An image provided by Johann Bruhn shows Armillaria gallica mushrooms growing on a decomposing log in Michigan. They are part of a single giant fungus that is older and larger than previously known.JOHANN BRUHN/The New York Times News Service
Scientists first reported finding it in 1992: a giant mushroom that weighed as much as a blue whale and sprawled across more than 12 hectares of forest in Michigan’s upper peninsula. It wasn’t some Alice in Wonderland-type of toadstool but a 1,500-year-old parasitic mould, with growing tentacles that foraged beneath the soil for roots and decaying wood to devour.
More than 30 years later, the same scientists – using new technology for genetic analysis – wanted to know whether they had properly measured this unusual example of fungal life.
“We made this outlandish prediction that the fungus is more than 1,000 years old,” said James Anderson, now a retired mycologist and emeritus professor at the University of Toronto. “And so an obvious outcome of that, is after three decades, it ought still be there, and if not, we’d have some explaining to do.”
Recently they published what they uncovered in Proceedings of the Royal Society B: Biological Sciences. Their original humongous fungus, Armillaria gallica, is even older and bigger than first estimated: the 2,500-year-old parasite spreads across 75 hectares of forest. And its genome harbours a mysterious survival strategy: an extremely low mutation rate.
From 2015 through 2017, Prof. Anderson and his colleagues tested soil from nearly 250 sites on the peninsula. They connected dots on a forestwide canvas and painted an impressionist portrait of this monster beneath the dirt.
And it had some surprising features.
First, it covered more space than first measured.
Second, based on observations of how much it grew over a season, the scientists figured the fungus had to be 1,000 years older than they had originally estimated.
And when they began considering that age against their genetic analysis, something seemed strange.
As an organism grows and cells start splitting and copying DNA during mitosis – which is how they make new, identical body cells – you expect to see mutations emerge in copies that are passed on from one generation to the next. But this old beast harboured only about 160 mutations, orders of magnitude lower than expected.
More than two millenniums was plenty of time for cells to divide, copy and paste their DNA and send it – mistakes and all – from one generation to the next. But to get so few mutations, the fungus must have had very few cell divisions, which is crazy for a giant fungus made of microscopic cells. The researchers couldn’t measure how many cell divisions separate the bits of fungus spanning the length of nine football fields side by side, and so they couldn’t measure the mutation rate directly. It should have been huge, but it wasn’t.
“I think it’s a really interesting result with cutting-edge technology, and it opens up new questions about how organisms can remain stable over that length of time,” said Tom Bruns, a fungal ecologist at the University of California, Berkeley, who reviewed the study.
But he and Prof. Anderson agree that it’s still unclear how the fungus genome ended up this way.
Their paper offers some speculation. The infecting tips of the fungi’s rhizomes could have low rates of cell division. Or the fungus could be really good at repairing damage inside its cells, passing healthy traits onto the next generation. Even more bizarrely, the cells may be selective about which copies of DNA they send on to the next generation. Maybe it’s a combination of these factors – or something else entirely, Prof. Anderson said.
Prof. Bruns said it was also possible that the analysis missed some mutations. When they do show up within a big pool of cells, they’re so rare, they’re presumed to be errors.
But if this extremely low mutation rate is indeed the case – and it seems to be, according to Prof. Bruns, it poses other interesting questions.
How widespread among fungi and other life is this low mutation rate? What can it tell us about cancer, which seems to be on the opposite end of the genetic-stability spectrum? And if this thing is so good at living, who wins in an apocalypse: the cockroach or Armillaria?
“I’ll bet on Armillaria,” Prof. Bruns said.