As I reported last year when it was first announced, Nature proclaimed the existence of “unexpected structures” in soils that appear to gather and share genetic know-how between microbes.
Jillian Banfield coined the term “Borgs” for the structures after the mythical Star Trek beings that grow by assimilating other beings. Now, Nature has published work by Banfield and her colleagues on this phenomenon, using her creative term in the title: “Borgs are giant genetic elements with potential to expand metabolic capacity.” One of the co-authors is Jennifer Doudna who discovered the CRISPR-Cas9 gene splicing mechanism.
A “Borg” is described as a “storage locker” for genes in news from Lawrence Berkeley National Lab. See, Aliyah Kovner, “Methane-Eating ‘Borgs’ Have Been Assimilating Earth’s Microbes.” And they might just help save the planet. How? They can share information with microbes on how to “eat” methane — a potent greenhouse gas believed to be contributing to climate change. If we can breed them, we might be able to reduce methane emissions.
Wondering why some archaea microbes in the Methanoperedens family contain Borgs in their genomes and some do not, Kovner suggests the following reason:
One likely explanation is that Borgs act as a storage locker for metabolic genes that are only needed at certain times. Ongoing methane monitoring research has shown that methane concentrations can vary significantly throughout the year, usually peaking in the fall and dropping to the lowest levels in early spring. The Borgs therefore provide a competitive advantage to methane-eating microbes like Methanoperedens during periods of abundance when there is more methane than their native cellular machinery can break down. [Emphasis added.]
Whether the advantage is “competitive” or not in a Darwinian sense, it would certainly be convenient for an organism to stash its methane-digesting tools in a storage locker in the off season. This implies the ability to find the tools later.
Banfield and colleagues were gathering mud samples in the East River near Crested Butte, Colorado, when they discovered the Borgs. She classifies them with plasmids as “extra-chromosomal elements” (ECEs), except that Borgs tend to be much larger. Some of them can have a million base pairs of information — a third the size of a Methanoperedens genome.
The Borgs are not separate entities; they live within cells but appear to be giant “DNA packages” that can shuttle between microbes. How the sharing is accomplished for such large collections of data remains to be seen.
Where did Borgs come from? Banfield is not sure; she compares it to examples of endosymbiosis that supposedly gave rise to chloroplasts in plants and mitochondria in animals. There are differences, though; “the overall diversity of genes found in the Borgs indicates that these DNA packages were assimilated from a wide range of organisms.”
Voyages of Discovery
The paper ends on a “land-ho!” note with curiosity about what the new world will be like:
We can neither prove that they are archaeal viruses or plasmids or minichromosomes, nor prove that they are not. Although they may ultimately be classified as megaplasmids, they are clearly different from anything that has been previously reported. It is fascinating to ponder their possible evolutionary origins. Borg homologous recombination may indicate movement among hosts, thus their possible roles as gene transfer agents.
Must Borgs have “evolutionary” origins? Maybe they co-existed with the microbes from the start. It’s incredibly exciting to discover a new biological paradigm coming to light that, to the microbes at least, is not new at all.
No matter the origin, it is clear that Borgs have existed alongside these archaea, shuttling genes back and forth, for a very long time.
Plasmids were already known as ECEs that contain genes for antibiotic resistance that microbes can share within soils. By comparison, Borgs are like large diverse libraries of information open to a wide variety of organisms.
Plasmids are known to serve a similar purpose, quickly spreading genes for resistance to toxic molecules (like heavy metals and antibiotics) when the toxins are present in high enough concentrations to exert evolutionary pressure.
“There is evidence that different types of Borgs sometimes coexist in the same host Methanopreredens cell. This opens the possibility that Borgs could be spreading genes across lineages,” said Banfield.
With national labs and prestigious journals starting to read these storage libraries of genetic information, more surprises are sure to come.
Notably, some Methanoperedens were found with no Borgs. And, in addition to recognizable genes, the Borgs also contain unique genes encoding other metabolic proteins, membrane proteins, and extracellular proteinsalmost certainly involved in electron conduction required for energy generation, as well as other proteins that have unknown effects on their hosts.
A New Paradigm
The finding that some archaea contain “storage lockers” of genetic information is giving rise to a new paradigm about genetic inheritance. Now, Borgs can be added to my list from last month of gene sharing processes, enlarging the thought of “immense implications for neo-Darwinian theory that are not yet fully recognized.”