From “As Mushrooms Evolve to Live With Trees, They Give Up DNA Associated With Decomposing Cellulose” (ScienceDaily, July 18, 2012), we learn,
Armed with their family tree, Pringle and Wolfe were able to determine that Amanita evolution has largely been away from species that help decompose organic material and toward those that live symbiotically on trees and their roots. More interestingly, they found that the transition came at a steep price — the loss of the genes associated with breaking down cellulose.
“There had been earlier suggestions that this type of gene loss might be taking place, but our study is the first precise test of that hypothesis,” Pringle said. “The idea makes sense — if you’re going to actively form a cooperative relationship with a tree, you probably shouldn’t simultaneously be trying to break it apart and eat it. But it’s a very tricky dance to form these kinds of tight, cooperative interactions, and I think this work shows there is a cost associated with that. You have to change, you have to commit, and it can become a sort of gilded cage — these mushrooms are very successful, but they’re stuck where they are.”
Curiously, it’s always called “evolution” and it should be called “devolution.”