In the unending arms race between hosts and parasites, there are few rules. Now, researchers say they've found hints of a new type of foul play: bacteria stealing genes from their animal hosts.
Bacteria are among the most adaptable of living creatures, notorious for shuffling genes amongst themselves, passing on good ideas and discarding others as the environment changes around them. But gene transfer between kingdoms--from animal to bacteria for example, or vice versa, has been much harder to document. But in a study published online 26 May in Genome Biology, a team of computational biologists and malaria researchers from the European Molecular Biology Laboratory in Heidelberg, Germany, report finding a bacterial version of the gene that produces α2macroglobulin (α2m) in animals. α2m is a handy molecule--it captures the protein-destroyers the bacteria use to push their way deeper into the host's body. Because it is such a versatile player in the immune system, the gene appears to have been preserved in all animal lineages, from horseshoe crabs to humans. Outside the animal kingdom, however, the gene seemed absent.
So the researchers were surprised when a database search turned up versions of the gene in more than 30 bacteria species, says team member Aidan Budd. Although the bacteria shared common features (most live inside animals and many cause disease), the pattern of species carrying and lacking the gene suggested that the gene had been shuffled horizontally in the family tree, between distantly related species, not passed down from an ancient ancestor. Even dissimilar species had similar versions of the α2m gene, they found.
The most likely explanation for these findings, together with the apparent absence of the gene in plants and other kingdoms, is that some ancient bacteria acquired the gene from a primitive animal (metazoan) host and then shared it with other bacteria, Budd and colleagues say. The researchers admit the case for theft isn't airtight, though, because they can't rule out the possibility that the gene has simply been lost from plant, fungi, and other kingdoms.
"But if I were a betting person, I'd bet on the metazoan-to-bacteria transfer," says Peter Armstrong, a comparative immunologist at the University of California, Davis. "The chance of the gene being lost over and over again in the other lineages seems vanishingly small compared with a single transfer from a metazoan to a bacteria." The really interesting question, says Armstrong, is what the gene does in bacteria: "Is it part of the infection cycle? Does it suppress other bacteria? It will be very interesting to find out."