21st Century Guidebook to Fungi, SECOND EDITION, by David Moore, Geoffrey D. Robson and Anthony P. J. Trinci

Table of Contents

1. Chapter 8: Sexual reproduction: the basis of diversity and taxonomy
2. Chapter 9: Continuing the diversity theme: cell and tissue differentiation
   1. What is diversity?
   2. Mycelial differentiation
   3. Making spores
   4. Aspergillus conidiophores
   5. Conidiation in Neurospora crassa
   6. Conidiomata
   7. Linear structures: strands, cords, rhizomorphs and stipes
   8. Globose structures: sclerotia, stromata, ascomata and basidiomata
   9. Chapter 9 References and further reading
   10. Buy a PDF of Chapter 9
3. Chapter 10: Fungi in ecosystems

9.1 What is diversity?

Much of the success of fungi is related to their ability to produce vast numbers of asexual, dispersal spores that spread the organism in space. A by-product of this productivity is that the air we breathe contains very large numbers of fungal spores (which can be potential allergens in the air of our urban environments). In contrast to the sexual spores of zygomycetes and Ascomycota (but like the basidiospores of Basidiomycota), asexual fungal spores are usually:

• produced in very large numbers (much of the biomass of the fungus being converted into asexual spores);
• relatively small;
• thin walled;
• non-dormant;
• germinate rapidly on suitable substrates; and
• short-lived.

Though there are, of course, exceptions to these generalisations: for example, chlamydospores, which are large, thick-walled, resting spores of organisms like Candida and Fusarium.

These spores need to be efficiently and effectively dispersed, and as we describe more examples we will encounter a range of mechanisms and processes that have been developed by fungi to contribute in some way to dispersal. This is part of fungal diversity.

The biodiversity of fungi starts with the numerous ways that fungal mycelia differentiate morphologically. This diversity in form is related to the ecological function(s) of the mycelium. The accepted interpretation is that fungal mycelia can express a range of alternative phenotypes that enable them to:

• explore (rapid extension of sparsely-branched hyphal systems);
• assimilate (locally enhanced branching);
• conserve (cells differentiated to store metabolites and cope with adverse conditions);
• redistribute the resources within ecological niches that are heterogeneous in both space and time (via strands and similar hyphal systems adapted to bulk translocation).

Most of this mycelial biodiversity results from variation in the kinetics of hyphal growth parameters like rate of extension growth, branching frequency and branching angle (Trinci et al., 1994). Together these can have enormous impact on the macroscopic morphology of the mycelium and the functional efficiency of the mycelium in the various roles it fulfils (see Section 4.9). In the 1990s Alan Rayner and his colleagues (Rayner, 1997; and other references in the Mycelial Differentiation Resources Box) argued that different local morphologies in the mycelium result from changes in the hyphal membranes and walls affecting the flow and passage of molecules. Discussions of this sort raise the important point that differentiation of hyphal cells is, at least in part, a reaction to the local environment. Response to the environment (and the word ‘environment’ must be taken to cover other cells, the substrate, the substratum, the gaseous atmosphere and physical environmental conditions) has a large measure of control over differentiation of the hyphae. The genetic constitution of the hyphal cell will determine the nature of the differentiation of which the cell is capable, but the local environment determines where, whether, and to what degree that differentiation will occur. The ways in which mycelia differentiate morphologically have been recorded in a very extensive literature, some of which can be found in the publications referenced in the Mycelial Differentiation Resources Box.

Updated July, 2019