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The Case of the Half-Blood Bug
10 April 2008 (All day)
It's a good thing Darwin studied finches instead of bacteria. The ubiquitous microbes have such a messy family tree--with various types of sexual reproduction and genes jumping between distantly related bacteria--that the very concept of a microbial species is in doubt. Now, a genetic study of the microbes that cause food poisoning suggests that the evolution of bacterial species might not be so different from that of finches after all.
If you're suffering from horrible stomach pains and diarrhea, blame Campylobacter jejuni or C. coli. These two bacteria account for the majority of food poisoning cases worldwide. As bacteria go, the two species are quite distinct, sharing only 86.5% of their most conserved genes. (By comparison, humans share more than 99% of their entire genome sequence with chimpanzees.) Chalk the difference up to millions of years of life in separate host animals. But over the past 10,000 years, this barrier has begun to fade: The two bacteria encounter each other regularly on farms, within the guts of domesticated animals such as chickens and cows, which they can both infect, and in soil and water contaminated with farm animal feces. When such ecological barriers break down for animals--for example, when two species of Darwin's finches colonize the same Galápagos island--hybridization can occur, blurring the lines between species. Can such classic evolutionary dynamics be seen today between C. jejuni and C. coli?
To find out, a team led by Martin Maiden, a microbial geneticist at the University of Oxford, U.K., used multilocus sequence typing (MLST). The method creates a genetic fingerprint for bacteria using DNA sequences from seven highly conserved genes. When the researchers analyzed bacteria isolated from farms, they found telltale signs of hybridization in about 1 in every 10 bacterial isolates, with C. jejuni MLST markers appearing in the genome of C. coli and vice versa. The team concludes that this is a blurring of the species lines, as happens with Darwin's finches.
The hybrids may not last long. Instead of being an equal genetic exchange between the species, C. coli hybrids with C. jejuni genes were nearly 20 times more common than vice versa, the team reports 11 April in Science. The team predicts that if the trend continues, C. coli will "despeciate," becoming more and more like C. jejuni until it is genetically indistinguishable.
That conclusion isn't shared by everyone. "In order for species to truly fuse," says Frederick Cohan, a microbial geneticist at Wesleyan University in Middletown, Connecticut, their "niche-defining" genes must be lost. An example from Darwin's finches are the genes responsible for differently shaped bills. The jury may be out until the functions of Campylobacter's genes are better understood.