Chapter 4: Hyphal cell biology and growth on solid substrates

Although their mode of nutrition is important in defining members of Kingdom Fungi, the fundamental aspect of cell biology which sets most fungi off from most members of the other major kingdoms is the apical extension of their tubular hyphae. These possess controls which ensure that hyphae normally grow away from one another to form the typical ‘colony’ with an outwardly-migrating growing front. Extension growth of the hypha is limited to the apex and this pattern of growth makes the vegetative fungal mycelium an exploratory, invasive organism; and exploration and invasion is the fundamental lifestyle of fungi. This lifestyle allows filamentous fungi to dominate their ecosystems because it gives them the tools they need to find and colonise new substrates rapidly. The success of this growth habit can be judged from the extraordinary diversity of fungal species, their distribution in virtually every habitat on the planet and the parallel evolution of a similar growth strategy by other important soil microorganisms, the prokaryotic streptomycetes and some of the Oomycota in Kingdom Straminipila (e.g. Saprolegnia and Achlya; see Section 3.10). A general introduction to the fungal life style can be found in Chapter 3 of the book Slayers, Saviors, Servants, and Sex: An Exposé of Kingdom Fungi (Moore, 2001).

In this chapter we will discuss the hyphal mode of growth in some detail, explaining how hyphae emerge during spore germination and how hyphae contribute to colony formation. Mycelium growth kinetics is a key topic in understanding the nature of fungi; here we show how that understanding has been built from experiments with living fungi. We consider how fungal colonies grow to maturity and the morphological differentiation that can be seen in fungal colonies. At the cellular, or hyphal, level we show the meaning of ‘duplication cycle’ in moulds, how this depends on regulation of nuclear migration, and how it contributes to hyphal growth kinetics. Turning then to communities of hyphae, we explain autotropic reactions, and consider hyphal branching and septation. Finally, we draw it all together by discussing the ecological advantage of mycelial growth in colonising solid substrates.

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Updated July, 2019