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Is it possible for a virus to be a descendant of a bacterium that was not through horizontal gene transfer?

How I think this could happen:

Suppose a species of bacteria lives in an environment where it gets all the resources it needs to reproduce (and let's assume that there will be no major changes in the environment). That is, if it loses a gene that synthesizes some substance (or regulates some process), it will not be at an evolutionary disadvantage. Because it can achieve this from the environment. Even depending on the gene, it can even be an evolutionary advantage, as it can become more efficient when it comes to reproducing. And I have seen that an experiment was carried out related to this question, it was shown that viruses can lose some genes in an environment where it can naturally find the protein that is associated with those genes. Thus, there is a chance that this species will become so simple, that it will end up becoming a virus.

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    $\begingroup$ The fundamental difficulty with this theory is the bacterium would somehow have to learn to make multiple proteins to encapsulate and deliver the genetic material. This is a classic chicken-or-the-egg problem for evolution: to iterate and to evolve the mechanism, the mechanism must exist. This is an open problem for evolution, so I don't want to nix your theory on this aspect alone. But -- you must realize this idea requires knowledge far outside what we understand well about evolution. Is it possible? No one knows. Is it plausible? Yes, but very difficult $\endgroup$ – BAMF4bacon May 6 at 17:11
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    $\begingroup$ Mitochondria and chloroplasts are descended from endosymbiotic prokaryotes that can no longer reproduce outside a eukaryotic host, and in billions of years they've not simplified anywhere near that much. As with @BAMF4bacon's point, this is a non-fatal but significant difficulty. $\endgroup$ – J.G. May 7 at 12:01
  • $\begingroup$ Could you be a lot more realistic, Raphael? Your bacterium does get all the resources to reproduce, with no changes in the environment. If it loses (an important) gene it can achieve (this?) from the environment? That could be an evolutionary advantage; it could become more efficient at reproducing? Then, how did the experiment you saw switch from bacteria to anything about viruses? Perhaps viruses can lose genes when they find proteins in the environment and how does that change bacteria into viruses? Is your thesis simply that “there is a chance” or what? $\endgroup$ – Robbie Goodwin May 8 at 19:47
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This virology site has a post about a 2017 paper about membrane-vesicled plasmids that act in ways that are theorized to be precursors to how viruses work:

It is likely that the plasmid-containing membrane vesicles are precursors of what we know today as virus particles. It is thought that viruses originated from selfish genetic elements such as plasmids and transposons when these nucleic acids acquired structural proteins (pictured; image credit). Phylogenetic analyses of the structural proteins of many enveloped and naked viruses reveal that they likely originated from cell proteins on multiple occasions (link to paper).

The membrane-encased Archaeal plasmid seems well on its way to becoming a virus, pending acquisition of viral structural proteins.

Though plasmids are not viruses and are not bacteria, bacteria are hosts for plasmids, and plasmids add indirectly to the genetic constitution of bacteria. Expression of plasmid genes can help bacteria survive hostile or extreme environments, and they reproduce along with bacterial chromosomes. Via the Wikipedia link:

Unlike viruses, which encase their genetic material in a protective protein coat called a capsid, plasmids are "naked" DNA and do not encode genes necessary to encase the genetic material for transfer to a new host; however, some classes of plasmids encode the conjugative "sex" pilus necessary for their own transfer.

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There are giant viruses that some people think could be degenerate bacteria.

https://en.wikipedia.org/wiki/Mimivirus

Mimivirus shows many characteristics which place it at the boundary between living and non-living. It is as large as several bacterial species, such as Rickettsia conorii and Tropheryma whipplei, possesses a genomic size comparable to that of several bacteria, including those above, and codes for products previously not thought to be encoded by viruses (including a kind of collagen[20]). In addition, mimivirus has genes coding for nucleotide and amino acid synthesis, which even some small obligate intracellular bacteria lack. They do, however, lack any genes for ribosomal proteins, making mimivirus dependent on a host cell for protein translation and energy metabolism.

It is plausible that organisms like this started as cells and dispensed with various things they did not need in their parasitic lifestyle, resulting in something that fits the definition of virus. Other parasitic bacteria (like those mentioned in the article) lost other unnecessary components but retained such that they still fit the definition of a cell.

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    $\begingroup$ Can you reference your claim that "some people think (Mimivirus) could be degenerate bacteria"? Preferably someone with the relevant expertise? The Wikipedia page does not include this point of speculation and it seems likely to be false. $\endgroup$ – Bryan Krause May 6 at 17:12
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    $\begingroup$ The giant viruses like minivirus may be degenerate free-living organisms, possibly a bacterium but more likely an archeon or eukaryote (based on number of homologous genes). One paper you might reference is Abrahão [2014] doi: 10.1186/1743-422X-11-120 $\endgroup$ – Ethan May 6 at 18:56
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    $\begingroup$ @BryanKrause - saying it seems likely to be false means you did not check at all past reading wikipedia, and I suspect you are able. But if not, here is the link to the first hit for mimivirus on google: nature.com/articles/nrmicro.2016.197. Here is text from that link: Furthermore, there is disagreement as to whether the large genomes of giant viruses are a result of smaller viruses acquiring genes or of a genome with cellular ancestry adapting to escape the cell nucleus2,9,10,11,12,13. $\endgroup$ – Willk May 7 at 18:39
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    $\begingroup$ @Willk It's up to you to provide a complete answer, not up to me to research the entire question for you. It's also not acceptable (and I think of it as academically dishonest) to cite a source for a claim that doesn't make that claim - it makes it look like you have support that you don't have. So far you've only cited Wikipedia which does not make that claim. Please edit proper citations into your answer for claims you are making rather than having them in comments as comments are ephemeral. $\endgroup$ – Bryan Krause May 7 at 18:50
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    $\begingroup$ @TCooper The Wikipedia page includes statements like "its lineage is very old and could have emerged prior to cellular organisms" and "It has been suggested that Mimivirus and similar kinds are remnants of a "fourth domain" of life, and that other giant virus may represent other ancient domains" as well as being part of en.wikipedia.org/wiki/Nucleocytoviricota which are genetically related, so if one of them is derived from a bacterium than they all are. My statement about "likely to be false" was mostly based on OP's own source ranging from saying nothing to contradicting it $\endgroup$ – Bryan Krause May 7 at 23:50

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