Three mushrooms known as the Destroying Angel, the Deadly Prowess, and the Mourning Bell all have something in common: the fabulously deadly toxin alpha-Amanitin. If you eat one of these mushrooms, the symptoms may not occur for several hours. But soon the toxin begins wreaking havoc on your body’s ability to transcribe genes. By the fourth day or so after consumption, your liver and kidneys begin to fail. You may die after about a week.
This uncanny lethality has a mystery at its core: These mushrooms are from three separate genera or groups of fungal species that aren’t closely related. How did they come to make the exact same poison?
In an article published Monday in the Proceedings of the National Academy of Sciences, scientists who sequenced the genomes of 15 species of mushrooms from these three groups make an intriguing claim: the genes to make alpha-Amanitin, rather than to are inherited from a shared ancestors of these groups, were transferred directly to them from an unknown, probably extinct mushroom.
This kind of gene transfer, called horizontal gene transfer, is common in bacteria, said Hong Luo, a researcher at the Kunming Institute of Botany in China and an author of the new paper. Small pieces of DNA are passed from one microbe to another and then passed on to their offspring. However, there is mounting evidence that genes can somehow also move between complex, multicellular beings, perhaps with the help of pathogens. In April, another group of scientists reported that genes had been moved between snakes and frogs living in the same forest habitat by piggybacking on shared parasites. It sounds bizarre, but it can help explain other mind-boggling observations in the tree of life.
The team behind the fungal paper already suspected that horizontal gene transfer had created the identical toxins in these mushrooms. However, there were some surprises when they completed their research. They had expected that their look at the mushroom genetics would confirm that one of the groups had passed on the genes to the others. Instead, the gent toxin clusters all seemed equidistant from their origin.
“It surprised us,” said Dr. luo.
While discussing it, the paper’s authors concluded that the simplest explanation was that horizontal gene transfer had occurred — but not necessarily between these three groups.
“Then we started thinking that there must be another, possibly extinct species,” said Francis Martin, a scientist at the French National Research Institute for Agriculture, Food and Environment and an author of the paper.
This long-ago mushroom would have had the genetic toolkit to make the toxin and passed it on, in some unknown way, to the surviving varieties. The affected mushrooms are not their offspring — just the carriers of a small packet of its genes, released as a message in a bottle, that gives the fungi their extraordinarily toxic powers.
Scientists may never know much about this proposed donor for the toxin genes, if it existed at all. But the researchers are curious as to why these three groups, of all mushrooms, inherited and used it. Do the toxins play a special role in the ecology of these particular mushrooms? Or are the mushrooms just particularly good at any mysterious techniques for bringing genes from the environment into their own genome?
As scientists learn more about how horizontal gene transfer goes beyond bacteria, some of these answers may become clearer.
“We know it’s happening,” said Dr. Martin, “but we don’t know how.”