Phylogeny and Ecology of Mycotrophic Achlorophyllous Angiosperms

Achlorophyllous genera of angiosperms, such as Galeola (Orchidaceae), Leiphaimos (Gentianaceae), and Monotropa (Pyrolaceae), have evolved highly specialized, mycotrophic root systems, along with chlorosis and complex floral morphologies. Productivity rates of most such plants cannot be satisfactorily explained by the saprophytic niche which is often assumed as theirs but is inadequately nutritious. Rather, they parasitize their mycorrhizal fungi. When a mycorrhizal fungus is shared by roots of an achlorophyllous angiosperm an those of a photosynthesizing plant, the former can indirectly parasitize the latter via the connecting mycelium (epiparasitism). The functional mycelial connection between the mycorrhizae of epiparasites and those of associated host trees has been demonstrated by radiotracer techniques. Some epiparasites are very locally distributed. The ecological requirements of their mycorrhizal fungi, such a suitable host trees, may explain much of this localization. A taxonomic survey of the Orchidaceae and Pyrolaceae reveals coherent, continous lines of evolution from green, leafy taxa with fibrous root systems to progressively more chlorotic taxa with reduced root systems. The widely distributed genera Hypopitys and Montropa of north temperate forests represent highly successful occupants of a widespread, mycotrophic-epiparasitic niche for angiosperms.