Some insects and roundworms pick up DNA from bacteria living within their cells, new research shows.
The DNA transfer occurs in the animals’ egg cells, so the genetic modification passes between generations. The mechanism therefore provides an alternative to mutation of existing DNA as a way for the species to acquire new genetic traits.
Gene swapping is ubiquitous among bacteria and other single-celled organisms. Even plants and fungi are known to occasionally adopt a piece of foreign DNA. But scientists thought that multicellular animals picked up genes from bacteria only rarely.
“Our data are indicating that [DNA transfer] is going on all the time,” says John H. Werren of the University of Rochester in New York, who led the research team.
The discovery challenges the prevailing view of animal evolution, in which genetic information is passed exclusively from parents to offspring. The transfer of DNA from bacteria means that an individual could acquire and pass on genes that it had not inherited.
“We’re sort of on the edge of a transformation in the field” of animal evolution, comments Laura A. Katz of Smith College in Northampton, Mass. “These sorts of data allow us to redefine the field to capture this other process going on.”
Werren’s team looked at several species of insects and roundworms infected by a parasitic bacterium called Wolbachia pipientis, which afflicts about 20 percent of insect species as well as many other invertebrates. The bacterium lives inside the animals’ cells, including their egg cells, giving it ready access to the chromosomes that are passed on to the animals’ offspring.
“I think that physical access is the key to allowing this [DNA transfer] to happen,” Werren says. The way in which animals’ bodies insulate their eggs and sperm from foreign bacteria is the main barrier to heritable-DNA transfer in animals, he says.
The researchers compared the genetic code of the bacterium with the code of 11 other species: four roundworms, four fruit flies, and three wasps. The team found that all but three of the fruit fly species had segments of the bacterium’s genetic code embedded in their DNA. The report appears online and in an upcoming Science.
Some of this transferred DNA is active in the host species’ cells, the researchers found, but they didn’t determine whether the genes serve a biological function in the host.
The team also scanned an archive of published genomes for 21 other invertebrate species and found bacterial genes in nine of them.
Such bacterial genetic code is routinely ignored during the sequencing of animals’ genomes because most scientists have assumed that the foreign DNA is a sign of contamination, Werren says. However, the new research rules out the possibility of contamination, Katz says. “I think it’s a really beautifully done, elegant study.”
Julie C. Dunning Hotopp, a member of the research team and a scientist at the J. Craig Venter Institute in Rockville, Md., says that the mechanism by which DNA leaves the bacteria and becomes inserted into the host species’ chromosomes remains uncertain.
While in-cell parasites such as W. pipientis are common among invertebrates, none is known to infect people or other mammals, Werren says.