Daniel Molter (University of Utah), Bryn Dentinger (University of Utah)
Is the Humongous Fungus really the world’s largest organism?
‘World’s largest organism’ is often referenced in philosophy of biology, where it serves as something of a type specimen for the organism category, so it’s important to make sure the biological individual which holds this title really is one organism. The Humongous Fungus (HF), a 3.7 square mile patch of honey mushrooms (Armillaria solidipes) in Oregon’s Blue Mountains, is said to be the world’s largest organism. To determine if it really is will require both new empirical work (currently being planned) and philosophical clarification about what it means to be an organism. At question empirically is whether the HF is physiologically integrated; at question philosophically is whether physiological integration is necessary for organismality. Ferguson et al (2003) reported that all samples collected inside a 3.7 square mile patch were genetically homogenous and somatically compatible, indicating common descent from a single reproductive event and the potential to fuse into a single mycelium. Their results are consistent both with a single humongous mycelium and with a swarm of fragmented clones that periodically flair up and die out as they spread from tree to tree. Tests to see if the HF is all connected have not yet been done.
If “organism” is defined in terms of evolutionary individuality, then the HF does not need to be connected in order to function as a discontinuous evolutionary organism, but it would not be the largest discontinuous evolutionary organism; that title instead probably* goes to Cavendish bananas (the common yellow variety), which are clones of a single genet cultivated on millions of hectares around the world. If, on the other hand, organismality is defined in terms of physiological integration, then the HF would have to be continuous for it to count as one organism. Interestingly, the distinction between fragmented and continuous might be blurred if the HF periodically breaks apart and comes back together, as mycelia sometimes do. If the HF really is physiologically integrated, then it is the world’s largest physiological organism, beating out Pando, an aspen grove in Utah, and another Humongous Fungus in Michigan (yes, they fight over the name).
The first planned test for physiological integration involves sampling eDNA in soil along transects through the genet. This will tell us how far from infected trees the Armillaria extends, and it will help to locate areas of concentration that might represent physiologically isolated individuals. Further testing might include a stable isotope transplantation study to see if tracers absorbed by the mycelium in one region of the genet make their way to distal regions.
Ferguson, B. A., Dreisbach, T. A., Parks, C. G., Filip, G. M., & Schmitt, C. L. (2003). Coarse-Scale Population Structure of Pathogenic Armillaria Species in a Mixed-Conifer Forest in the Blue Mountains of Northeast Oregon. Canadian Journal of Forest Research, 33(4), 612-623.
* Other plants, such as dandelions, might also be contenders for the world’s largest genet.
