Plasmid, Virus or Other? DNA ‘Borgs’ Blur Boundaries.

Some archaeal cells seem to carry unusual DNA outside their chromosomes that straddles the familiar definitions for genomic structures.

When the microbiologist Jillian Banfield and her colleagues started combing through samples of mud from wetland environments three years ago, they had a specific goal in mind: to recover and analyze fragments of DNA from large bacteria-killing viruses. And they did. But they also found something unexpected. Some of the DNA in those samples wasn’t immediately recognizable as coming from viruses, archaea or other known cellular sources. Instead, the genomic structures seemed to be “distinct from anything that’s ever been seen before,” Banfield said.

In a new study posted earlier this month on the biorxiv.org preprint server, Banfield and her colleagues at the University of California, Berkeley announced the existence of these strange entities. (The work still needs to be peer-reviewed.) The researchers are upfront in acknowledging that what they have found could be some new type of giant virus, plasmid or bizarre chromosome. But they also suggest that the DNA could belong to something else entirely: what they have dubbed Borgs. Just as the alien species in Star Trek “assimilates” individuals into a hive mind, these unknown elements may integrate genes from their host cells into their own DNA, according to Basem Al-Shayeb, a graduate student in Banfield’s lab.

So far, other scientists generally seem reluctant to embrace the most extraordinary interpretation of what Borgs are. But they do agree that even the more mundane interpretations show how little is known about the categories of genetic diversity in nature. It highlights a need “to start thinking about different genetic entities [on] a continuum,” said Cédric Feschotte, a molecular biologist at Cornell University who was not involved in the new work. This kind of thinking could open up a world of possibilities for what we understand about the nature of the genome, how microbes function, and where to look for potential biotechnological applications.

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