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Canadian scientists have identified microscopic creatures that differ so much from everything that has been seen before. So they had to create an entirely new branch of the evolutionary tree of life to be able to insert it.
A new article published this week in Nature offers the first genetic analysis of hemimastigotes – a rare and poorly understood group of unicellular microorganisms. Biologists are aware of the existence of these little animals for over a century, but it is only now that hemimastigotes can be officially incorporated into the evolutionary tree of life, a process more formally called phylogeny. And in doing so, scientists have come across a completely new branch of the tree of life – a branch dating back billions of years.
Yana Eglit, Ph.D. student at Dalhousie University and co-author of the new study, discovered two different hemimastigote species – one already known and one completely new scientifically – in soil samples collected along the Bluff Wilderness Trail in Nova Scotia, Canada. . The previously known species is called Spironema and the new one was nicknamed Hemimastix kukwesjijk (pronounced "ku-ga-wes-jij-k"), which means "hairy ogre rapacious" in the language of the Mi'kmaq First Nations peoples of Nova Scotia.
Raptor and hairy, indeed. These unicellular and free predators use two rows of hair-like flagella, which they use to move and grab their prey. Hemimastix feeds by connecting its mouth – if we can call it that – to the surface of its microscopic prey, sucking the victim's cytoplasm, according to the observations made by Eglit and his colleagues.
Hemimastigotes can not be classified as animals, plants, fungi or bacteria. But they are eukaryotes, having complex cells and a clearly defined nucleus. Eukaryotes that can not be inserted into these conventional groups are called protists – a sort of clustering with sometimes unclassifiable eukaryotes. Such as hemimastigotes.
In addition to describing them as eukaryotic protists, scientists have been unable to locate them anywhere in the phylogenetic tree of life. Part of the problem was that scientists were not able to perform meaningful genetic tests on hemimastigotes – but that has now changed, thanks to new research.
"This discovery literally redefines our branch of" The Tree of Life "at one of its deepest points," explained Alastair Simpson, lead author of the study and professor of biology at Dalhousie, in a statement. "This opens a new door to understanding the evolution of complex cells – and their ancient origins – long before animals and plants emerge on Earth."
Using a new technique called unicellular transcriptonomy and with the help of doctoral candidate Gordon Lax, the Dalhousie team was able to extract large amounts of genetic material from both species of hemimastigotes. Whole genomes could not be extracted, but enough information was collected to allow researchers to perform a phylogenetic analysis.
Almost all eukaryotes belong to the animal, plant or fungal kingdoms, but some eukaryotes can not be classified as such and are considered protists. Dissatisfied with these general designations, scientists have created six "supergroups" for the eukaryotic domain: Sar / Telonemie, Haptophyta / Centrohelida, Archaeplastida / Picozoa (this group contains plants), Cryptista (this group contains algae), Discoba and Amorphea. (this group contains animals and mushrooms).
To further group these eukaryotes at the kingdom level, scientists have created a "supra-group", called Diaphoretickes, which groups together four similar supergroups (see diagram below). The purpose of this organizational scheme is to sort and group species according to their relationship with common ancestors, rather than according to their physical characteristics or other attributes.
Previously, scientists had designated Hemimastigophora at the phylum (below the kingdom), but the new study suggests that they belong to a distinct supra-group, or to a "new line of eukaryotes at the of the supra-kingdom ", in the words of the researchers. .
"Our analyzes clearly showed that hemimastigotes did not belong to any known group at the kingdom level, nor even to a known" supergroup "of several kingdoms combined, such as the one that includes both animals and fungi. , "Simpson said. "This small collection of organizations is an entirely new group on this level, all by itself.This is a branch of the Tree of Life that has been separated for a very long time, perhaps more than a year ago. 39; one billion years, and we had no information about it. "
In addition to being the first to perform a genetic analysis of hemimastigotes, the Dalhousie team is also the first to cultivate these organisms. Armed with a constant number of hemimastigotes on which to experiment, researchers now hope to get more genetic information about these remarkable creatures.
Andreas Hejnol, head of the Sars International Center for Marine Molecular Biology at the University of Bergen, said the document was valuable in that it provides a phylogenetic analysis that ultimately places these organisms in the background. tree of Life.
"The methods are standard and the major achievement is the discovery of organisms in sufficient quantities to isolate the molecules and get their sequence information," said Hejnol, who was not affiliated with the new study, to Gizmodo. "Another great progress is that researchers are able to cultivate them."
Mr. Hejnol pointed out that it was important to remember that categories such as "supra-reign" and "super-group" are simply "anthropogenic categories" that scientists use to classify and organize organisms. as our understanding of biology changes.
"The criteria are not well defined and rely mainly on the" distinctiveness "of other bodies," he said. "This distinctiveness is mainly based on randomly chosen criteria, and prior to that, evolutionary biologists normally used similarity as a criterion for assigning organisms together."
In the case of the new study, phylogenetic analysis placed hemimastigotes on a major new branch, which now coexists with a preexisting sister branch of eukaryotes known as Diaphoretickes.
"Although this position is interesting and useful for reconstructing the evolution (…) of these organisms, the exclusion of hemimastigotes is arbitrary," Hejnol told Gizmodo. "They [hemimastigotes] could also be included [in the Diaphoretickes group] – this is in the eye of the viewer. This shows once again how arbitrary missions such as "supra-kingdom" are ".
To which he added: "The" novel of the supra-kingdom "looks classy and exciting – but has no scientific value. Too bad that it hides the main achievement. authors, namely that they can place this group in the tree of life – which is really a scientific breakthrough. "
In the future, it will be interesting to see if other scientists agree with this classification. As indicated, there are other species of hemimastigotes; perhaps subsequent phylogenetic research will support the creation of this new supra-group or may further ignite the debate. On the contrary, this discovery shows the need to improve the methods of description and classification of organisms along the highest branches of the tree of life.
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