage/plasmids, and the amount of lysogeny out there, I think its probably no contest. So while it's probably hideously difficult to figure out how many of each there are, I think we can be pretty confident that unique phage outnumber unique plasmids. Now if you add in transposable elements...hmm, based on Elio's (?) post about transposases from a few days ago, that might be another interesting question.

Merry comments: Yes, more interesting questions. Either way, the amount of information in circulation "out there" is stupendous.

I agree with Merry - in fact, I think it's a trick question! The hazy nature of viral species and the patchwork composition of plasmids would encourage considerable gerrymandering on both sides. And don't some of them get counted twice, once in each column?

And Merry agrees with Welkin. :-)
Perhaps we should institute a decadal phage and plasmid census, going door-to-door, and attempt to avoid residents being counted twice. And perhaps then one could also compare the numbers of different genes on both sides.

I have no idea. I can't even figure out how many different phages and plasmids are known to inhabit E. coli. Which is a bit sad, since I used to work on plasmid replication in coli. Even back then, I don't remember ever seeing a "complete" list of all known coli plasmids or phages.

(We're not counting plasmids or phages engineered in labs, right?)

Merry responds: Right! But mind-boggling nonetheless.

Is this a question about packaging?

Merry replies: I don't think so. I think Elio meant different genomes, but that leads to the question of how different two genomes have to be in order to be counted as two types rather than one.

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