| Fungal endophytes play an important role in mediating many ecological processes within their community, essentially providing an extended phenotype for their hosts. Although endophytes have been found in every plant species examined to date, the extent of diversity and symbiotic function in natural forest habitats is poorly known. In this study, we investigated the ecological context of these symbiotic microbes by examining and characterizing the fungal endophytes of a forest tree species, Pinus monticola. A total of 2013 fungal endophytes were isolated from 750 surfaced-sterilized needles and 16 endophytes were isolated from 800 surfaced-sterilized seeds sampled. Sequencing of the ITS region from a selection of the endophytes revealed 82 unique phylotypes, representing at least 21 discrete, novel taxa and 10 different orders of fungi. The most abundant taxa isolated were rhytimataceous endophytes. Transmission of the endophytes was found to be predominantly horizontal, and, in addition, the high frequency of endophytes isolated was not attributable to propagation of one genetic clone suggesting multiple colonization events. No evidence was found to support the hypothesis that the endophytes of P. monticola are merely cryptic or latent versions of known parasites of this same tree species, as often claimed. Instead, they were found to be most closely related to parasites of other species of Pinus. Distinctiveness of the fungal endophytes from their closest relatives was suggested by any one or all of the following: morphological distinctiveness, extent of sequence divergence in the ITS region, or allopatry. A mutualistic interaction between the fungal endophytes and their hosts was revealed. Host plants challenged with the pathogen Cronartium ribicola were found to have prolonging survival when previously inoculated with fungal endophytes. Our findings, combined with the ubiquity and diversity of endophytes in all plants studied to date, suggest that fungal endophytes may be an important, but widely unrecognized, component of forest communities. |
