Two bacterial species have apparently been caught in the act of merging back into one. This case of de-speciation may be a consequence of the two species being thrown together within domestic animals.

Campylobacter coli and C. jejuni are the most common causes of bacterial food poisoning worldwide. The two species differ in 13.5% of the nucleotide bases that spell out their genomes.

That is a level of genetic differentiation comparable to that seen between other bacteria that diverged over 100 million years ago – plenty of time to have developed into different species or even different genera.

But when Daniel Falush, a bacterial geneticist at University College Cork, Ireland, and his colleagues looked more closely, they found that the two species have recently started exchanging genetic material at a high rate.


Wholesale exchange

In particular, they found that one sub-lineage of C. coli, known as clade 1, carries many more C. jejuni genes than the other two C. coli sub-lineages.

Moreover, 49 out of 50 such genes had sequences that had not changed by even a single base from the C. jejuni form — strong evidence that the exchange happened very recently.

In fact, Falush’s analysis shows that the two species’ genomes are converging through gene exchange at least four times as quickly as they are diverging through mutation.

In the wild, C. coli and C. jejuni tend to infect different species, but the clade 1 form of C. coli can co-occur alongside C. jejuni in farm animals such as chickens and cattle.

By eliminating the ecological barriers that once kept the two bacteria separate, agriculture may have given them the opportunity for wholesale genetic exchange.

Evolutionary dead end?

“When these two bacteria get together in the same niche, they are perfectly capable of exchanging DNA at a very high rate,” says Falush. “Differences that happened over a long time period can be reversed over a very short time period.”

If Falush is right, then different niches may play a crucial role in speciation in bacteria, just as they do in higher organisms. Evolutionary biologists have suspected this on theoretical grounds, but Falush’s study is the first concrete evidence to support this idea.

Falush’s study clearly shows that the two Campylobacter species are swapping genes at a high rate, says William Hanage, an evolutionary biologist at Imperial College in London.

However, the researchers have yet to show that the resulting hybrids are fit enough to continue the process of convergence, rather than just being an evolutionary dead end. “Call me 75% convinced,” says Hanage. “There’s a ways to go yet.”

Journal reference: Science (DOI: 10.1126/science.1155532)

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