Evolution: review of eukaryote origin (Evolution)

by David Turell , Thursday, October 27, 2022, 19:03 (756 days ago) @ David Turell

Using genetic studies of comparative genes:

https://www.the-scientist.com/features/the-long-and-winding-road-to-eukaryotic-cells-70556

"Most of the details of these evolutionary leaps, however, remain unsettled. Researchers do not uniformly agree on which branch of life eukaryotes sprang from, which microbial players might have contributed to the process, or on the order of specific evolutionary milestones along the way. But the recent identification of the Asgard archaea, thought to be the closest living relatives to modern eukaryotes, has enlivened discussions about eukaryogenesis.

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"Already, the discovery of the Asgards has solidified certain aspects of eukaryogenesis while raising new questions about others. “I think this is the most exciting development in biology right now. So much is being discovered and so many predictions are being met,” says Daniel Mills, a geobiologist and postdoctoral researcher at Ludwig-Maximilians-Universität München who recently coauthored a paper suggesting that eukaryotes likely evolved in the absence of oxygen. Eukaryogenesis, he adds, is “arguably one of the most important events in the history of life, after the origin of life itself.”

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"Spang had studied a group of archaea called the Thaumarchaeota (now Nitrososphaerota), and during that work, she’d picked up hints that the genomes of these and other archaea contained code for genes that produce what are known as eukaryotic signature proteins, or ESPs. These proteins should not have had recognizable counterparts in archaea, and yet, there they were.

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"In addition, they found that Ca. P. syntrophicum lives in close association with another archaeon called Methanogenium. Ca. P. syntrophicum gets its energy by digesting amino acids and peptides for their nitrogen, and in turn, Methanogenium uses the hydrogen produced during that process to create its own fuel and at the same time reduce environmental hydrogen, which can induce cellular stress. This partnership confirms that Asgards engage in the type of relationships that researchers suspect gave rise to eukaryotes.

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"Researchers who spoke to The Scientist say that many scientists have rallied behind the idea that the first eukaryotes evolved out of a syntrophy between an archaeal host and bacteria that somehow found their way inside to become the organelles, such as nuclei and mitochondria, that distinguish eukaryotes. The details of these relationships remain murky, but mitochondria provide the most tantalizing clues to their origin story. “There’s DNA in mitochondria that we can somewhat clearly connect or trace back to alphaproteobacteria,” says Laura Eme, an evolutionary microbiologist at France’s National Centre for Scientific Research (CNRS). “Even if we don’t know exactly which lineage, we have a smoking gun.”

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"In the 1990s, López-García proposed the Syntrophy hypothesis for the origin of eukaryotes, which posited a three-party metabolic symbiosis between two bacteria and an archaeon. She maintains that this hypothesis is the only one that explains not only the origins of the nucleus, but also the so-called lipid divide, another unsettled aspect of eukaryogenesis in which the lipids that make up the cell membranes of eukaryotes are more similar to those in bacteria than to those in archaea.

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"A couple of years ago, López-García and her Paris-Saclay colleague David Moreira, also affiliated with the CNRS, updated the hypothesis to reflect the discovery of Asgards, but rather than place an archaeon as the original host, they propose than an archaeon—specifically a hydrogen-producing, Asgard-like archaeon—was the original nucleus. The host, they suggest, was likely a deltaproteobacterium, and the ancestor of mitochondria an alphaproteobacterium. This idea is supported, they say, by the fact that most genes in modern eukaryotes are actually bacterial, and not archaeal, in origin, and that eukaryotic membranes are made up of phospholipids that more closely resemble bacterial ones.

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"Leger agrees that our understanding of eukaryogenesis is likely to advance with baby steps. “Part of the nature of these deep evolutionary questions is that we will never know, we will never have a clear proof of some of the hypotheses that we’re trying to develop,” she says. “But we can keep refining our ideas.”

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"Valerie De Anda, a microbiologist at the University of Texas at Austin Marine Science Institute who studies early prokaryotic metabolism, isn’t dissuaded by the current lack of evidence from the idea that a virus may well be the source of the eukaryotic nucleus. She and her colleagues are currently looking for mRNA-capping genes involved in transcription and translation that were suggested by Bell to have been derived from a long-ago “first eukaryotic nuclear ancestor.”

Comment: no conclusions but slow progress. I've boiled down a giant article.