Herbivory tolerance and coevolution: an alternative to the arms race?

Commentary The surprising diversity of ascomycetous mycorrhizas Aside from a few well-characterized types such as Cenococcum geophilum , most ascomycetous mycorrhizas, with their typically thin mantles and sparse emanating hyphae, have received less attention than their more robust basidiomycetous counterparts. As a result, we know much less about the taxonomy and ecology of these fungi, and we have undoubtedly underestimated their importance in mycorrhizal systems. The paper by Tedersoo et al. (pp. 581–596) in this issue confirms many suspected, and describes several new, ascomycetous mycorrhizas in the Pezizales based upon morphotyping and sequencing of ectomycorrhizal root tips from forests in Estonia and Denmark. As well as providing detailed morphological descriptions and photomicrographs, they have inferred phylogenetic affinities based upon DNA sequences from roots and fruiting bodies, and described ecological characteristics. These ascomycetous mycorrhizas were a significant component of the ectomycorrhizal community, comprising 33 species and colonizing between 4.5 and 6.1% of the root tips, further confirming the high diversity of pezizalean mycorrhizas in northern forests. 'Clearly, looking only through the basidiomycetous lens greatly restricts our appreciation of the physiological and ecological diversity of ectomycorrhizal fungi.' The mutualistic paradigm: is bigger better? It is somewhat ironic that the first rigorous study of ectotrophic mycorrhizas, by none other than A. B. Frank, was stimulated by an interest in truffle cultivation. It was Frank who proposed the term mycorrhiza, and who championed the concept that these fungi were beneficial to their hosts. After Frank's pioneering work and the acceptance of the mutualistic nature of mycorrhizas, although truffles remained a research priority because of their economic value, the focus quickly shifted to the study of basidiomycetous ectomycorrhizas. The paradigm of mutualism that developed from the time of Frank until the 1980s was that mutualisms evolve to 'benefit the association' (Law, 1985) rather than the individual interests of each partner. An implicit assumption stemming from this paradigm was that bigger and more robust mycor-rhizas must be providing bigger and better benefits to their hosts. With their prominent mantles and conspicuous extramatrical mycelium and mycelial strands, basidiomycetes seemed the most likely representatives of highly evolved mutualistic associations, providing maximum benefits to their hosts in the search for nutrients. If bigger is better, then mycorrhiza researchers can be excused for assuming that the thin-mantled and sparse ascomycetous forms were less important and therefore not a priority for study. By the turn of this century, the concept of …