According to this article in Scientific American, scientists have discovered that mycorrhizal Amanita mushrooms (those that live in a symbiotic relationship with trees) – while having evolved from saprotrophic fungi (which process decaying organic matter) – can no longer rot other things for their food source. They are now inextricably bound to trees.
“So the mycorrhizal Amanitas tested, at least, have irreversibly lost the genes needed for feeding themselves on their own. Their host trees have become, quite literally, their sugar daddies. What they cannot determine from their data, the scientists wrote, was whether the loss of the ability to digest cellulose preceded or followed the symbiotic living arrangement…..Can symbiotic fungi ever “escape” their lot in life, if conditions warrant? This study suggests that, for Amanitas at least, the answer is no. Still, it’s hard to feel too sorry for them. In making their deal with trees, they surrendered their freedom for some cozy digs and three squares a day. Though symbiosis may be a trap, it’s a gilded cage.”