12.1 Development and morphogenesis
In a great many fungi hyphae differentiate from the vegetative form that ordinarily composes a mycelium and aggregate to form tissues of multihyphal structures. These may be linear organs (that emphasise parallel arrangements of hyphae), such as strands, rhizomorphs and fruit body stems (CLICK HERE to view the page in Chapter 9); or globose masses (that emphasise interweaving of hyphae), such as sclerotia, fruit bodies and other sporulating structures of the larger Ascomycota and Basidiomycota (CLICK HERE to view the page in Chapter 9).
Development of any of these fungal multicellular structures requires that hyphal growth takes on a particular ‘pattern’. A ‘pattern’ that, time after time, produces the same species-specific structure and morphology of that structure, a process that demands precise control and regulation. Formation of a multicellular structure begins with a localised association of aerial hyphae into a hyphal tuft (also called a hyphal knot), which gradually enlarges and differentiates into a primordium of the fruit body (or other structure, according to circumstances) from which the fruit body (etc.) finally emerges.
The differential growth represented in this morphogenesis, and which gives rise to the development of the variety of tissues that make up a fungal multicellular structure involves detailed control and regulation of wall synthesis. Most of the descriptions of wall formation we have given so far have concentrated on hyphal tip (apical) growth, but in development of multicellular structures, 'mature' hyphal wall distant from its hyphal apex can restart wall formation to remodel and reshape the cell. In addition, two adjacent hyphal branches can be joined together by synthesis of a joint wall, which can be stronger than the original. Also, there are many observations of hyphal walls being thickened internally by synthesis of a secondary wall, mostly made up of thick fibrils, which are probably constructed of glucans accumulated as a nutritional reserve. Fungal wall synthesis, resynthesis and secondary wall formation are topics worthy of separate treatment and we have dealt with them in detail in Chapter 6.
Differentiation events can be limited to particular hyphae within the structure (indeed, to particular cells in individual hyphae), and differential growth of tissues can generate mechanical forces that change the macroscopic shape of the whole structure. In developmental terminology this is pattern formation (creation of a particular spatial arrangement of tissues that will generate the final morphology of the structure or organ) caused by regional patterning (regional specification) of the differentiation pathways followed by the cells within those patterns. That these processes take place can be deduced from relatively simple experiments and observations even though it is not yet known how the hyphae that will differentiate are specified.
Here we will illustrate how the formal principles of fungal developmental biology have been established by experiment and observation of the patterns of hyphal growth, branching and interactions that achieve the tissue patterns represented in the diverse morphologies of fungal fruit bodies and similar multicellular structures; and then theorise about how these patterns are achieved. But first we have a few words about the terminology that is employed.
We have already used some development-specific terminology; namely pattern formation and regional specification. We have also mentioned some important, indeed crucial, features that uniquely characterise fungal development. This is the dependence of fungal multicellular development on control and adaptation of the normal growth and branching of vegetative hyphae, and in particular the fact that formation of any multicellular structure in fungi requires reversal of the outward, exploratory growth habit that characterises vegetative hyphae (specifically, altered autotropisms, CLICK HERE to view the discussion in Chapter 4). In order to contribute to the aggregations that become multicellular structures the hyphae concerned must convert the negative autotropism that ensures outward growth of hyphal tips in mycelia into a positive autotropism that permits the tips of branches to approach each other and other hyphae and create the hyphal tuft that is the initiation point for the multicellular structure.
Updated December 17, 2016