On 2021-07-16 14:56:45, user Claudiu Bandea wrote:
Will Borgs Illuminate the Evolutionary Origin of Ancestral Viral Lineages?
Borgs - another remarkable discovery by Banfield Lab that could illuminate the origin of ancestral viral lineages (1); the other discoveries I have in mind are the huge phages (2) and ARMAN/Thermoplasmatales inter-species connections (3).
True to their data, Al-Shayeb et al. (1) seem, at least for a moment, to limit their speculations on the nature and evolutionary origin of Borgs to open questions: “Are they giant linear viruses or plasmids unlike anything previously reported? Alternatively, are they auxiliary chromosomes?” Then, to my big surprise, the authors, rather casually, write: “Perhaps they were once a sibling Methanoperedens lineage that underwent gene loss and established a symbiotic association within Methnoperedens …” (1). So, why is this a big surprise?
Over the last four decades or so, I have been searching for data and observations that are consistent with, or support, the Fusion Hypothesis on the origin and nature of the ancestral or emerging viral linages (4-6). Although, it is clear that the extant viruses originated from other viruses, and there is compelling evidence that the endogenous viral elements, such as transposons and plasmids, originated from exogenous viral lineages, the evolutionary origin of the ancestral viral lineages has remained enigmatic.
According to the Fusion Hypothesis, the ancestral viral lineages originated from parasitic cellular organisms, including endo- and ecto-parasites that, to increase their access to the resources present in their environmental niche (i.e. the host cell), fused their cell membrane with the host cell membrane, thereby losing their own cellular organization within the host cell. However, after synthesizing their proteins and other specific molecules and replicating their genome, these novel type of organisms induced the morphogenesis/differentiation of cell-like reproductive forms (i.e. virus particle, or virions), which started a new life cycle by fusing with new host cells. [Metaphorically, the Fusion Hypothesis places the ancestral viruses at the intersection of Hollywood and Greek ‘mythologies,’ in which 'viral Borgs' assimilate their hosts, and reemerge just like Phoenix. Factually, within the host cell, viruses, which have been historically and conceptually misidentified with the virions (4-9), are considered to be in the eclipse phase designated as “The time between infection by (or induction of) a bacteriophage, or other virus, and the appearance of mature virus within the cell”(10)].
A fundamental premise of the Fusion Hypothesis is that only symbiotic/parasitic lineages that have a cellular and molecular composition, and processes compatible with those of their host cells (e.g. an archaeal lineage parasitizing another archaeal lineage) have the opportunity to evolve into a viral lineage (4-6); this implies that bacterial or archaeal lineages parasitizing eukaryotic host cells, for example, are unlikely to be able to evolve into viral lineages, regardless of the degree of their genome/proteome reduction (11). Another intriguing inference from this evolutionary model is that numerous cellular lineages evolved into viral lineages throughout the history of life, and that, remarkably, this process might still be active (5-6).
The Fusion Hypothesis is a radical departure from the conventional thinking on the evolutionary origin and nature of ancestral viral lineages, including the historical reductive hypothesis, which lost its appeal more than half of century ago because it could not explain the gradual evolutionary transition from a cellular organisms to viruses (15), which have been conceptually misidentified with the virions and have been erroneously defined based on their physical, biochemical and biological properties (4-9). Perhaps no one has questioned the dogma of viruses as virus particles more explicitly, and in stronger terms, than Jean-Michel Claverie, one of the leading researchers in the field of giant viruses, who asked: “what if we have totally missed the true nature of (at least some) viruses?” (8). Claverie answered this intriguing question in a rather revealing way: identifying viruses with the virus particles, he wrote, might “be a case of ‘when the finger points to the stars, the fool looks at the finger.” (8).
Nevertheless, likely, very few readers of this note are familiar with or even heard of these radical perspectives on the origin and nature of viruses. That might change, though, if the researchers realize that, as discussed next, these new perspectives might better explain the existing data and observations and might open new research venues and objectives for grant applications.
