In the late 1980s, a team led by James Anderson of the University of Toronto Mississauga discovered that a single honey mushroom (Armillaria gallica) occupied at least 37 hectares (91 acres) of forest in the upper Michigan Peninsula thanks to its enormous underground network of fungal fibers. Writing in Nature in 1992, the scientists proposed that the remarkable colonial organism – later dubbed the “humongous fungus” – was one of the world’s largest and most ancient organisms, estimating it to be at least 1,500 years old and 100,000 kilograms (220,462 pounds) in mass.

Now, over 20 years later, Anderson and his original co-authors have an update. Based on field surveys and advanced genetic analyses conducted between 2015 and 2017, they believe that the mushroom, known as C1, covers 70 hectares (173 acres), is at least 2,500 years old, and weighs 400,000 kilograms (880,000 pounds).

"I view these estimates as the lower bound," Anderson said in a statement. "The fungus could actually be much older. However, we think we have circumscribed its entire dimensions, which wasn't the case in 1992."

Found in temperate forests of North America, Asia, and Europe, Armillaria gallica has evolved to act as both a parasite and saprophyte of wood tissue, meaning it begins its feast by infecting a living tree and continues gaining nourishment until it breaks down the last remaining nutrients in its dead host. To find new food sources, the ambitious fungus continually weaves through the soil by growing networks of mycelium that are organized into larger, root-like aggregations called rhizomorphs. When rhizomorphs encounter wood, their hyphae (branching filaments) secrete enzymes that break down plant cell walls and chemicals that suppress the host’s immune system.

But verifying C1’s biographical details with modern technology was not the group’s only goal. The new investigation also sought to compare C1’s genome to fungal reference genomes and explore how fast mutations arise in its cells. The latter subject is particularly interesting given that the organism must clone itself repeatedly to reach its enormous size, yet despite the many opportunities for genetic mishaps to occur, it stays healthy for centuries.

According to their findings, currently available on bioRxiv, A. gallica individuals have incredibly stable genomes, and even when mutations occur, they rarely impact the fitness or appearance of subsequent mycelium generations.

“What we think that tells us is that there must be some mechanism by which the fungus protects itself from mutations," Anderson said.

In addition to any potential DNA repair tricks, he speculates a combination of other several factors may come into play. First, besides its fruiting bodies – the above-ground growths we think of as mushrooms – A. gallica exists entirely underground, where it is protected from mutation-causing UV radiation. Second, the rhizomorph tip is propelled forward through the soil in a way that necessitates the least possible amount of cell division, and third, it is possible that the fungus has a strategy for how DNA stands are distributed after they are replicated.

“In Armillaria, this would mean that cells in the rhizomorph tip would retain the old DNA, whereas the subtending cells (committed to local, dead-end development) would receive the new DNA," the team wrote. "In this way, the rhizomorph tips perpetuating the lineage would retain fewer mutations than cells committed to local differentiation.”

They conclude that it would be interesting to study the fungi's DNA maintenance processes to gain insights into cancer, but this will have to be done without Anderson. The emeritus biology professor retired after this research was complete.