The find renewed the debate about the place of giant viruses on the evolutionary tree. Are these viruses or living cells that have lost the ability to reproduce independently?
Austrian microbiologists have discovered a virus that stands out from its relatives in size and complexity of the device. The "record holder", found in the wastewater of the town of Klosterneuburg in eastern Austria, has a diameter of about 400 nanometers and a genome size of 1570 thousand base pairs (355 genes). In honor of the place of discovery, the virus was named Klosneuvirus. The find is reported by Science magazine.
The virus was detected using an arsenal of metagenomics methods. Simply put, the genome of the virus was assembled piece by piece from the fragments found in the samples taken. This approach allows researchers to study the set of genes of all microorganisms present in the sample environment without their cultivation in the laboratory. The authors of the work themselves assess the find as an accomplished fact and report the probable discovery of three related species in the nearby area.
The discovery reignited the debate about whether the giant viruses that have been periodically discovered over the past decade and a half are viruses in the strict sense of the word, or whether we have former living cells that once became parasites and on a long evolutionary path lost a fair amount of genetic information along with those adaptations, which encoded "discarded". In this case, it would seem logical to separate them into a separate domain, equal to bacteria, eukaryotes and archaea.
Sizes of some known viruses: Poliomyelitis virus ~ 30 nm. Zika virus ~ 45 nm. Adenovirus ~ 90 nm. HIV-1 ~ 120 nm. Mimivirus ~ 400 nm
Indeed, most viruses are much smaller than the cells into which they invade, and have a very modest size of genetic material. Some types of avian and swine viruses cost only two genes to reproduce, versus thousands of even an ordinary bacterium. For example, the well-known E. coli contains 4400 genes - and this is not a record yet. Known giant viruses (the first one was discovered in 2003, and about half a dozen of them have been described in total) contain about one to two thousand genes each, which clearly exceeds the minimum number required to "capture" a cell and then use it. Some organisms, obviously capable of independent life and reproduction, cost less.
Skeptics do not see the need for the fourth domain, explaining the genetic redundancy of giant viruses by borrowing host material. For example, Evgeny Kunin (National Center for Biotechnology Information, Bethesda, Maryland) calls it "crystal clear" that giant viruses belong to groups that include much smaller forms. In general, the authors of the discovery themselves are inclined to this point of view. Teaming up with colleagues, they showed that giant viruses gradually borrowed genetic material from different hosts. “There is no evidence for a fourth domain, and this document confirms it,” says Curtis Suttle, a virologist at the University of British Columbia in Vancouver, Canada.
In addition, skeptics question the very fact of the discovery. Microbiologist Didier Raoult of the University of Marseille notes that before drawing conclusions about evolution from the genome, it would be nice to see its owner. The sequence of genes itself is a minor argument: they often change places and undergo other changes that mask their origin.
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