Abstract:
Orchids depend on fungi for germination and protocorm development, but also form mycorrhizas during life time. Mycorrhizal fungi are, therefore, discussed as driving force in orchid evolution and speciation. However, for serious discussion reliable data on fungal identity are crucial. Application of molecular phylogeny and transmission electron microscopy congruently revealed distinct fungal groups as orchid mycobionts. Sebacinales Group B, Tulasnellales, and Ceratobasidiales were found associated with terrestrial orchids in open grasslands and arbuscular mycorrhizal forests and with epiphytes, while Sebacinales Group A, Thelephorales, Russulales, some Euagaricales and Tuberales form mycorrhizas with terrestrial orchids in ectomycorrhizal forests. Enzyme and isotope analyses revealed that the former obtain carbon from rotten organic material to nourish the protocorm while the latter take carbon from ectomycorrhizas of woody plants to feed protocorms and adult heterotrophic or mixotrophic plants. Mycobionts of terrestrial orchids appeared to be of narrower host range than previously expected, and co-speciation was discussed. The few investigations on mycobionts of epiphytic orchids so far indicated sharing of hosts. Further information is needed of mycobionts from tropical terrestrial and epiphytic orchids at well resolved molecular level of fungal identity, in situ prove, host range, and ecology to finally evaluate if association strategies differ between epiphytic and terrestrial orchids or between temperate and tropical habitats.
