Evolution of life forms and mycorrhizal relationships in Cymbidium

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Cymbidium Sw., an orchid genus distributed from. East and Southeast Asia to Australia, comprises about 50 species. The genus exhibits distinctive ecological diversification and occurs in terrestrial, epiphytic, and lithophytic life forms. Moreover, Cymbidium section Pachirhizanthe lacks foliage leaves and has an obligate mycotrophic nutritional existence. In this study correlation between various morphological and anatomical characters and ecological characters was tested for most species of Cymbidium and its sister groups. Furthermore, a molecular phylogenetic tree of Cymbidium (Yukawa et al.2002) was used to examine phylogenetic constraint of these characters. The following insights were provided : (1)In Cymbidium, lithophytic and terrestrial life forms were likely to be derived from epiphytic habitat at least three times. (2)The obligate mycotrophic lineage, Cymbidium section Pachyrhizanthe diverged from a terrestrial progenitor. (3)The seed of epiphytic species showed higher hydrop … More hilicity and larger dimensions than that of terrestrial species. (4)The seed size of obligate mycotrophic species decreased greatly in comparison with its sister photosynthetic species. (5)Development of ramified rhizomes was found only in terrestrial species. We found that this type of the rhizome is the principal dwelling for mycorrhizal fungi. (6)In this genus, development of sclerenchymatous fibers in leaves is correlated to the degree of epiphytism. (7)Differentiation of palisade tissue and amphistomaty are likely adapted to high light stress because the species possessing these characters inhabit trees at exposed positions. (8)Orchid-type endomycorrhizal fungi and tree ectomycorrhizal fungi were found in Cymbidium roots and/or rhizomes. Although terrestrial Cymbidium species symbiose with both types of fungi, epiphytic species show symbiosis only with Orchid-type endomycorrhizal fungi. On the other hand, obligate mycotrophic species symbiose exclusively with tree ectomycorrhizal fungi. Less