Chromosomal tricks
Russian biologists have discovered a new method of genome editing in animals
Edible frogs are famous for removing half of their DNA from their genome before forming germ cells. Now researchers from Russia and Poland have found out how they do it. The answer was unexpected. Biologists were surprised that the "extra" parts of DNA are destroyed not at once, but gradually, in the process of many cell divisions.
"Previously, gradual removal of genetic material was found only in plants," explains the first author of the study Dmitry Dedukh from St. Petersburg State University.
In addition to the fundamental value, the discovery can also be practically useful. After all, the discovered phenomenon can become the basis of new genome editing technologies.
When chromosomes are torn apart
To explain the essence of the discovery, let us recall how germ cells are formed in animals. Let's explain this by the example of frogs, since we are talking about them. However, everything happens in the same way in humans. The only difference is that the number of chromosomes: a person has 46 of them, and croaking creatures have 26.
So, an ordinary (non-sexual) frog cell contains 13 pairs of chromosomes. In each such pair, one chromosome is inherited from the father, and the other from the mother.
At the same time, there is a special type of cells in the animal's body, which are called germ line cells (KZL). They separate from the rest of the cells when the amphibian is still at the embryo stage. When the amphibian reaches puberty, spermatozoa or eggs begin to be produced from KZL. Both are combined under the name of germ cells, or gametes.
Before dividing each CDL, all the chromosomes in it are doubled (it was 26, it became 52). As a result, the germ line cell generates four germ cells with 13 chromosomes each.
However, there is an important nuance in this process. One would think that the 52 resulting chromosomes peacefully diverge into four gametes intact. But in fact, everything is much more complicated.
Consider two chromosomes from the same pair in a germ line cell. Let's denote the chromosome carrying the DNA of the father O, and the DNA of the mother – M.
Before the formation of gametes, each of the O and M chromosomes is torn into two parts: O1, O2, M1 and M2. Then the O1 fragment is glued to the M2 fragment and a new O1+M2 chromosome is formed. Similarly, the M1 fragment is glued to the O2 fragment and a new M1+O2 chromosome is formed. (This process of exchanging chromosome sections is called crossing over). And already these new chromosomes later fall into the gametes.
Evolution, why is it so difficult?
Why does nature need such a complex and fraught with unforeseen failures procedure as a crossover? Its meaning is that different children of the same parents have different genomes.
There are many reasons why the genetic dissimilarity of children of the same couple makes natural selection much more effective. Here is just one example.
Let the frog Vasya inherit from his father a useful mutation in the O1 region, and a harmful one in the O2 region. Thanks to the crossover, Vasya will have two types of spermatozoa: O1 + M2 and M1+ O2. Thus, a useful paternal mutation gets into the spermatozoa of the first type, and a harmful one gets into the spermatozoa of the second.
This means that Vasya will have "losers" descendants who inherited only a harmful gene change, and "lucky" descendants who received only a useful genetic gift. Natural selection will attack the former and support the latter. Thus, the harmful mutation will eventually disappear from the population, and the beneficial one will persist and spread. But this would not have been possible if Vasya's father's DNA had not split into different chromosomes during the crossing-over process.
Crossing Over victims
However, crossing over, which is a boon for the species as a whole, turned out to be a real disaster for interspecific hybrids. Recall that such hybrids are obtained by crossing individuals of different species. And crossing-over deprives almost all such half-breeds of the chance to continue the race.
Let's remember, for example, about mules. The mule is known to be the offspring of a donkey and a mare. Each pair of his chromosomes consists of the paternal donkey chromosome O and the maternal mare chromosome M.
During the formation of gametes, the chromosomes are torn in two. Their sections are trying to unite... and they can't. Chromosome O1+M2 is not formed, M1+O2 is not formed either. Because the maternal and paternal sections belong to different species and are too different from each other to connect into one chromosome.
That's why mules are infertile. Almost all other interspecific hybrids are the same. But there are exceptions. One of them is an edible frog (in honor of its ability to reproduce, it was even given a name like a real species: Pelophylax esculentus). These creatures, whose paws the French love to feast on, have found a wonderful way out of the situation.
