Mycorrhizal associations of an endemic terrestrial orchid

Background/Question/Methods Extending from southern Oregon into northern Mexico along the Pacific Coast in North America, the California Floristic Province is a zone of mediterranean-type climate with high plant endemism, where urbanization, commercial farming, and other human activities are identified as serious threats to the persistence of natural biodiversity. Among the endemic species is a terrestrial, federally endangered orchid, Piperia yadonii, which is restricted to a single county. Considering the importance of fungi in the life cycle of orchids, the persistence and distribution patterns of P. yadonii may be dependent on the distribution of suitable fungi in its habitats. We improved our previously used methods by amplifying the nuclear ribosomal ITS region with the specific orchid-fungus primers ITS1-OF and ITS4-OF. We identified the endomycorrhizal fungi associated with this species across its distribution range. Roots of adult plants were collected from eight populations to sample peloton-containing sections. PCR products yielding single bands were sequenced birectionally. Consensus sequences were constructed and similarity searches were conducted through the web portal at http://www.borealfungi.uaf.edu/.

Results/Conclusions

Across all populations, approximately 60% of the sequences belonged to Ceratobasidiaceae while the rest fell within the Tulasnellaceae. At least a third of the Tulasnellaceae sequences were closely related to Tulasnella sp. isolates known from adult plants of Tipularia discolor, a terrestrial orchid with a relatively wide distribution in the southeastern U.S. Another third were related to a Tulasnella calospora isolate previously know to occur in roots of several epiphytic orchids of Andean cloud forests. The fungi in the Ceratobacidiaceae most closely allied with Ceratobasidium obscurum, but isolates from several populations formed distinct clades regardless of the similarity of their habitats, i.e. chaparral or Monterey pine forest. A similar trend was observed in the Tulasnella sp. isolates, which segregated mostly by population but not by habitat type. Our results indicate that while certain populations associate with fungi belonging to both fungal groups, specific fungal isolates from each family are restricted to individual populations. Piperia yadonii appears to associate with narrow clades in both fungal families, suggesting complex but significant specificity.