Evolution: origin of eukaryotes: nucleus from a virus? (Evolution)

by David Turell , Wednesday, November 25, 2020, 22:55 (1247 days ago) @ David Turell

Like the addition of the mitochondria:

https://www.quantamagazine.org/did-viruses-create-the-nucleus-the-answer-may-be-near-20...

"Different as the cells from animals, plants, fungi and protozoa can be, they all share one prominent feature: a nucleus. They have other organelles, too, like the energy-producing mitochondria, but the presence of a nucleus — a well-defined porous pouch full of genetic material — is what inspired the biologist Édouard Chatton in 1925 to coin the term eukaryotes, which referred to living things with a “true kernel.” All the rest he labeled prokaryotes, for life “before kernel.” This dichotomy between nucleated and nonnucleated life became fundamental to biology.

"No one knows exactly how the nucleus evolved and created that division. Growing evidence has persuaded some researchers, however, that the nucleus might have arisen through a symbiotic partnership much like the one believed to have produced mitochondria. A crucial difference, though, is that the partner responsible for the nucleus might not have been a cell at all, but a virus.

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"Scientists generally think eukaryotes first came on the scene between 2.5 billion and 1.5 billion years ago, when evidence suggests that a bacterium took up residence inside a different kind of prokaryote, an archaeon, and became its mitochondrion. But a deeper mystery surrounds the emergence of the nucleus; no one even knows whether that ancient archaeon was already a kind of proto-eukaryote with a nucleus, or whether the nucleus came later.

"Any origin story for the eukaryotic nucleus needs to explain several of its features. There’s the nature of the structure, for starters: its nested inner and outer membranes, and the pores that connect its interior to the rest of the cell. There’s also the curious way it compartmentalizes the expression of genes within itself but leaves the construction of proteins outside. And a truly persuasive origin story must also explain why the nucleus exists at all — what evolutionary pressures pushed those ancient cells to wall up their genomes.

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"...scientists cited recent discoveries involving an extraordinary group of “giant viruses” as one of the main reasons for the updates. These viruses were totally unknown when Takemura and Bell published their initial hypotheses. Their genomes, which have more than 1 million base pairs, rival those of small, free-living bacteria in size, and they carry viral versions of genes for proteins involved in essential processes in cells. (There’s some evidence that the eukaryotic versions of several of these proteins came from these viruses.)

"But most importantly, these giant viruses replicate inside complex, self-constructed compartments in a host cell’s cytoplasm, which is why these viruses, like poxviruses, are classified as nucleocytoplasmic large DNA viruses (NCLDVs). For these giant viruses, the compartments they make are “viral factories which are as big as a eukaryotic nucleus,” said Patrick Forterre, an evolutionary biologist at the Pasteur Institute in Paris. Tellingly, the viral factories made by NCLDVs that infect eukaryotes also have inner and outer membranes like the nucleus. Giant viruses are what Forterre, Takemura and Bell say are responsible for the origin of the nucleus.

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"Until that kind of extraordinary evidence is in hand, viral eukaryogenesis will likely remain controversial. But even if it doesn’t end up winning the battle for acceptance, every test of the theory reveals bits and pieces of our evolutionary past — and because of that, we’re getting closer and closer to learning the truth about where we came from."

Comment: I've skipped over a review of the battle. It is not settled, but the mitochondria are described as happening in a similar way.