A community of unknown, endophytic fungi in western white pine

Abstract

The endophytic fungi of woody plants may be diverse as often claimed, and likewise, they may be functionally novel as demonstrated in a few studies. However, the endophyte taxa that are most frequently reported tend to belong to fungal groups composed of morphologically similar endophytes and parasites. Thus, it is plausible that endophytes are known (i.e., described) parasites in a latent phase within the host. If this null hypothesis were true, endophytes would represent neither additional fungal diversity distinct from parasite diversity nor a symbiont community likely to be novel ecologically. To be synonymous with parasites of the host, endophytes should at least be most closely related to those same parasites. Here we report that seven distinct parasites of Pinus monticola do not occur as endophytes. The majority of endophytes of P. monticola (90% of 2,019 cultures) belonged to one fungal family, the Rhytismataceae. However, not a single rhytismataceous endophyte was found to be most closely related by sequence homology to the three known rhytismataceous parasites of P. monticola. Similarly, neither endophytic Mycosphaerella nor endophytic Rhizosphaera isolates were most closely related to known parasites of P. monticola. Morphologically, the endophytes of P. monticola can be confounded with the parasites of the same host. However, they are actually most closely related to, but distinct from, parasites of other species of Pinus. If endophytes are generally unknown species, then estimates of 1 million endophytes (i.e., approximately 1 in 14 of all species of life) seem reasonable.