September 21, 2012 | 19
Amanita mushrooms — like all creatures — rot, but most of them can’t rot other things.
The fact that they don’t rot other things is not news to biologists, who have long known that many, if not most, fungi have become professional partners with trees, plants, or algae.
The fact that they can’t rot other things — as reported in July in PLoS ONE — is news, and provides a clue to how symbiotic partnerships can withstand the temptations of leaving and the sometimes dissonant interests of their symbiotic partners.
Amanitas count among their numbers some of the most beautiful and deadly mushrooms on Earth. How would you recognize an Amanita in the woods?
In general, they have sac- or cup-like enclosures called “volvas” from which the young mushrooms emerge. Pieces of this volva often stick to the cap of the mushroom, giving them the characteristic “warts” (or sometimes a little hat) that these fungi are famous for.
Amanitas typically also have a tall, graceful form, often with beautiful striations around the edge of the cap. For a sense of the beauty and diversity in this family, watch this:
Among the Amanitas are some mushrooms that can kill you with a few bites, like the pure white, eloquently-named “Destroying Angel”, Amanita virosa et al., and the equally lethal “Death Cap”, Amanita phalloides (which was likely introduced to the US on nursery rootstock). But the family also contains other mushrooms both reputedly delicious and consumed since antiquity, like the “Caesar’s Mushroom”, Amanita caesarea.
The Amanita family also includes some of the best-known tree-partnering fungi on Earth. Many of the mushrooms in this family are mycorrhizae — fungi that coil themselves in and around the roots of trees.
The tree provides them with food it makes topside in return for a vastly improved underground absorptive network. This network, made by the many searching filaments of the fungus, brings much more water and many more minerals to the tree than it would otherwise be able to procure for itself.
Symbioses are widespread on Earth. In fact, they seem to be the rule. In fungi alone, mycorrhizal symbioses seem to have evolved from decaying fungi on at least 11 occasions. Eleven.
But big questions have persisted. How do such living arrangements come to be? Fungi are generally supposed to have evolved from ancestors who were decayers, but mycorrhizal fungi generally lack the ability to decompose. Did the loss precede shacking up with trees, or did it follow it?
Further, what maintains symbioses, how often do symbioses disintegrate, and is it possible for the microbial symbionts to revert to their former lifestyles? For instance, might it still be possible to grow mycorrhizal fungi on leaf litter, or are they really and truly inseparable BFFs with the tree? If so, what keeps them locked in the marriage?
In order to answer those questions, scientists needed a large group of fungi that included both mycorrhizal and saprotrophic species. By studying the pattern of change in the DNA of dozens of different Amanita species, the team from Harvard University and the New York Botanic Garden determined that symbiosis has only arisen once in the Amanitas from decaying ancestors. All the symbiotic Amanitas fell into a single related group, while all the saprotrophic species fell into another, more ancestral group.
They first checked to see if Amanitas still make the enzymes that free-living decay fungi make to eat leaf litter. They looked specifically at three genes involved in one cellulose digestion pathway (fungi may have several redundant pathways). Cellulose is a major component of plant cell walls and a difficult molecule to digest. Fungi are among the only macro-organisms on Earth that have managed to do so.
The mycorrhizal Amanitas, the scientists found, have lost at least two major enzymes needed to start the break down of cellulose. They retain, however, an enzyme that helps convert small pieces of the polymer cellulose to its monomer glucose — perhaps a sign that the fungi are still content to help themselves to the leftovers of decomposition by sloppy neighboring fungi and microbes, even if they aren’t able to start the feast themselves.
Next, they asked whether Amanitas could actually subsist on dead stuff by testing Amanita‘s ability to grow on sterilized forest litter. Of nine mycorrhizal Amanitas, none could survive on dead plant bits, while all three saprotrophic Amanitas tucked in just fine.
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.
The authors suggest that the latter may be the case via “relaxed selection” — that is, when there’s no cost to offspring to allowing mutations to accumulate in genes whose products are already being provided by a host. I’m speculating here, but there may even be positive selection for individuals who don’t waste their energy making proteins they no longer need.
But not all mycorrhizal fungi are the same. Other mycorrhizal fungi have retained the same cellulose-degrading enzymes that Amanitas seem to have happily lost. The European black truffle and a small, lilac-peachy common forest mushroom called Laccaria bicolor, though both mycorrhizal, retain their versions of the cellulose breakdown enzymes lost by the mycorrhizal Amanitas. The black truffle expresses it extensively in the tree roots where it lives, implying both that it has an important role in symbiosis, and that different mycorrhizal fungi — having evolved seperately many times — have different biochemical relationships with their host trees and different methods that sustain them.
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.
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