???? These species systematically destroy much of their DNA

2023-09-07 11:00:07

Some animal species systematically destroy part of their DNA in their somatic cells. For what ? How ? This programmed elimination of the genome, discovered 150 years ago, remains mysterious due to a lack of suitable study means. An article to appear in the journal Current Biology describes a group of nematode species, genetically manipulable, for which almost a third of the genome (The genome is all the genetic material of an individual or a.. .) is eliminated, finally providing a model for studying this fascinating process.
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While in the majority of species sophisticated DNA protection and maintenance mechanisms exist, some species systematically excise and eliminate whole portions of their DNA in some of their cells. The cells passed on to the next generation, called germ cells (sperm and eggs), keep an intact genome. The other cells, called somatic, undergo this programmed elimination of the genome (called PDE in English for Programmed-DNA Elimination).

This programmed elimination phenomenon was first described more than 150 years ago, and has since been found in around a hundred animal species, belonging to very different branches (notably vertebrates such as the lamprey). It is therefore a process that has appeared several times during evolution. However, it remains to this day (The day or the day is the interval which separates sunrise from sunset; it is the…) mysterious because the species undergoing it are not genetically modifiable, not allowing experimentation (Experimentation is a scientific method which consists in testing by experiments…) in the laboratory. Thus, the scientific community (A scientist is a person who devotes himself to the study of a science or sciences and who…) still does not know how the targeted DNA is recognized, cut (A cut is a closed car, with two doors (sometimes three with tailgate or four as they have…), chromosomes removed and then physically destroyed, questions remain as to the role of such a mechanism and why certain species suffer from it and not others.

This phenomenon of programmed elimination of the genome has recently been found in all species of nematodes of the genus Mesorhabditis (belonging to the same family as the species (In the life sciences, the species (from the Latin species, “type. ..) Caenorhabditis elegans model) The scientists, in this article published in the journal Current Biology, then took advantage of the many experimental advantages offered by this model to describe the mechanisms of elimination of the genome.

By a combination of cytological and genomic approaches, they showed that the elimination takes place by a first phase (The word phase can have several meanings, it is used in several fields and…) of fragmentation of the chromosomes and the exclusion of certain fragments during cell division (Division is a law of composition which associates the product of the first by…) with two numbers in the somatic precursor cells from the beginning of embryonic development, at the stage (A stage (from Ancient Greek στ?διον stadion, from the verb…) where the embryo (An embryo (from Ancient Greek ἔμϐρυον / embruon) is…) still has only 5 cells. The fragments excluded, then destroyed, mainly contain non-coding repeated sequences, but also a few hundred genes coding for proteins. In total (Total is the quality of what is complete, without exception. From an accounting point of view, one…), it is about 30% of the genome that is eliminated from the somatic cells of these species.

Unexpectedly, the scientists showed that two close species of Mesorhabditis do not eliminate the same repeated sequences and do not eliminate the same genes. The genes eliminated in these species are generally very little conserved during evolution, suggesting that they have no essential role for the proper functioning of cells and for embryonic development. These results lead us to propose that the main function of the programmed elimination of the genome is to eliminate repeated elements and that when genes are eliminated, only the genes invisible to selection, because they are useless in the soma, can be eliminated.
An embryo in the process of DNA shedding is shown at the top of the image, the DNA is in white. In some cells (enlarged on the right), the DNA that is about to be eliminated (in red) is expelled from the nuclei (white dotted lines) during cell division. The germ cell (in yellow) does not undergo any fragmentation of its DNA. The embryo will give birth to a worm (Worms constitute a very heterogeneous group of invertebrate animals…) whose germ cells (in yellow) have intact chromosomes (still containing the red DNA) while the somatic cells ( in beige) have reduced chromosomes (having lost the red DNA).
© Marie DELATTRE