Mycorrhizal association is a common characteristic in a majority of land plants, and the survival and distribution of a species can depend on the distribution of suitable fungi in its habitat. Orchidaceae is one of the most species-rich angiosperm families, and all orchids are fully dependent on fungi for their seed germination and some also for subsequent growth and survival. Given this obligate dependence, at least in the early growth stages, elucidating the patterns of orchid-mycorrhizal relationships is critical to orchid biology, ecology and conservation. To assess whether rarity of an orchid is determined by its specificity towards its fungal hosts, we studied the spatial and temporal variability in the host fungi associated with one of the rarest North American terrestrial orchids, Piperia yadonii. The fungal internal transcribed spacer region was amplified and sequenced by sampling roots from eight populations of P. yadonii distributed across two habitats, Pinus radiata forest and maritime chaparral, in California. Across populations and sampling years, 26 operational taxonomic units representing three fungal families, the Ceratobasidiaceae, Sebacinaceae and Tulasnellaceae, were identified. Fungi belonging to the Sebacinaceae were documented in orchid roots only at P. radiata forest sites, while those from the Ceratobasidiaceae and Tulasnellaceae occurred in both habitats. Our results indicate that orchid rarity can be unrelated to the breadth of mycorrhizal associations. Our data also show that the dominance of various fungal families in mycorrhizal plants can be influenced by habitat preferences of mycorrhizal partners.

Highly endangered plants that are also narrow endemics are generally found to be genetically depauperate and thus are exceedingly susceptible to ecological and anthropological threats that can lead to their extinction. Piperia yadonii is restricted to a single California county within a biodiversity hotspot. We used nine primers to generate intersimple sequence repeat (ISSR) data to assess its genetic diversity and structure. Within each population, 99% of the loci were polymorphic, expected heterozygosity was low, and a majority of the loci were shared with few other populations. Forty percent of the total variation could be attributed to population differentiation while the rest (60%) resides within populations, and the genetic distances between populations were independent of the corresponding geographical distances. High divergence among populations is likely due to fragmentation and limited gene flow. Each population contains several private loci, and ideally, each should be protected to preserve the overall diversity of the species. Because P. yadonii currently retains a modest amount of genetic variation among individuals within populations, preserving and expanding the habitat at each site to allow natural expansion of populations would be additional strategies for its conservation before populations become too small to persist naturally.

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