Tuesday, December 4, 2007

Extreme Gene Transfer and Speciation, Part 3

The phylogeography of prokaryotes (indeed of microbial forms in general) has received scant study. Relatively little is known about the forces that shape microbial biogeography. Nevertheless, we do know that at the family and genus level, certain prokaryotic "regulars" are very widely distributed (geographically), despite obstacles to physical transport (and obstacles to survival during transport). For example, we can find methanogenic anaerobes belonging to the same family in anoxic lake sediments on different continents. Given the inaccessible habitats of these organisms (i.e., deep lake sediments), the fragility of the organisms with respect to exposure to air, and the unlikelihood of an organism the size of a methane bacterium migrating thousands of kilometers on its own, it's hard to explain the ubiquity of certain signature species of microorganisms around the world. Finding the same families of bacteria in the deep sediments of a lake in China, and a similar lake in North America, is tantamount to finding turtles on Mars.

The temporal dimension of the problem is just as baffling in its own way. Many landlocked microbial habitats ("disjunct refugia") have supported microbial populations for thousands, even millions of years. That's astronomical numbers of generations. Applying the concept of dog-years, we can imagine that a bacterial-year is on the order of a few human-minutes. To put it another way: in bacterial time, a month is eons. The potential for genetic drift is enormous.

And yet we find the same signature families of microorganisms over and over again, despite the huge time scales and distances involved.

Against this backdrop, it's a bit of a challenge to explain how speciation occurs in microbial flora and why the same species seem to emerge in the same types of habitats the world over. (We shouldn't get sidetracked on the precise meaning of the word "species" here. The point is that we can identify the same genomic and phenotypic motifs, packaged in readily identifiable cell types with familiar names, in different points in the biosphere.) Did today's species evolve from common ancestors who were somehow physically distributed uniformly around the world? What was the mechanism of that distribution? More to the point, what happened after the ancestral organisms were laid down? How do you get from there to today's ecosystem of commonly seen microbial communities, with its many self-similarities around the world?

I'll leave as an exercise for the reader the question of whether evolution occurred along parallel paths. I, for one, don't rule out that pseudomonads in Taiwan evolved to their present-day form independently of pseudomonads in Ohio.

I think the amazing taxonomic regularity seen in the microbial world demands flexible thinking when it comes to explaining the emergence of new species. Survival pressure keeps bacterial genomes from drifting very far outside an evolutionary "noise" zone. A substantial barrier has to be crossed in order to arrive at a new species. Accumulation of point mutations probably won't do the job. That just gives "noise." Transfection by viruses probably isn't an important mechanism, either, although the jury is certainly still out on what role (if any) viruses play in speciation.

My suspicion is that "extreme gene transfer" (including inter-species DNA transfer) plays a greater role in microbial speciation than is presently assumed. The bacterial genome inside Drosophila (see prior blog) is a clue that shouldn't be dismissed. DNA is probably more promiscuous than most of us are willing to consider.

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