The first time I came across Boletinellus merulioides, I was rather perplexed. I was just really getting into mycology as an undergrad, so I was a beginner mushroom identifier at best. At the time, I was getting familiar with just the major orders of fungi so you can imagine my confusion when looking at a species without true gills or pores. This species holds a special place in my mycological heart because it was my first to properly identify, and it also carries out quite an extraordinary ecology.
I found this species gregariously scattered across the forest floor heavily populated by ash saplings. This was a key identifier, as its common name suggests-the ash tree bolete. After having the cap of one specimen sit on some notebook paper for an hour, an olive green spore print emerged that confirmed my initial presumption. Boletinellus merulioides, kind of rolls off the tongue, doesn’t it?
As soon as I read it associated with ash trees, and that it was in fact a bolete, I assumed it was mycorrhizal. Even though I finally identified my first mushroom, I was swiftly knocked off my high horse when I found out that this fungus does receive plant sugars, but not directly. This is not a mutualism involving two species, but a more complex tripartite relationship.
Up until 1987, Boletinellus merulioides was thought to be mycorrhizal. Even if you isotopically trace the sugars generated in the plant, you will find those same sugars in the fruiting body. This is a testament in how using new technologies can still be misused, and highlights the importance of good old-fashioned morphological studies. Just because the fungus receives plant sugars from ash trees, doesn’t mean it is receiving them directly through a mutualism or parasitism. As it turns out, the fungus is forming a mutualism, but not with the ash tree.
A few rock stars in the field of mycology, Mark Brundrett and Bryce Kendrick didn’t agree with this mycorrhizal categorization. Ash trees utilize arbuscular mycorrhizal fungi (AMF) and even I know that AMF doesn’t produce fruiting bodies like ectomycorrhizal fungi. What they found and published in 1987 was that the fungus created underground tissues around the roots of ash trees. When Mark and Bryce looked inside the fungal sclerotia, they found dense groupings of Meliarhizophagus fraxinifolii, aphids that specialize in feeding on ash roots and leaves.
So, in return for housing the aphids in melanized sclerotia, the fungus receives sugars from the feeding aphids. Aphids that consume more than they metabolically require excrete excess sugars in the form of honeydew. This physiological response to excess energy has driven some crazy mutualisms across terrestrial habitats for millennia. I knew that ants actively ‘farm’ aphids to collect a sugar reward and protect them in return, but we now know fungi do the same. Boletinellus merulioides and the species it engages with really do show the complexity of life living from the forest floor.