Become a different species
The edible frog is a hybrid of the lake frog (P.ridibundus) and the pond frog (P.lessonae). Half of the chromosomes in the cell of such an amphibian belong to one species, and half to another. It would seem that this condemns crossing over to failure, and amphibians to infertility.
However, edible frogs successfully produce gametes. But... it's like it's not their gametes. The sperm or egg of such an amphibian contains either exclusively the genes of the lake frog, or exclusively the genes of the pond frog. The other half of the hybrid genome just disappears somewhere.
Biologists have been aware of this phenomenon for a long time. But only now they have figured out how animals manage to pull off such a trick.
How do they do it
Scientists from Russia together with colleagues from Poland have shown that the cells of the germ line of edible frogs gradually lose the chromosomes of one of the species, leaving the chromosomes of the other intact.
The researchers created hybrids by artificially fertilizing frog eggs. Then they tracked the composition of chromosomes in the KZL, checking it weekly.
Biologists have discovered that special structures – microkernels - are formed in the cores of edible frogs. There are no such components in the cells of either lake or pond frogs. Each microkernel contains one chromosome, and it is the one that will soon be lost.
Micronuclei forming in the cells of hybrid frogs (a) and their three-dimensional reconstruction (b). Illustration by Dmitry Dedukh.
"Selective utilization of chromosomes occurs, most likely, directly during the division of germline cells. Chromosomes are located in the center, or equator, of the cell to attach to the spindle of division, which distributes them between two daughter cells. But part of the chromosomes belonging to one of the parent species is lagging behind," explains the Grandfather.
Chromosomes that do not have time to attach to the spindle of division are doomed to death. They do not enter the nucleus of either of the two daughter cells. The genes working in them are trying to create a separate core around themselves with all the comforts. But they get only those microkernels that are defective and unable to support the work of chromosomes. Over time, these chromosomes die and are disposed of by the cell.
Such a fate always befalls only the chromosomes of one of the species. An edible frog living in a lake population "gives up for slaughter" only the chromosomes of its pond ancestors, and vice versa.
Scientists have yet to figure out how the amphibian "explains" to the cell which chromosomes it does not need. Such mechanisms are still known only for unicellular organisms (in particular, infusoria).
Subsequently, the germ line cell undergoes division after division, and gradually loses all the "pond" (or, conversely, all the "lake") chromosomes. When the "pond" chromosome dies, the paired "lake" chromosome doubles, and vice versa. Thus, by the time of puberty of the animal, its KZL contains DNA of only one species. This means that the chromosomes cross over without any problems, giving rise to gametes.
So an edible frog living among pond frogs produces pond frogs, and one living among lake frogs produces lake frogs. Among other things, this means that her offspring will not be eaten by the French, and is this not a reason for joy?
Why is it important
But biologists have other reasons for joy. Firstly, the mechanism of an interesting biological phenomenon has been clarified. Secondly, our understanding of the diversity of life has expanded. We repeat that a gradual "reset" of genetic material has never been observed in animals before.
Genetic engineers may also be interested in this topic. After all, the phenomenon discovered by the authors can give rise to new methods of genome editing that change not the genes individually, but the number of chromosomes.
Recall that breeders have long bred polyploid (that is, having not a double, but a triple or more set of chromosomes) varieties of cultivated plants. But in a certain sense, it was luck: experts took advantage of the results of random mutations. And if, thanks to the new discovery, the creation of such varieties will be put on stream, then edible frogs can be fed not only to lovers of French cuisine.
Help a person?
In humans, changing the number of chromosomes does not lead to anything good. It is enough to recall Down syndrome associated with the presence of an extra chromosome in cells. But, who knows, perhaps in the future biologists will teach human cells to take an example from frogs and get rid of unnecessary genetic burden during division.
The details of the study are described in a scientific article published in the journal Scientific Reports (Dedukh et al., Micronuclei in germ cells of hybrid frogs from Pelophylax esculentus complex contain progressively eliminated chromosomes).
The work was supported by a grant The Presidential Program of research projects of the Russian Science Foundation.
Portal "Eternal youth" http://vechnayamolodost.ru