Together, but not for ever: Ectomycorrhizal symbiosis is an unstable affair

摘要

Ectomycorrhizal symbiosis continues to pose a number of intriguing and so far partially unanswered questions. These concern not only general aspects of mycorrhizal physiology, host specificity, or the impact of ectomycorrhizas on forest ecosystems, but also the evolutionary ecology of the ectomycorrhizal symbiosis itself and the reasons why it has evolved. Classically, mutualistic symbioses are considered to evolve from parasitic relationships, and to be vertically transmitted. But mycorrhizal mutualism escapes these tenets by being, for example, horizontally (or 'pseudo-vertically') transmitted (Wilkinson 1997, Genkai-Kato & Yamamura 1999). Even the general evolutionary progression from parasitism to mutualism seems not to apply to mycorrhizas. This is shown by a recent report by David Hibbett and colleagues from the Harvard University Herbaria, Massachusetts, that provides some fresh insight into the evolutionary ecology of ectomycorrhizal fungi (Hibbet et al. 2000).