Journal Number 97
November 2005

EDITORIAL

Myco-heterotrophy
By Ian St George

There are 200 wholly nongreen orchids, and Martin Bidartondo of the Imperial College, and
Kew Gardens, London, has written a Tansley Review [1] on the evolutionary ecology of myco-
heterotrophy. In it he makes the interesting point that most nongreen plants that form
mycorrhizal relationships with soil fungi, are understorey woodland inhabitants. That habitat
is only sustainable when sunlight is no longer necessary for nutrition. Furthermore most nongreen plants have evolved from green mycorrhizal plants that inhabited similar places.

All nongreen mycorrhizal plants except Gastrodia and Galeola are partners in tripartite assemblages (two plants, at least one green, linked by a common fungus). Gastrodia are colonized by Armillaria which is destructive, and is not known to form mycorrhizal associations with any other green plants. That is what Ella Campbell's research found, but her work has not yet been verified by direct molecular analysis.

Orchids are specialist cheats or generalist cheats of course - both in their selection of insect pollinators and their selection of mycorrhizal fungus. But you wouldn't expect an orchid that specialised in cheating only one fungus to take the reproductive risk of having a single pollinator, and in practice that is borne out.

Mycorrhizas may thus be the strongest determinant of the reproductive biology of myco-heterotrophic plants. "A myco-heterotroph specialised on a narrow set of closely related fungi should rely disproportionately upon: (i) the most generalist pollinators available; (ii) allocation of resources to seed production rather than to pollinators; and/or (iii) self-pollination. Contrariwise, a plant that relies on specialised deceptive pollination should be a mycorrhizal generalist."

However, "In the Orchidaceae in particular, the imbalance in our knowledge of reproduction vs mycorrhizas is staggering, even though fungi are widely thought to have shaped orchid characteristics of great evolutionary importance: root structure, seed morphology and seedling physiology. Consequently virtually nothing is known about the ecological and evolutionary interplay of the two dominant forces that both constrain and diversify this huge family."

Reference

1. Bidartondo M.I. The evolutionary ecology of myco-heterotrophy. New Phytologist (2005). www.newphytologist.org. 

A high proportion of the NZ orchids are self-fertile, which, if the above is true in reverse, suggests mycorrhizal specialisation - Ed.