The largest living thing on Earth is not a blue whale or a giant sequoia tree. Instead, it is a specimen of Armillaria fungi that covers the area of a small city and weighs hundreds, or possibly thousands, of tonnes. When discovered in the late 1980s this amazing organism was quickly dubbed the "humongous fungus" and attracted something of a cult following. Now we're starting to understand the genetics that allowed it to get so big.
Armillaria are parasitic fungi. They can colonize more than 500 plant species, killing the host's roots and then spreading through the root tissue to form giant underground bodies, which reproduce through spore-producing honey mushrooms. They can also infect the lower trunks of trees, making their way under the bark, and blocking the movement of water and nutrients, eventually killing even the mightiest forest giant.
A team led by Dr György Sipos of the University of Sopron, Hungary, sequenced the genome of four of the numerous Armillaria species and compared them with genomes from 22 related fungal species to see what allows these few to grow so big.
In Nature Ecology and Evolution, Sipos and his co-authors report that these Armillaria have unusually large genomes for rotting fungi. Moreover, all of them, but particularly A. ostoyae, the species from which the humungous fungus comes, are packed with genes that enhance their capacity to infect trees. These include genes that produce enzymes for breaking down lignocellulose, allowing a fungus to eat away at the roots of the forest. These genes give their possessor access to dead wood before degrading microbes can digest it, offering a huge competitive advantage.
Armillaria have previously been shown to have mutation rates approximately a thousand times lower than other filament-producing fungi. And why not? When your genes are this well suited to allowing you to take over the world, why would you want to change them?
Although the name humongous fungus has also been given to other Armillaria that are giants by local standards, the original and biggest (that we know of) is in the Malheur National Forest, Oregon. The giant has continued to grow since it was first identified and today stretches for almost 10 square kilometers (4 square miles). Estimates of its weight vary from 550 tonnes (600 tons) to several times this.
The forest service has had samples taken throughout the fungus' range and DNA measurements confirm its status as a single living thing. From its rate of growth, the humongous fungus is estimated to be at least 2,400 years old. If this sounds terrifying, the good news is you can get your own back, because the mushrooms are safe to eat, although opinions vary as to their palatability.