Fortunately, there are only 2 broad ways of thinking about the evolution of viruses, and these paradigms could critically inform the hypotheses on the origin and nature of ancestral viral lineages: (i) viruses have evolved and diversified from simple to more complex entities by increasing the size of their genome/proteome/virions, or (ii) vice versa, they have diversified by reductive evolution. The first paradigm supports the hypothesis that the ancestral or incipient viral lineages were simple genetic entities, usually referred as ‘replicons’, which apparently preceded the cellular organisms at the dawn of life (13-14), and the second paradigm supports the hypothesis that the incipient viruses originated from more complex organisms as suggested in the Fusion Hypothesis.
Because of the high rate of genome evolution and rampant sequence exchanges among various viruses and their hosts, the current sequence analyses cannot clearly differentiate between the two broad evolutionary pathways. Nevertheless, currently, the hypothesis that the complex viruses have evolved from simpler siblings dominates the literature and discussions in the field (e.g.13-14). This perception, though, is in stark contrast to the well-established fact that all intracellular parasitic or symbiotic microorganisms, which count into thousands of species, have evolved toward a smaller genome/proteome/cell size. Although, similar to their free-living ancestors or relatives, these parasitic and symbiotic cellular organisms do occasionally acquire new genetic material, there is overwhelming evidence that, overall, these species have experienced reductive evolution; and this principle apparently also applies to many free-living species. If this is indeed the case, why would viral lineages evolve in opposite direction? Without addressing this critical question, the dominance of the simple-to-complex hypothesis on the origin and evolution of viruses is questionable.
Although, just like any symbiotic/parasitic cellular species, viruses can occasionally increase the size of their genome/proteome (the ‘accordion model’ on viral evolution) it is difficult to define the selective forces leading to the overall evolution of a parasitic organism towards complexity within an intracellular environment. Also, it would be difficult to envision the development of experimental approaches addressing the evolution of ‘replicons’ into simple and, eventually, into more complex viruses; interestingly, Howard Temin’s protovirus hypothesis on the origin of extracellular viruses from endogenous viruses (15) was abandoned when it became clear that the millions of endogenous viruses present in humans and other species originated from exogenous viral lineages, not vice versa.
On the contrary, the Fusion Hypothesis on origin and diversification of viral lineages by reductive evolution is consistent with the life cycle of many viruses, which fuse with their host cells to start their intracellular development (4-6). Given the nature of their intracellular environment, which can provide basically unlimited resources, including ribosomes and other components of the metabolic and informational machineries, and considering the dominance of deleterious mutations over those beneficial, as well as the strong selection for increasing their reproductive rate, it is likely that, overall, viruses have experienced reductive evolution. And, very importantly, this reductive evolution is in line with that of all symbiotic and parasitic cellular species.
Nevertheless, the huge advantage and appeal of the Fusion Hypothesis is that it can be addressed experimentally in the laboratory using various experimental models (5, 6). Even more thrilling is that, as I previously made the case (5), some parasitic/symbiotic cellular lineages are currently in the process of natural transition from a cellular to a viral type of biological organization. To realign this discussion with Al-Shayeb et al. study and intuition (1), it is likely indeed that the ancestor of the 'colorful Borg' was “a sibling Methanoperedens lineage that underwent gene loss and established a symbiotic association within Methnoperedens”, after fusing with it and losing its cellular organization. So are the Borgs viral lineages?
To answer this question, we need to add a few more ‘dimensions’ to the Fusion Hypothesis. As I previously discussed (4-5), the paradigm behind this hypothesis is the ‘cellular fusion’ or ‘hybridization’ phenomena. In principle, two cellular organisms can interact and co-evolve in multiple ways: (i) one cell enters the other, keeps its individualizing membrane (i.e. cell-like structure), and integrates its symbiotic life style and life cycle in synchrony with those of the host cell, as has been the case with the mitochondria and chloroplasts lineages; (ii) a parasitic cellular organism enters its host cell, maintains its cellular structure, and after reproduction it leaves the host cell, which is a very common phenomenon; (iii) a parasitic cellular organism enters the host cell by a membrane fusion mechanism, synthesize its components using the host’s resources, and induce the assembly a cell-like progenies (i.e. virions) that leave the host cell and restart the viral life cycle by fusing with new host cells (iv) in an analogous case, a parasitic cellular organism enters the host cell by a membrane fusion mechanism, ‘assimilates’ the host cell, synthesize its components using the host’s resources and induce the host cell to divide and fuse with other cells, which is another putative viral type of biological organization; (v) and, finally, two related/compatible cellular organisms fuse with each other (i.e. hybridize), and integrate their metabolism and life cycle, generating a new hybrid organism; likely, this has been a very common phenomenon in the history of life, but because of the integration of the sibling partners, it is difficult to detect.
