A single fungus consists of a number of identifiable components. In most cases, the most common element is the hyphal network (mycelium) which ramifies through the substrate from which the fungus is gaining its organic energy. The fungus is likely to reproduce asexually and sexually at some period during its existence. The reproductive structures are usually morphologically distinct from the mycelium.
Asexual reproductive structures may simply be compartments which have separated from the thallus, or identifiable structures where the spores are formed and then released. The fungus may also form somatic structures which enable the fungus to survive a period of stress. In addition, some fungi form sclerotia, or stromata. LINK The fungus may escape from reduced resources using aggregating hyphal structures called strands or rhizomorphs. The aggregated hyphae grow beyond the zone of depletion and in doing so, may locate further resources. LINK Each of these structures is constructed from modified hyphae.
Many fungi do not form hyphae. LINK Yeasts are single celled fungi. Their life cycle in analogous to the filamentous lifecycle, but much simpler. The thallus of chytrids may be simple: some chytrids have only two forms, sporangium and zoospore, and the sexual and asexual stages are encompassed within this simple staging.
Fungal hyphae are tubular structures. The cytoplasm is surrounded by a rigid wall LINK which separates the fungus, physically and functionally, from the external environment.
The tube is divided into compartments in most cases. The dividing wall is called a septum, and the septum is incomplete in the early stages of the life of the hypha. Elongation of the hypha takes place at the apex. LINK The tip region extends as the cytoskeleton delivers vesicles containing materials and enzymes for dissolution of the old and construction of the new wall at the apex. The precise mechanism whereby the tip extends is still unclear. However, turgor pressure is thought to drive the process and the process appears to be under the control of the cytoskeleton.
In all fungi, septa form, either adventitiously in all filamentous fungi, or at regular intervals along the hypha in most members of the Ascomycota and Basidiomycota. Septa are barriers across the filament. In the fungi with primary septa, the septa are regularly spaced; pores in the septum enable exchange of at least cytoplasm between adjacent compartments. Nuclei and most organelles within the cytoplasm can pass through pores. LINK In all regularly septate fungi, cytoplasmic continuity within the thallus is a feature regardless of the structure of the pore. This suggests that the linked compartments have the potential to communicate rapidly. Closure of the pore enables differentiation of function within the thallus. In adventitious or secondary septa, entire barriers prevent movement of cytoplasm from one compartment to the adjacent section. Secondary septa are found where hyphae are dieing or damaged. The barriers prevent loss of cytoplasm, and septae enable the separation of the moribund hyphae from the living compartments.Thus secondary septa tend to be irregularly spaced because of the multiple forces acting on the mycelium. Secondary septa also form between the thallus and its sexual structures.
Septum formation is a simple process. Wall ingrowth towards the centre of the compartment results in a complete or incomplete blockage of the hypha. Inward growth may be followed by modification of the outer wall. The septum also increases rigidity of the hypha as it provides structural support for the turgor pressure within the compartment.
The septum in yeast cells has a function during reproduction. Replication of yeast cells differs from hyphal fungi. In the ascomycetous yeasts, where bud cells separate from the parent, a septum divides the two during growth of the bud. A wall is deposited within the septum and dissolution of the septum then proceeds. The bud separates following septum dissolution. The parent cell is left with a scar at the point where the bud separates. This scar will not be able to bud again, and as a consequence, the number of buds formed by any one yeast cell is limited.
Closure of pores takes place by one of several processes. The pores may become blocked by a variety of materials, initially by means of electron dense materials, crystals or Woronin bodies. Complete closure follows. Blockage can be very rapid, in a few seconds in some cases. The effect is to separate adjacent compartments, establishing two functional units on either side of the blocked septum. The separation enables development of the thallus to proceed differently on either side of the blockage. Closure of pores also prevents or reduces loss of cytoplasm from the thallus should a hypha be damaged. Cytoplasmic continuity within the thallus can be re-established by growth of laterals followed by anastomosis of hyphae from either side of the closure. An analagous process takes place in aseptate fungi. The blockages form extremely rapidly, effectively separating the damaged part from the living thallus. Closure of pores appears to have an important role in the life cycle of fungi enabling spatial separation of function and as a response to the environment.
Development of a variety of structures follows closure of pores. Asexual spores develop within a small number of cells above an adventitious septum. Sexual structures and stromata are more complex. These structures continue to grow as separate units after closure, before completion of the structure. The contribution of the thallus to the development of these complex and large structures is still unclear. LINK Even within the sexual structure, different elements of the structure appear to have different developmental pathways suggesting complex signalling and physiological processes.
The structure of hyphae found in complex structures such as fruit bodies, rhizomorphs and sclerotia may differ. These differences were used taxonomically to differentiate various fruit bodies of some groups within the Basidiomycota. A system was developed where three different types of hyphae were used to differentiate polyporoid fungal taxa.
Monomitic fungi contained only generative hyphae. Dimitic fungi contained both generative and skeletal hyphae. Trimitic fungi had all three types of hyphae. This system has been extended with the recognition that a variety of hyphal types exist, some are inflated, others of a specific shape and apparent function.
The point to understand here is that hyphae may develop a variety of forms under genetic control triggered by environmental cues. Overall, fungi are able to radically alter the development pathway of individual hyphae within a mycelium as the environment changes. These influences are realised at the level of the hypha.
The basis of all fungi is the hypha. The hypha may become modified such as in yeast cells, and it may take on various forms in complex tissues. Further, modified hyphae bud off from existing hyphae, or the hypha may fragment, enabling dispersal of the fungus through space and time. While apparently very simple in structure, modification of hyphae, and separation by closure of pores in a thallus enable complex physiological and biochemical processes to take place in a cytoplasmic ocean of the thallus.
Gow NAR & Gadd GM 1995 The Growing Fungus. Chapman Hall.