7.7 Devolution? a vanishing bacterial species boundary

http://arstechnica.com/journals/science.ars/2008/04/12/devolution-a-vanishing-bacterial-species-boundary

By John Timmer | Published: April 12, 2008 - 02:40PM CT

Those of you who made it through a high school biology class where evolution was discussed undoubtedly were introduced to the idea of geographic isolation. Keep two populations of a single species from mating for a few million years and, even in the absence of selective pressure, they are likely to pick up enough genetic differences to keep them from mating should they ever be rejoined. But this model really applies to larger species—geography really doesn't make much sense on the bacterial level, and bacterial exchange DNA through a process called conjugation that really isn't picky about the what species the organism on the receiving end is.

The frequency of conjugation and other forms of horizontal gene transfer in bacteria has led some to question whether it makes sense to talk of bacterial species at all. Some research published in Science last week describes a case that will definitely inform that discussion, as it describes a species boundary that may be vanishing due to horizontal gene transfer.

The work looked at gene flow between two species, Campylobacter jejuni and Campylobacter coli; the latter is responsible for human gastroenteritis. Both species inhabit the digestive tracts of warm-blooded animals, and sequence differences in key genes suggest that jejuni and coli have been been going their own way for over 100 million years.

The new work tracks a set of 3,000 global Campylobacter samples through seven genes. The work suggests that C. jejuni is a distinct species, while C. coli is composed of three recognizable groups. It also finds a distinct pattern of horizontal gene transfer among them, as nearly 20 percent of the alleles transferred into one of the C. coli groups originated in C. jejuni. In essence, the distinction between the two species is being erased in this one population. Other instances of horizontal transfer suggest the three groups of C. coli still exchange DNA, so it's possible that this process might eventually reach an endpoint where all species distinctions are gone.

After 100 million years apart, what could have caused this genetic reintegration? The researchers suggest it happened only very recently; none of the C. jejuni DNA has picked up mutations since it has been picked up by the other species. They propose that agriculture, which has led to radical changes in animal species distributions and brought things like pigs and chickens into close contact, is responsible for bridging the barriers that once separated the bacterial species.

Science, 2008. DOI: 10.1126/science.1155532

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