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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.

References:

Dunning Hotopp, J.C. . . . and J.H. Werren. In press. Widespread lateral gene transfer from intracellular bacteria to multicellular eukaryotes. Science. Abstract available at http://dx.doi.org/10.1126/science.1142490.

Some Note About DNA
Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. The main role of DNA molecules is the long-term storage of information and DNA is often compared to a set of blueprints, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.

Chemically, DNA is a long polymer of simple units called nucleotides, with a backbone made of sugars and phosphate groups joined by ester bonds. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription. Most of these RNA molecules are used to synthesize proteins, but others are used directly in structures such as ribosomes and spliceosomes.

Within cells, DNA is organized into structures called chromosomes and the set of chromosomes within a cell make up a genome. These chromosomes are duplicated before cells divide, in a process called DNA replication. Eukaryotic organisms such as animals, plants, and fungi store their DNA inside the cell nucleus, while in prokaryotes such as bacteria it is found in the cell's cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA, which helps control its interactions with other proteins and thereby control which genes are transcribed.

Further Readings:

Travis, J. 2004. Genetic pickup: Did animals get brain genes from bacteria? Science News 165(June 12):372. Available to subscribers at http://www.sciencenews.org/articles/20040612/fob4.asp.
______. 2000. Pass the genes, please. Science News 158(July 22):60-61. Available at http://www.sciencenews.org/articles/20000722/bob10.asp.


About Researchers:

Jan Andersson
Institute of Cell and Molecular Biology
Uppsala University
Biomedical Center
Box 596
75124 Uppsala
Sweden

Julie C. Dunning Hotopp
Institute for Genomic Research
J. Craig Venter Institute
9712 Medical Center Drive
Rockville, MD 20850

Laura A. Katz
Smith College
Burton Hall 201
Northampton, MA 01063

Russell F. Watkins
Centre for Molecular Medicine and Therapeutics
950 West 28th Avenue
Vancouver, B.C. V5Z 4H4
Canada

John H. Werren
University of Rochester
Hutchison Hall 306
Rochester, NY 146273