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Mycelial Fungi




The traditional view of mycelial fungi is that the development cycle can be split into two stages: vegetative and reproductive. The former relates to the mycelial spread within the host and the latter the production of sexual or asexual structures that produce spores.


However, a more contemporary approach to discerning the development cycle of fungi reveals four distinct stages: arrival, establishment, exploitation, and exit. Such development stages are considered to be triggered by the changing conditions of the substrate (such as drying, cell degradation, etc), as in meeting the changing conditions the fungus must adapt its own behaviour (dubbed 'functional modes') to successfully persist within the host. Curiously, a fungus can compartmentalise itself so that, at different stages of decay within the same host, it adopts a different functional mode.


Each mode is briefly described below:


(1) Arrival: two known mechanisms exist: arrival as propagules (airborne and seemingly only favourable when localised conditions are optimised at the arrival surface - good supply of nutrients, a good microclimate, and a lack of competitors), or arrival as migratory mycelium (contact of an infected host to a non-infected host, such as with H. annosum and A. mellea).


(2) Establishment & exploitation: three principal concepts of this mode are understood: the fungus must successfully gain access to the host and begin to command available resources; the fungus must begin to successfully convert potential energy resources into actual energy sources, and; the fungus must successfully 'wall-off' an area to resist against competitors or the host tree itself, or in turn have a rapid exit strategy (panic fruiting on the wound surface or within the inside of the hollow(ing) host, for example).


(3) Exit: in the rawest sense, exit can be achieved either through the formation of reproductive structures or by the outgrowth of mycelium (relating back to the arrival strategies). The efficacy and extent of the exit mode is dictated by two factors: the extent to which resources are re-allocated from the mycelial biomass within the wood structure to the biomass of external structures, and; the effect of the environment on the form these exit structures adopt.


At a slight tangent, the r-K continuum further dictates exit strategy, with r-strategists (Deuteromycotina, Zygomycotina) taking a more rapid, economical stance to reproduction (not developing massive or durable exit structures), whilst K-strategists (Ascomycotina, Basidiomycotina) limit commitment of non-reproductive biomass, though also possess greater ability to develop exit structures that persist and are thus durable and do so at more 'fixed' stages. Within the continuum, certain species do however hold the ability to possess more than one mode of exit (such as with F. hepatica and L. sulphureus having the ability to develop either as a basidiospore on primary fruiting structures and as a conidiospore on auxiliary structures).


Source: Rayner, A. & Boddy, L. (1988) Chapter 5: Development Cycles. In Fungal Decomposition of Wood: Its Biology and Ecology. UK: John Wiley & Sons.




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