Horizontal Gene Transfer: Bacteria Transmit Genetic Code without Sex In the absence of sex, how are bacteria able to adapt so fast to changing conditions? Gene transfer By David Biello | September 30, 2010 Scientific American
PROMISCUOUS GENETICS: Microbes, like Rhodobacter capsulatus pictured here, can eject packets of genetic material that allow them to swap genetic code and adapt on the fly.Image: Courtesy of Microbe Wikie / Bacterio-Web
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Oceans are highly dynamic habitats: nutrients flood in from a river only to dwindle away over intervening days or weeks; currents shift the mix of waters; an oil spill suddenly makes hundreds of millions of liters of hydrocarbons available to eat. Without sex—and many bacteria don't have sex thank you very much—it's harder for marine microbes to mix it up and achieve the genetic diversity that's key to population success. So how to
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Marine biologist Lauren McDaniel of the University of South Florida and her colleagues
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adapt quickly rather than wait for the long, slow process of mutation in a species? The answer is so-called horizontal gene transfer (HGT)—which actually comprises a number of processes that enable bacteria to swap genetic code.
tested the gene transfer abilities of nine alphaproteobacteria. These specific strains of bacteria have so-called gene transfer agents" (GTAs) or, as McDaniel calls them, "little genetic escape pods." In essence, Rhodobacter capsulatus, Roseovarius nubinhibens ISM and Reugeria mobilis 45A6, among others, could create packets of genetic material and then eject them into the surrounding waters. According to the findings, published October 1 in Science, these packets were then absorbed by their fellow bacteria and incorporated into their own genetic code. "These particles were able to transfer genes from a donor strain to wild-type bacterial strains as well as natural populations," McDaniel explains. In fact, the bacteria in the wild—the researchers tested microbes in coastal, estuary, reef and open ocean environments—were quite promiscuous with their DNA, busily transferring genes to not only their own species but also other closely related bacteria and even other genera. They were also doing it thousands to hundreds of millions of times more frequently than previously estimated for other methods of gene transfer, such as via phages or bacterial viruses (the method also employed by human gene transfer agents, also known as synthetic biologists). "The other processes of gene transfer that were studied were more 'accidental' methods of gene transfer," McDaniel says. "This mechanism appears to be more 'intentional' in that it is under the control of the host promoters." That said, the overall impact of such gene transfer agents may be limited, says molecular biologist Ford Doolittle of Dalhousie University, given that the DNA fragments transferred were short, on the order of 500 to 1,000 base pairs in length. The smallest known genome, or complete set of genetic instructions, is roughly 1 million base pairs long—the genetic blueprint for Mycoplasma mycoides, a goat pathogen. "GTAs are funny because they don't preferentially transmit the genes for making GTAs, and it is hard to see how natural selection favors their presence," he adds.
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antibiotic-resistant superbugs," Doolittle says. In fact, sequencing bacterial genomes has revealed that such gene transfer is responsible for many of the genes present in today's microbes. "Many already accept that HGT is very important to marine microbial adaptation."
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Nevertheless, such horizontal gene transfer is a potent weapon in the bacterial evolutionary arsenal. "Truly novel genes can be taken on instantly. This is why we have
And not just for bacteria. Given the apparently vast numbers of such genetic packets— and viruses, plasmids and other bits of genetic material—the ocean can be seen as a bit of a microbial DNA soup. Some preliminary experiments by McDaniel and her colleagues show that the presence of such gene transfer agents helps increase the
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number of new coral larvae that settle on a given reef and make a new home. "This is very preliminary work," she says. "But it is very exciting."