One Way Street to Mycorhizzal Mutualism

Perhaps the most recognizable mushroom genus in the world, Amanita, has a vast variation of special lifestyles. Some of them live free, growing wherever they can find nutrients; others live in symbiotic relationships with plants, tethered to their host.

Amanita species

A new study, published in the July 18 PLoS One Journal, demonstrates that those Amanita species that have become symbiotic with plants have truly lost their ability to exist independently.

The Irreversible Loss of a Decomposition Pathway Marks the Single Origin of an Ectomycorrhizal Symbiosis

The team looked at 100 different species in the Genus containing only around 600, and created an extensive phylogenetic tree to show their relation to one another. They focused on what happened to the genes allowing decomposition of plant materials as they constructed the tree.

A key feature of Amanita living saprotrophic lives is that they can efficiently transform cellulose in dead plant materials into simple sugars. However, ectomycorrhizal fungi get most of their carbon from the roots of their host plant, so they don't need to keep the ability to eat cellulose.  Keeping this in mind, the scientists monitored what happened to the genes associated with these skill sets as the family tree moved from those living freely, to those species that have entered into the bonds of Mycorrhizal marriage.

As it turns out, the genes required to make two out of the three enzymes needed to decompose cellulose are lost as the species become intertwined with their host. Further experimentation demonstrated that Amanita species that have developed symbiotic relationships have lost the ability to grow on complex organic matter without supplied carbon. This means they would be unable to live in forest soil without a feed from their host plant.

The possibility of fungi to move from saprophytes to mycorrhyzals and potentially return to the former is a subject that has had much debate. The team on this study believes themselves to be the first group to combine fine-scale phylogenetic, functional gene, and experimental data in regards to this question. Their determination is that even though Amanita species retained one of the enzyme production pathways for cellulose degradation, they would be unable to return to saprophytic lives due to the loss of the other two.

There are no backsies for Mycorrhizal Amanitas.



Study and Photo credit: Benjamin E. Wolfe, Rodham E. Tulloss, Anne Pringle via PLoS One