Small Things Considered

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?

Talmudic Question #83

Assume the Earth is hit by a large asteroid that lowers the average temperature of the planet to the extent that would eliminate most multicellular life. What would be the short term consequences for the microbial world? And later on?

Talmudic Question #82

Are there any surface structures or components on bacteria that are NOT used as receptors by some phage?

A Gathering of Comments on a Talmudic Question

by Elio

First it may be appropriate to repeat what we mean by Talmudic Questions (and therefore Talmudic Answers). We use the term loosely to denote questions that may lack definitive answers but that open the door to ingenuity and dialectic. These questions embody the musings: “I have often wondered about…” We are fond of saying that answers to such questions are not likely to be found by a Google search.

Our Talmudic Question #81 (Given that most mutations are deleterious, why does the mutation rate not evolve to zero?) elicited especially lucid comments. Here we summarize a few only (and apologize to those whose comments are not mentioned).

Jan Spitzer points out that the human mutation rate is very slow indeed. But, he notes: something else is afoot: very fast growth and mutations of ‘memes.’

Daniel Smith posits that most mutations are in fact not deleterious. He says: I think the mutations we notice are certainly more likely to be deleterious. But what's science without a little sample bias?

John Wilkins cleverly points out that: Any mechanism that might keep the mutation rate low would be subjected to deleterious mutations...

Ramy Aziz tells us: Problem is: it cannot evolve to zero unless it is not zero. If mutation rate slows down to near zero, evolution will also slow down, and the probability of a ‘beneficial’ mutation of ‘no mutation’ will be astronomically low.

MB thinks that: A mutation rate of zero can be good for an individual organism, but deleterious to a population because of selective pressures. Hence good for population, bad for individual, and evolution/selection works on populations.

Liz says something similar: Zero mutation rate means no variation and thus the halt of evolution through the loss of environmental adaptability. This decreases the fitness of an organism that achieves this state versus one that allows some mutations, even if most of these would be deleterious.

Ami Bachar goes further: In earth or earth-like environments, where evolution is the base for all life, there cannot be such a thing as evolution to zero mutation rate.

Thinking about possible mechanisms, Mike Jones writes: Is there a level of noise that is unavoidable during replication?

And, similarly, Nick Matzke offers this: Because error-checking and correcting the DNA takes energy, organisms have a limited amount of energy, and thus at some point the cost of error-checking outweighs the benefits.

Mark Martin suggests that we pay attention to viruses: It also reminds me of the ‘no proofreading’ problem with RNA polymerases. RNA viruses using that enzyme for replication are extremely prone to mutation, and make many defective progeny (do we have a number for that?). Yet the ‘lottery winners’ have their shot, as well.

And lastly, Nathan Myers muses: So, here we are, bobbing about in a seething cauldron of mutating viruses with our exons and memes and vaccines and recombinant gene therapy. The Joneses, next cauldron over, have all that and mutations besides. Eventually the viruses cough up a mutation our civilized library of adaptability cannot best. Soon only Joneses bob about in our cauldron.

Talmudic Question #81

Given that most mutations are deleterious, why does the mutation rate not evolve to zero?

Talmudic Question #80

Within the microbial world, is the degree of pleiotropy displayed by an organism inversely related to its genome size?

Talmudic Question #79

What's your guess as to the number of different kinds of viruses that can infect a single species on average?

Talmudic Question #78

Why are there so many species of microbes on earth?

Talmudic Question #77

Both marine phages and predatory protists prey on bacteria. Something has been written about how phages may help bacteria escape protist predation. Click here for an erudite treatment. Is something known about the converse, namely communication between protists and phages? For instance, do protists make compounds that diminish predation by phages, e.g., by making phage-inhibitory substances?

Talmudic Question #76

Do you think that our species is exempt from "kill-the-winner" predator-prey dynamics? (KTW is the colloquial term for the cyclical dynamic by which phages promote community evenness by reducing the numbers of the most abundant bacterial strains.)