Orchids possess a complex, but poorly understood, relationship with their obligate mycorrhizal partners. The specificity of orchid mycorrhizal relationships and the availability of suitable mycorrhizal fungal are considered key factors influencing the distribution of orchids, whether it be the spread of potentially weedy orchids or the conservation of rare species. We examined the specificity of fungal endophytes associated with Microtis media, a terrestrial orchid endemic to southwestern Australia. In natural systems, M. media rarely dominates the plant community, but in disturbed sites, M. media grows in profusion and is removed manually or by herbicide application. Because the degree of specificity in orchid mycorrhizal associations depends on the ecological coincidence of the orchid and fungi, we hypothesized that M. media has fairly low specificity and associates with a range of compatible fungal endophytes and habitats. Alternatively, M. media may associate with a very specific, but broadly distributed, endophyte that confers a competitive advantage in a variety of environments. To test these hypotheses, DNA nLSU sequence variation was surveyed in pelotons isolated and cultured from the roots of M. media collected from differing geographical locales and habitat types, including undisturbed natural systems, anthropogenically- altered sites, and greenhouses. We also used ex-situ fungal baiting to determine if the endophytes necessary for seed germination were present in each habitat Results/Conclusions
Sequencing of pelotons showed that M. media was associated predominantly with rhizoctonia-forming Basidiomycete fungi (Ceratobasidium, Sebacina, Tulasnella) across all habitats and localities. Most, if not all, of these rhizoctonia- forming fungi, are cosmopolitan soil saprophytes. Thus, our results support the hypothesis that M. media as low specificity and exhibits weedy behavior because of its ability to utilize a broad range of fungal endophytes. Because Sebacina and Tulasnella also contain some ectomycorrhizal taxa, it is possible that M. media may be connected to both orchid endophytes (i.e., the rhizoctonia- forming fungi) and to Basidiomycete-forming ectomycorrhizas in trees. The next step is therefore to determine those plant and/or soil factor(s) that control the colonization of M. media in relatively undisturbed habitats, and how these factors might be modified to promote weediness in an unlikely weed.
