Fungi took on a deadly toxin from a mysterious source

Three mushrooms known as the destroying angel, the deadly dapperling, and the funeral bell have one thing in common: the fabulously lethal toxin alpha-amanitin. If you eat one of these mushrooms, symptoms may not appear for several hours. But soon enough, the toxin begins to wreak havoc on your body’s ability to transcribe genes. Around the fourth day after consumption, the liver and kidneys begin to fail. After about a week, it may die.

This staggering lethality has a mystery at its core: These fungi are from three separate genera, or groups of fungal species, that are not closely related. How did they come to make the exact same toxin?

In a paper published in the Proceedings of the National Academy of Sciences on Monday, scientists who have sequenced the genomes of 15 fungal species from these three groups make an intriguing claim: The genes for making alpha-amanitin, rather than being inherited from an ancestor of these groups, they were transferred to them directly from an unknown fungus, probably extinct.

This type of gene transfer, called horizontal gene transfer, is common among 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, mounting evidence suggests that genes can also somehow move between complex multicellular creatures, perhaps with the help of pathogens. In April, another group of scientists reported that genes had moved between snakes and frogs living in the same forest habitat by traveling on shared parasites. It sounds outlandish, but it might help explain some puzzling observations on the tree of life.

The team behind the mushroom paper already suspected that horizontal gene transfer had created identical toxins in these mushrooms. However, there were some surprises when they completed their investigation. They hoped that their glimpses into the genetics of the fungi would confirm that one of the groups had passed the genes on to the others. Instead, the genetic toxin clusters all seemed equidistant from their origin.

“It baffled us,” Dr. Luo said.

Speaking of which, the paper’s authors decided that the simplest explanation was that horizontal gene transfer had occurred, but not necessarily between these three groups.

“That’s when we started to consider that there had to be another possibly extinct species,” said Francis Martin, a scientist at France’s National Research Institute for Agriculture, Food and the Environment and an author of the paper.

This long-ago fungus would have possessed the genetic toolkit to produce the toxin and transmitted it, by as yet unknown means, to still-living varieties. The affected fungi are not his descendants, merely the carriers of a small package of his genes, released like a message in a bottle, which gives the fungi their extraordinarily poisonous 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 fungi, received and used his legacy. Do toxins play a special role in the ecologies of these particular fungi? Or are fungi particularly good at some mysterious technique that brings genes from the environment into their own genomes?

As scientists learn more about how horizontal gene transfer works beyond bacteria, perhaps some of these answers will become clearer.

“We know it happens,” Dr. Martin said, “but we don’t know how.”

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