A missing link sheds light on the origin of nucleated cells

The first stages of the evolution of living organisms have not ceased to intrigue scientists. However, the discovery of a new strain of archaea has allowed researchers to take a step closer to understanding the origin of eukaryotes, this vast branch of life to which we belong.

The tree of life is traditionally divided into three main branches: eubacteria or true bacteria; archaea, another group of single-celled microorganisms; and eukaryotes. This last group, even if it is not the largest, remains the most studied, because it is the only one to have seen the emergence of pluricellularity. Eukaryotes, that is to say both humans and planktonic algae or fungi, are distinguished by a complex cellular architecture.

A debated origin

The fundamental characteristic of eukaryotic cells is the presence of a nucleus, a pocket delimited by a double membrane containing the genetic material. But they also have other organelles (structures specialized in a given cellular function) such as mitochondria (the cells’ energy factories) and an internal membrane network called the endoplasmic reticulum. Finally, they are equipped with an actin cytoskeleton, a network of long filamentous proteins that structure and give shape to cells like microscopic cables.

However, the origin of eukaryotes remains debated. Recent data from phylogenetics, the study of kinship between species during evolution, suggest that this group emerged from a line of archaea called “Asgard”. Indeed, hundreds of genes thought to be specific to eukaryotes are found in the Asgard genome. To explain this astonishing relationship, Japanese researchers had proposed in 2020 a hypothetical model according to which eukaryotes would come from an Asgard having emitted long membrane protrusions (extensions of the cell) which, by merging, would have, on the one hand, formed the different internal membrane compartments and, on the other hand, included a eubacterium at the origin of the current mitochondria.

Model validation

Until then, however, scientists were unable to validate this model, struggling to isolate and therefore observe these potential cousins. It is now done: researchers from Vienna and Zurich succeeded in cultivating a strain of Asgard from Slovenian marine sediments in the laboratory. By observing these single-celled cells under an electron microscope, the researchers noticed that the Asgard did indeed emit long, budding membrane protrusions. Even more interestingly, they showed that these protrusions were structured by actin filaments, similar to what one might observe in a eukaryote. This complex cellular architecture, unprecedented in a non-eukaryotic, goes well in the direction of the model proposed by the Japanese researchers.

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