And to finish, I agree for the same reason, that it is the DNA of retrovirus or bornavirus, it is not the body which is requesting of new genetic sequences, it is the virus which seeks to attack. Whether the sequences become useful to the body is obviously a mystery. That the multitude of unused sequences is indeed the fruit of the repression of the body.
Let me put the text here, because it is very likely that on this site, this very instructive text is put in archive: Thank you again Bernardd!
Unearthing new Homo sapiens ancestors does not necessarily mean scratching the earth. You can also search your chromosomes for exotic genes and find unique branches in your family tree. Witness the work of American and Japanese researchers, led by Masayuki Horie and Tomoyuki Honda (University of Osaka), published Thursday January 7 in the journal Nature, which document the existence of one of our ancestors so far unknown.
This is a small virus, from the bornavirus family; he bequeathed, some 40 million years ago, four genetic sequences to the ancestor of primates.
That a virus is one of the ancestors of animals or plants is not, however, a surprise in itself. We already know that around 8% of the human genome is made up of viral genetic sequences. But these come exclusively from retroviruses: these indeed have the property of inserting their genome into that of the cells they infect.
Imagine that the germ cells (sperm or ova) of an animal are infected with such viruses: the descendants of the animal in question receive a modified genetic heritage, embellished with genes of the microscopic parasite. This genetic modification will then be transmitted vertically to all of the descendants of the being affected by the infection ...
"Here, the novelty is that the sequences detected in the genomes of mammals come from a virus which is not a retrovirus, explains the biologist Cédric Feschotte (University of Texas at Arlington). The first question posed by this work is therefore to know how a bornavirus was able to integrate into the genome of our ancestors. "
First, unlike retroviruses, bornaviruses are not equipped to carry out this genetic invasion. They lack the ability to produce themselves an enzyme necessary to transform their genetic makeup into DNA "compatible" with that of their host. To do this, it is likely, the authors explain, that the small bornavirus has managed to use the tools of the very genome that it was going to colonize to its benefit. Such an ability to "hack" the genetic material of its host has already been demonstrated in another virus.
On the other hand, bornaviruses are known to infect the brain of their victim, not their germ cells. Similar viruses circulating today are thus responsible for lightning encephalitis in horses and other large herbivores ... For several years, some research teams have even suggested a link between infection by a bornavirus and appearance in humans , severe psychiatric disorders, such as schizophrenia. However, this work is still preliminary and much debated.
How did a virus so prone to infect brain cells end up in the germ cells of its host, to be transmitted to the descendants of the latter? This question remains open. "It is all the more disturbing since the introduction, 40 million years ago, of a bornavirus into the genome of primates is not an isolated case, says biologist Christophe Terzian (Ecole Pratique des Hautes Etudes The same team shows, for example, the insertion of a bornavirus, 10 million years ago, into the squirrel genome ... "
Another astonishing fact, adds Cédric Feschotte, "of the four genetic sequences identified (in primates) as originating from the bornavirus, two are potentially functional genes and are therefore theoretically capable of producing proteins". Are these proteins well synthesized? What are they for ? "The published work does not allow to say it, answers Christophe Terzian. But if they are preserved during the evolution, it is probably that they had a utility: perhaps they offer a form of immunity. against bornaviruses, or perhaps their function has been "domesticated" by the host. " Such examples of "domestication" do exist. Thus, the genes which allow the synthesis of the placenta in humans (this is also the case in the majority of mammals) are directly inherited from retroviruses.
However, if certain genetic functions inherited from the viral world are indeed active - and useful - for the functioning of our organism, the majority of them are silenced by mechanisms of repression of gene expression. However, certain studies show that, in the event of a defect in these repressive processes, certain diseases can be triggered. "It's very speculative, but it is now possible to imagine that disturbances in the expression of the bornavirus sequences inserted into our chromosomes could cause certain neurological diseases", concludes Mr. Terzian.
