Monday, December 3, 2007

Extreme Gene Transfer and Speciation, Part 2

A basic riddle of biology is how members of the same prokaryotic species can be found in so many far-removed places. For example, sulfate-reducing members of the genus Desulfotomaculum have been found in South African gold mines as well as deep basalt aquifers of Washington State. (See Baker et al., below.) On a bacterial scale, Washington State is about as far from South Africa as Earth is from Mars for you or me. Considering that the bacteria in question are bound in rock thousands of feet underground, it seems implausible that the Washington State bacteria somehow propagated to their current location from ancestors living in South Africa (or vice versa).

What are the possible explanations, then?

The easiest is creationism: A Higher Force created these organisms in situ, just as they are, when the Earth itself was created.

Another is panspermia: Some natural force (as yet unknown) caused all of Earth's microhabitats to be seeded with the same types of organisms, at the same time.

A third possibility is genetic convergence: All of the bacterial species we see today evolved independently, in separate locations, in parallel manner, starting from some unknown number of (possibly common) ancestors.

I say possibly common ancestors because yet another possibility exists, which is that given a sufficiently complex local ecosystem, a new member of the ecosystem can emerge on its own through mixing and matching of "borrowed genes" from existing species. Here's the thought-experiment: Imagine that we have a soil sample, and imagine that through some combination of suitable experimental techniques (remember, this is just a thought experiment) we can enumerate all of the different microbial species present in the soil sample. Homogenize the soil sample and divide it in two. Suppose there are 357 prokaryotic species in the sample, and 10 of them are Bacillus species. Now suppose you can completely eradicate all 10 Bacillus species from one of the two samples. (Pretty hard to do, but again, this is a thought experiment.)

Add water and nutrients to each soil sample (separately so as not to cross-contaminate them) on a daily basis. Prediction: After a sufficient period of time, one or more Bacillus species reappears in the soil that previously had none.

A bacteriologist will complain that this is not a terribly strict experiment, because even if a Bacillus cell were to evolve "out of nothing," it probably actually would come about through modification of a preexisting Clostridium species in the soil. (Clostridia are close relatives of Bacillus.)

Fair enough. Repeat the experiment with Pseudomonas instead of Bacillus.

The point is, if the environment favors the existence of Bacillus, the experiment will eventually find Bacillus emerging "from nothing." Or at least that's the hypothesis. A new organism, from borrowed genes.

Sounds a bit fanciful, doesn't it?

It does, until you start to read about things like an entire bacterial genome having been found within the genome of a fruit fly (Dunning-Hotopp et al., cited below.)

(to be continued)


1. Brett J. Baker, Duane P. Moser, Barbara J. MacGregor, Susan Fishbain, Michael Wagner, Norman K. Fry, Brad Jackson, Nico Speolstra, Steffen Loos, Ken Takai, Barbara Sherwood Lollar, Jim Fredrickson, David Balkwill, Tullis C. Onstott, Charles F. Wimpee, David A. Stahl (2003): "Related assemblages of sulphate-reducing bacteria associated with ultradeep gold mines of South Africa and deep basalt aquifers of Washington State," Environmental Microbiology 5 (4), 267–277. doi:10.1046/j.1462-2920.2003.00408.x

2. Dunning-Hotopp, Clark, Oliveira, Foster, Fischer, Torres, Giebel, Kumar, Ishmael, Wang, Ingram, Nene, Shepard, Tomkins, Richards, Spiro, Ghedin, Slatko, Tettelin & Werren. Widespread lateral gene transfer from intracellular bacteria to multicellular eukaryotes. Science doi:10.1126/science.1142490

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