It is found everywhere in the environment with a size of less than a millimeter. A rotifer is an extraordinarily complex animal. Being able to survive in extreme conditions is one of its characteristics. Absolute dryness, deep freeze, ionizing radiation: nothing bothered. It seems to keep itself ready when conditions require it, and resumes its life as if nothing had happened when conditions are most favorable to it.
A team of Belgian scientists under the supervision of Karine van Doninck (ULB-UNamur) together with Bernard Hallet (UCLouvain) studied the small rotifers. She made two major discoveries that challenged everything that had ever been written.
“This kind of invincibility is the first thing that makes the rotifer so interesting to scientists, explains Mathieu Dervagne, a doctor of biological sciences who participated in the research. But it is also true that its population is not made up only of females. In any case, until recently, it was thought that they cloned themselves to reproduce identically.
Rotifer, an evolutionary scandal
Since being studied, the rotifer has been considered an “evolutionary scandal”. How did this animal that existed for millions of years become hundreds of species by cloning itself? Mathieu Dervagne explains that “if a species is made up of perfect clones, in the slightest favorable event, the entire species will disappear. However, in the case of rotifers, it is quite the opposite. The animal adapts, it can survive changing conditions and there is a very high diversity of individuals.”
A specific meiosis for rotifer
To understand the following, we need to talk more deeply about sexual reproduction and its effects: bringing variation to the individuals of a species.
In sexual reproduction, both males and females produce gametes (in humans, sperm and egg) whose genetic content is halved. When they meet, the two gametes form the first cell of the “offspring,” which contains two halves of the genetic heritage. It keeps the same number of chromosomes in the species. This cell division is called meiosis. By combining cells with different genetic heritage, we create variation. But that’s not all. During meiosis, when the number of chromosomes is halved, they change slightly, with small exchanges between identical chromosomes from each parent.
“During our research, we discovered that rotifers retain some of the characteristics of meiosis, explains Mathieu Dervagne. During rotifer ovulation, homologous chromosomes come very close, then separate, but always stay together in the same cell. The meeting between a male gamete and a female gamete If not, there are changes at the level of the chromosomes. The exchanges take place in the same way as in a classic meiosis. Therefore, the offspring is not an exact copy of its mother.”
Where does rodiferin’s hyper-resistance come from?
The scientific team did not stop with this first discovery. She learned more about rotifer’s hyper-resistance. It is associated with this particular meiosis.
The extreme conditions in which the rotifer finds itself create breaks in its DNA. Its entire genome is fragmented. Rotifer can repair all of these breakdowns. But he doesn’t do it all the time.
“For somatic cells, all the cells of the body, except the reproductive system, repair their DNA very quickly, but not perfectly,” explains Mathieu Dervagne. “For the cells of the reproductive system, on the other hand, they wait. For the time being to repair meiotic breaks. This is when identical chromosomes join together, Using copy as such allows them to correct themselves as much as possible.
In books on biology and evolution, there is a change to add to the chapter on rotifers. “This does not prevent all mysteries from being solved, concludes Mathieu Dervagne. Which proteins regenerate DNA? Why are they not broken?” The research surrounding this tiny animal is far from over.