Microorganisms such as bacteria enjoy swapping genes, and the trades have made a big difference in how they've evolved. Now new research suggests that bacteria are also easygoing about passing genes on to more complex organisms. The findings have researchers rethinking the prevalence of interspecies gene transfer and its role in evolution; they may also change the way geneticists filter out bacterial "contamination" when they sequence a new genome.
So-called lateral gene transfer is ubiquitous among bacteria--they can acquire antibiotic resistance by swapping genes with species that have evolved it--but transfers between bacteria and multicellular organisms were thought to be rare. Some of the few known cases involve genes from parasitic bacteria called Wolbachia, which infect 20% to 75% of insects, as well as other invertebrates. The parasitic bacteria live within their hosts' cells, including the germ cells that give rise to eggs, and in past studies scientists found its genes in the genomes of two worm and insect hosts.
However, according to microbiologist Julie Dunning Hotopp of the J. Craig Venter Institute in Rockville, Maryland, common wisdom holds that these transfers are uncommon, and so genetic sequences from bacteria like Wolbachia are considered to be contamination when they're found in insect genomes. Suspecting that their treatment as contaminants was masking transfers' true frequency, Dunning Hotopp and her colleagues screened animal genetic databases for Wolbachia sequences. Reporting online 30 August in Science, the team found them in three wasp and four worm species. In the wasps, the DNA was a 96% match to each wasp's resident Wolbachia strain.
Even more surprising, by resequencing genes from a tropical fruit fly called Drosophila ananassae, the team discovered that the insect was carrying nearly the entire Wolbachia genome of more than 1 million DNA base pairs on one of its chromosomes. Most of the DNA appears to be nonfunctional, but the researchers found RNA transcripts from 30 Wolbachia genes. That indicated that these genes were being transcribed, the first step toward making a working protein. To ensure that the Wolbachia sequences really come from the fly genome, the team treated the flies with antibiotics and then peered at the cells using microscopes to ensure that all Wolbachia DNA had been destroyed.
"Lateral gene transfers ... might be much more widespread than previously envisioned," says Takema Fukatsu, an evolutionary biologist at the University of Tokyo in Japan. Fukatsu calls the paper "exciting" but questions whether the Wolbachia genes are active because they didn't display any biological function. However, if future experiments confirm that expression really is occurring, he adds that it's an "unprecedented insight" into the process of evolution.