It remains to be seen exactly in which group of biological organization and co-evolutionary pathway the Borgs and their apparent ‘partners,’ the Methanoperedens lineage, fall in, but the discovery of Borgs, and the mystery surrounding their nature and evolutionary origin, should stimulate the interest in developing experimental approaches for addressing the Fusion Hypothesis on the origin of viruses. Additionally, studding the fusion/hybridization of various cellular lineages should open new venues for studying cellular evolution and for dissecting various metabolic and information machineries.
I think it is meaningful to end this note with the inspiring remarks by Jill Banfield (16), the senior author of the Al-Shayeb et al. (1) article:
“I repeat- I haven’t been this excited about a discovery since CRISPR. We found something enigmatic that, like CRISPR, is associated with microbial genomes. We have named these unique entities #BORGs.
*Imagine a strange foreign entity, neither alive nor dead, that assimilates and shares important genes... A floating toolbox, likely full of blueprints, some that we may one day harness, like CRISPR… Wait- wouldn’t that just be a virus? a megaplasmid? a mini-chromosome? No… #BORGs are unique..<br />
.
BORGs are huge, a third the size of their methane-eating hosts, they have assimilated many metabolism-relevant genes, and they have combinations of features not seen before... #BORGs are like turbo boosters for their host’s methane metabolism. This means they could have significant climate impacts...*
This discovery started in deep mud and was brought to light by an analysis of around 10 billion DNA snippets. That such an approach could reveal something with potentially global ramifications!
In 2021, I will again sit across the table from Jennifer Doudna (@doudnalab) and we will talk about how we might begin to explore the technological and environmental importance of this discovery...
This may be an example of the type of basic, discovery-based science that can ultimately tackle the big problems that face our world, the type of discoveries that @elonmusk is seeking through his current 100M @xprize
Basic science, starting with fieldwork and looking at what nature has invented, is important if we are to discover things that we could not imagine. This type of science deserves more funding. Without it, the world would not be meeting the #BORGs”
References:
- Al-Shayeb et al. 2021. Borgs are giant extrachromosomal elements with the potential to augment methane oxidation. bioRxiv: https://www.biorxiv.org/con... doi: https://doi.org/10.1101/202....
- Al-Shayeb et al. 2020. Clades of huge phage from across Earth’s ecosystems. bioRxiv: https://www.biorxiv.org/con... doi: https://doi.org/10.1101/572362.
- L.R. Comolli, J.F. Banfield, 2014. Inter-species interconnections in acid mine drainage microbial communities. Front Microbiol. 5:367.
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- Forterre P. 2010. Giant viruses: conflicts in revisiting the virus concept. Intervirology. 53:362-78.
- Claverie JM. 2006. Viruses take center stage in cellular evolution. Genome Biol. 7, 110.
- V. Racaniello, The virus and the virion. 2010. Virology Blog. http://www.virology.ws/2010...
- Definition of “Eclipse phase.” 2021. Biologyonline. https://www.biologyonline.c...
- Husnik et al. 2021. Bacterial and archaeal symbioses with protists. Current Biology. doi: 10.1016/j.cub.2021.05.049
- Luria SE and Darnell JE. 1967. General Virology. Wiley. New-York.
- Koonin et al. 2006. The ancient Virus World and evolution of cells. Biol Direct. 1-27
- Krupovic et al. 2019. Origin of viruses: primordial replicators recruiting capsids from hosts. Nat Rev Microbiol. 17(7):449-458.
- Temin HM. 1976. The DNA provirus hypothesis. Science. 192(4244):1075-80.
- Banfield J. 2021. Comments on the discovery of Borgs. https://twitter.com/banfiel... ; https://twitter.com/hashtag...