This article is taken from the monthly Sciences et Avenir – La Recherche n ° 908, dated October 2022.
Here comes the return of mushroom hunting. Thunderstorms following a long summer drought have already cheered people up by encouraging the growth of Bordeaux boletus since September. But be careful! The National Agency for Health Security (Anses) is used to cooling the heat this season.
Every year one thousand people in France are victims of poisoning linked to the consumption of mushrooms: on average, thirty are seriously ill and three die. This is why understanding where these toxins come from is a public health concern. Part of the veil has just been lifted by a French-Chinese team on the origin of amanitin, the substance that attacks the functioning of the kidneys and liver in humans.
A study that is the result of boring work
Amanitin has long been recognized as a deadly toxin. “But what is surprising is that it is present in the amanita phalloides as well as in the galea marginata and in the crested lepiote, three common forest mushrooms in our woods, which are genetically very different.“Explains Francis Martin, researcher at the National Agronomic Research Institute (Inrae) in Nancy and co-author of the study.
The authors of the article, published this year in the journal PNAS, compared the genome of 15 toxic fungi to identify the genes responsible for the production of the different forms of amanitin and to characterize the enzymatic activity driven by these genes. A tedious job, which lasted nearly a decade, but which produced promising results: four identical genes could be identified in all deadly species.
The degree of toxicity, which for example explains why amanita phalloides is much more dangerous than other members of its genus, is related to the number of copies of these genes. “They are present in dozens of copies in this mushroom. However, the more copies there are, the more abundant and dangerous the toxin produced is, according to the principle that it is the dose that makes the poison “, Continues Francis Martin.
How is it possible that such different fungal species carry all the genes that code for the amanitin biosynthetic pathway? The usual evolutionary scenario would make them all descend from a common ancestor. Impossible here, because the divergences in the evolutionary history of these species are too great. The authors then put forward another hypothesis: a horizontal transfer of all these genes between fungi that share the forest floor. In other words, as they got closer to each other, the species swapped genes and passed them on to their offspring.
Researchers are discovering more and more cases in nature where genes travel between sometimes very different occupants of the same ecological niche. Scientists have recently even described the genetic exchanges between snakes and frogs in a forest habitat, or between marine molluscs and the algae they feed on; Transfers have also been observed on flowering plants, in particular by grafting.
“We know that these exchanges between different species exist, but the genetic mechanisms involved are unknown. The hypothesis is that 40 million years ago, a now extinct mushroom passed these genes to its neighbors through viruses, transposons or bacteria “, explique Francis Martin. Les champignons les ont adoptés et transmis au fil des générations jusqu’à nos jours, ce qui indique qu’ils leur donnaient un avantage évolutif. to feed.
3 to 5 million species are still unknown
The visible fungus is only the fruit of an underground organism consisting of filaments, the mycelium. This growth serves to spread the spores that will allow the species to conquer new spaces. But these spores must reach a certain maturity before being disseminated and fertilized. The toxin could therefore discourage greedy predators. It’s just a guess.
In 2022, a Chinese team listed 643 toxic species worldwide belonging to 51 families and 148 genera, which demonstrates the wide distribution of these toxins. “The number of edible species is estimated at 300; therefore toxic and edible mushrooms are a very small minority compared to the 100,000 species described by science that are neither good nor dangerous, not to mention the 3-5 million that are still unknown to us. “Francis Martin agreed.
Unlike poisonous mushrooms, the truffle emits odors that attract animals to dig it, thus favoring the dispersion of its spores. These different strategies that mix odors, toxins, colors, shapes are still largely unknown to us and difficult to understand as it is difficult to observe in its natural environment organisms made up of thin filaments of several tens of meters that allow their symbiosis with trees. . The mushroom picker certainly frequents a place full of mysteries.
