On the evolution of ectomycorrhizal fungi

Ectomycorrhiza is a symbiosis between plants and fungi. It is the type of mycorrhiza that involves the highest number of fungal species and it involves many stand forming tree species. As the ectomycorrhizal trees include the majority of all the worlds tree stems, it has a huge impact on the nutrient and carbon cycles. Here, we review the evolution of ectomycorrhiza within fungi. It has evolved many times in different clades. The rate of evolution has not varied much through time, but has varied between clades. Thus, no time period seems to have been more important than others for the evolution of new ectomycorrhizal lineages. It seems like once becoming ectomycorrhizal the probability of becoming saprotrophic again is very low. Despite the large change in nutritional strategy, it does not seem like becoming ectomycorrhizal is a key innovation that by itself gives a high rate of speciation and/or adaptive radiations. Genomic studies have shown little commonality in the evolution of ectomycorrhiza except for the loss of decay genes, expansions of transposable elements, and expansions of Mycorrhiza-induced Small Secreted Proteins. Although this fits with a biotrophic lifestyle and possibly an elevated rate of genome evolution, it does not hint at any other functional similarity. As ectomycorrhizal lineages have evolved many times during the evolutionary history of fungi, and it is not unlikely that there are ectomycorrhizal lineages that have gone extinct, the available functions provided by ectomycorrhiza to plants may have varied over time.