Sweet-toothed Fungus

Ash bolete, with its eccentric stem partially emerging from the ground - Mark Bower

I came across these mushrooms while reading Fascinating Fungi of the Ozarks,*  It looks like a giant clam coming out of the ground but is actually an ash bolete fungus with the catchy name of Boletinellus merulioides. Other sources list it as Gyrodon meluliodes, which I will use, as the abbreviation GM sounds better that the alternative BM.

   Wikimedia

GM only occurs with ash trees but unlike other fungi that are either saprophytic (eating dead wood) or mycorrhizal (sharing nutrition with the plant's roots) this one has a sweet tooth.  It typically has an off-center or even lateral stem.  Rather than invading the ash tree for nutrition, it wraps its mycelium into tiny cups called sclerotia which fungi use to store food reserves.  

  GM sclerotia containing aphids  - Mark Brundrett

PF aphid -  Claude Pilon

In the case of GM, its sclerotia are protecting aphids.  These are not just any aphids but Prociphilus (Meliarhizophagus) fraxinifolii (PF), the "leafcurl ash aphid."  They are specialists that attack only ash trees. 
They feed on the ash trees and and can cause significant damage. They also produce honeydew that is the source of food for the GM.  In trade, the bolete provides the aphids shelter.  

Honeydew is produced by aphids, some scale insects and even the caterpillars of Lycaenidae butterflies.  Some species of ants farm aphids, protecting them from harm in exchange for their honeydew.  An article in Wikipedia explains, "When their (aphids') mouthpart penetrates the phloem, the sugary, high-pressure liquid is forced out of the gut's terminal opening," an icky fact that doesn't seem to bother the fungus.  That gives new meaning to the saying "no guts, no glory."

Exactly how the fungus manages to find the tree and the aphid is unknown but it manages to travel, including to China and Europe, (possibly in an international invasive species exchange program?)  As Mark Brundrett pointed out to me while giving permission to use his photograph above, "I think this fungus may become extinct along with its host tree due to ash borer." On the other hand, it might just possibly survive in its invaded lands.

* Fascinating Fungi of the Ozarks, Mark Bower, 2015.