Microbiologists may have taken a big step toward resolving the earliest branches on the tree of life and unraveling one of its great mysteries: how the complex cells that make up the human body are made. The advance reported today in Nature marks the first time that such microbes have been grown in high concentrations to study their guts.
The results of the electron microscope images show complex internal structures that are similar to those in our own cells. The complex cytoskeleton of the microbe is made of the actin, which is thought to have arisen in archaea before becoming an important part of plants and animals. Recent work shows that genes once thought to only be found in more complex organisms are actually found in Asgard archaea.
Buzz Baum is an evolutionary cell biologists at the Medical Research Council. The images are stunningly beautiful. Baum thinks a lot of researchers will want to study the new microbes.
A third domain of life, archaea are distinct from other life forms. Archaea andbacteria have some similarities, but typically neither has core features such as mitochondria, cells' internal powerhouses, or DNA encased inside a nucleus. Many researchers think that early cells arose after an archaeon engulfed a bacterium that became the mitochondria, but they haven't been able to figure out how other parts of the cell's structure evolved. Masaru Nobu is a Microbiologist at the National Institute of Advanced Industrial Science and Technology.
In 2015, Thijs Ettema, an environmental microbiologist at Wageningen University, discovered genes in strange archaea that looked like they were from the ancestors of eukaryotes. The Asgards are made up of several different groups of archaea.
Skeptics argued that he couldn't be sure the genes really belonged to archaea because he only cobbled together the genomes from environmentalDNA. Nobu's team cultured the first Asgard microbe, which was isolated from ocean mud off Japan, and discovered that it had genes from other organisms.
The California Institute of Technology and her colleagues isolated enough of two other Asgard species to sequence their complete genomes. The case was strengthened by the genes in those genomes. The genes that were found in the genomes were found to be involved in metabolism, suggesting that they played a role in transferring genes among life's major branches.
The two domains were connected in another way by comparing the genes that are found in Asgard archaea and the genes that are found in Eukaryotes. They looked at how cells bend, cut up, and sew together their membranes to link internal compartments. Only two of those complexes were found in archaea at that time. The instructions for making four of them can be found in the Asgard genomes.
The group synthesised some of the molecule in the lab and showed they worked the same as the eukaryotic version. The scientists think that this machinery is from the past.
It took 12 years of trial and error to culture the first Asgard, and the second one was even more difficult. He didn't know how difficult it would be.
The fragile nature of the Asgard cells made it difficult to concentrate them by transferring them from one flask to another or staining them before using a microscope. Thiago Rodrigues-Oliveira came up with a way to grow them in high concentrations and then use an electron microscope to look at them in a different way. The team's studies were complicated by the fact that the samples it cultured also contained other organisms. The discovery in 2020 helped them get home.
Two years ago, a scientist at the Georgia Institute of Technology noticed something strange about the ribosomes, the cellular structures that translate genetic information into proteins. The genes for a key part of these structures were longer, and so the resulting ribosomes were larger than those in other prokaryotes. The two men at ETH Zrich were able to pick out the cells with the help of big ribosomes. It took Wollweber 36 hours to find 17 of theMicrobes in the images.
The new Asgard, which is different from the one isolated by Nobu's team and the ones studied by Orphan to be put into a separate genus with the tentative name Lokiarchaeum ossiferum, has small bubbles poking out from its skin. As if to sample the environment, its cell wall is made of lollipop structures. The cell structures look like they came from another planet.
A key component of a eukaryotic cell's internal skeleton, actin, is one of the genes that is included in its DNA. It is possible that the skeleton is capable of being rearranged because it varies from cell to cell. The emergence of the first eukaryotic cells was thought to be the result of the invention of the eukaryotic cytoskeleton.
Hiroyuki Imachi is a microbiologist at the Japan Agency for Marine-Earth Science and Technology.
Some scientists think that the most likely scenario for the emergence of eukaryotes is that an oxygen-using bacterium can be turned into an energy producer by an Asgard-like microbe. The combined cell may have been made from otherbacteria.
Some people disagree. Some evolutionary biologists, including Patrick Forterre of the Pasteur Institute, argue that the family trees built based on comparing genes don't support the idea that Asgard archaea played a major role in the creation of eukaryotes. Last year, Sven Gould, an evolutionary cell biologist at Heinrich Heine University Dsseldorf calculated that Asgard archaea did not contribute much to the first eukaryotes.
Gould doesn't think the archaeal partner was like the newly cultured microbes. The evidence points to a simpler host. Gould and colleagues propose that the presence of bacterium inside these cells caused new stresses on the Archaea's cellular processes that led to the evolution of the Golgi apparatus.
The full story has yet to be told. The two cultured Asgards are just a small portion of the group's diversity. He points out that recent work shows that the Eukaryotes branched off of the Asgard archaea. He and other people are trying to culture and describe other Asgards.
Results of those efforts are eagerly awaiting. It will be interesting to see what other archaea look like.
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