Two bacterial species found in the guts of chickens, pigs and other animals are merging into a single species after the domestication of livestock brought the two microbes together, according to a study published today in Science. The research indicates that "despeciation" can be an important consequence of environmental changes in bacterial evolution.

Bacteria swap genes so often that it can be difficult to define species boundaries and determine the mechanisms that drive bacterial speciation. Nonetheless, biologists commonly distinguish bacterial species based on limited gene flow. The two pathogens, Campylobacter jejuni and Campylobacter coli, which live in the digestive tracts of many animals and are major causes of human gastroenteritis, are indeed recognized as distinct species — their nucleotide sequences have diverged by about 13%, suggesting that the two species probably split over 100 million years ago. Now, however, researchers at the University of Oxford have a gut feeling they might be coming back together.

Daniel Falush and his colleagues analyzed 2953 distinct multi-locus haplotype sequences based on seven housekeeping gene fragments from approximately 10,000 Campylobacter strains, and looked for signs of genetic exchange. They found the haplotypes largely clustered according to the two species, but there was evidence of one-sided gene flow — mostly from C. jejuni to C. coli. The imported alleles that C. coli adopted were indistinguishable from C. jejuni alleles from ruminants and poultry, but distinct from those found in wild birds, suggesting that human-mediated agriculture brought the two species together. Furthermore, the high rates of import of C. jejuni alleles by C. coli appear to be a relatively recent change from historically low rates of gene flow.

"If this process continues at the rate it does, then the two species will probably merge rather quickly... even decades is possible," said Falush, now at University College Cork in Ireland.

Jim Staley of the University of Washington in Seattle, who was not involved in the study, said that the approach is "very powerful," but he is not convinced that the two species will necessarily merge completely. Staley noted that only one of three major C. coli clades was incorporating large amounts of C. jejuni DNA, so the other two clades could remain distinct.

Falush was studying how species merge rather than split, but he thinks the study has broader implications for bacterial speciation. By looking backwards at how bacteria species converge, we can learn about the forward-process by which new species arise, he told The Scientist. Because taking away obstacles to gene flow in the novel farm environment is causing despeciation, he thinks ecological barriers to genetic exchange between the two species must have originally driven the two species apart.