4.1 Mycelium: the hyphal mode of growth

Members of Kingdom Fungi have made a major success of filamentous extension, even though the mycelial growth strategy is not unique to them. In all organisms in which it is used high rates of filament extension are achieved by generating biomass, most importantly membrane and cell wall precursors, over a long length of filament behind the tip. This biomass is transported to the tip to extend the filament, and as long as the food-gathering activities of the rest of the mycelium can supply the nutrients, tip extension can continue.

The strategy has arisen, by convergent evolution, in Oomycota and streptomycete bacteria but there are major differences between these organisms in the mechanisms used to put the strategy into effect, so don’t be misled (as early biologists were) into confusing similar morphology with phylogenetic relationship. Apical extension of pollen tubes in flowering plants and several systems in developing animals (neurons, blood vessels, insect tracheary systems, ducts in lungs, kidneys and glands) are also based upon a similar approach of apically-extending branching filaments, but for different purposes (Davies, 2006). Indeed, although the fundamental kinetics of all these different filamentous systems are similar, they are ‘fine-tuned’ in their extension rates, branching frequencies and tropisms to suit their specific biological function(s).

As far as members of Kingdom Fungi are concerned, hyphal tip growth is usually thought of as the primary mode of growth in filamentous fungi but though apical extension is the crucial way in which the hyphal structure is established, it is only the start of a multifaceted chain of events that extend over the full length of the hypha (Read, 2011). Overall, the characteristic behaviour pattern of fungi is to explore the habitat with rapidly-growing, sparsely-branched hyphae, then, when some of those hyphae find a nutrient resource, the extension rate declines, rate of branching increases, and the mycelium captures and exploits the resource, from which it subsequently send out a new generation of exploratory hyphae and/or populations of spores. This pattern of behaviour can be recognised from the microscopic scale as tiny saprotrophic colonies find minute fragments of nutrients on plant surfaces, to the landscape scale as pathogens of trees and wood-decay saprotrophs search across the forest floor for new hosts or freshly-felled timber (Carlile, 1995; Lindahl & Olsson, 2004; Money, 2004, 2008; Watkinson et al., 2005).

The polarised growth of the hyphal tip requires a progressive supply of proteins, lipids and cell wall precursors to the hyphal tip. This transport is managed by vesicle trafficking by way of the actin and microtubule cytoskeleton and polarity marker proteins; so, the arrangement, and rearrangement (tip polarity is maintained by repeated transient assembly and disassembly of polarity sites), of the cytoskeleton is crucial to maintain hyphal polarity (Takeshita, 2016). In the next few Chapters we will describe and explain the aspects of fungal cell biology that enable this pattern of behaviour. In this Chapter we will concentrate on the macroscopic aspects of hyphal extension and what can be deduced and established from observation of whole mycelia. In the next our attention will turn to the microscopic and molecular cell biology that characterises fungal hyphae and yeast cells, before turning to the more ‘population aspects’ of the meaning of fungal individuality and the consequences of interactions between hyphae from the same and from different fungal individuals, culminating in the outcome of genetic crosses between individuals.

Updated July, 2019