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The ability to reproduce asexually is common to almost all fungi.
1. Bud formation in yeasts
In its simplest form asexual reproduction is by budding or binary fission. The onset of the cellular events is accompanied by the nuclear events of mitosis. If you have forgotten these events please check in a basic text book.
The initial events of budding can be seen as the development of a ring of chitin around the point where the bud is about to appear. This reinforces and stabilizes the cell wall. Enzymatic activity and turgor pressure the act to weaken and extrude the cell wall. New cell wall material is incorporated during this phase. Cell contents are forced into the progeny cell, and as the final phase of mitosis ends a cell plate, the point at which a new cell wall will grow inwards from, forms.
Separation of the bud from the parent leaves a scar. When chains of yeast cells do not fully separated this can create a pseudo-mycelium.
Many fungi can reproduce by fragmentation. Any mycelium that is fragmented or disrupted, provided that the fragment contains the equivalent of the peripheral growth zone, can grow into a new colony. Many fungi are sub-cultured using this hyphal fragment technique. All of this weeks practical plates have been inoculated in this way with a cork bore taken from a colonized donor plate. Cut mycelial tips do not regenerate, but branches can form some distance from the damage point.
By far the most important type of asexual reproduction is that of spore formation. Asexual reproduction is extremely important to fungi. It is responsible for the production of large numbers of spores throughout the year. These asexual spores are formed on a phase of the fungal life cycle termed in some texts as the mitosporic, or anamorphic phase. There can be more than one mitosporic state for each species of fungus, and in some cases the mitosporic state of very different species can look very similar. This has contributed to the problems of creating a taxonomy for the fungi that only possess mitosporic states. The sexual stage of the fungus can be termed the teleomorph, and the characteristics of this phase of the life cycle are much more stable and reliable for taxonomic purposes.
The onset of asexual reproduction is controlled by many different things. Some are environmental, like nutrient levels, CO2 levels, light levels. Others can fungi have internal time clocks and sporulate anyway in a preset part of the fungal life cycle designed to spread and maximize colonization during one season.
Chytrids are quite distinct from other fungi as they have extremely simple thalli and motile zoospores. Species within this group are very simple in structure and may only consist of a single cell, perhaps with rhizoids to anchor it on to a substrate.
Asexual reproduction in the chytrids is by the production of motile zoospores, with a single, posterior flagellum, in sporangia (Fig. 2). There will be a film of chytrid reproduction available in the practical.
Figure 2. Holocarpic chytrid
This is a diagram of a holocarpic chytrid, one where the entire thallus consists
of only one cell with rhizoids. These are usually parasitic on aquatic plants or fish.
The fungus 'feeds' from its substrate via its rhizoids. The entire cell contents will
convert to motile zoospores
Zygomycete Asexual Reproduction
Zygomycete fungal mycelium is coenocytic. At the onset of sporulation large amounts of aerial hyphae are produced. The tips of these aerial hyphae fill with cytoplasmic contents, and the nuclei undergo repeated mitosis. Around each of the nuclei cytoplasm and organelles collect, and by the formation of copious vesicles from the Golgi, each nucleus becomes isolated from the next by a plasma membrane. Within the spaces created by this cytoplasmic cleavage, spore walls begin to form, again by the fusion of Golgi vesicles containing cell wall monomers and enzymes with the spore membrane. A sporangium forms. As these events occur so there is considerable water uptake by the forming sporangium, and as the columella forms the structure comes under considerable turgor pressure. The large sporangia can contain up to 100,000 spores (Fig 3).
Fig. 3 Formation of a sporangium
This diagram is redrawn from Brackers original and shows the development of a sporangium through time. As nuclei undergo repeated mitosis so the Golgi produces membrane bound vesicles filled with spore wall building materials. These coalesce around the nuclei to form a spore. Eventually they are released.
Not all sporangia are as large as this, there are many species with smaller, specialized sporangia, called sporangiola, merosporangia and some are almost conidial, forming single spored sporangia that are only distinguished from the conidium of the higher fungi by the possession of a double spore wall. We will look at some of these structures in the practical.
The process of spore formation in most members of the higher fungal groups is again based largely on the formation of aerial mycelium and the differentiation of the hyphal tip. However, unlike the process seen in the Zygomycetes, the process here involves something much more like the budding we see in the yeasts. This is termed a blastic process, which involves the blowing out or blebbing of the hyphal tip wall. The blastic process can involve all wall layers, or there can be a new cell wall synthesized which is extruded from within the old wall. As asexual reproduction is sometimes the only form of reproduction seen in some fungi, we have in the past tried to construct elaborate taxonomic schemes based on spore structure and production. However, as I said earlier, these features are notoriously plastic and such schemes have largely been abandoned. The hypha that creates the sporing (conidiating) tip can be very similar to the normal hyphal tip, or it can be differentiated. The commonest differentiation is the formation of a bottle shaped cell called a phialide, from which the spores are produced. Phialide formation in the Ascomycete fungi:
Not all of these asexual structures are single hyphae. In some groups the conidiophores (the structures that bear the conidia) are aggregated. In the Moniliales all are single with the exception of the aggregations termed coremia or synnema. These produce structures rather like corn-stooks, with many conidia being produced in a mass from the aggregated conidiophores.
Other species of Ascomycetes and Deuteromycetes form their structures within plant tissue, either as parasite and saprophytes. These fungi have evolved more complex asexual sporing structures, probably influenced by the cultural conditions of plant tissue as a substrate. These structures are called the sporodochium. This is a cushion of conidiophores created from a psuedoparenchymatous stroma in plant tissue. The pycnidium is a globose to flask-shaped parenchymatous structure, lined on its inner wall with conidiophores. The acervulus is a flat saucer shaped bed of conidiophores produced under a plant cuticle, which eventually erupt through the cuticle for dispersal.
Sexual reproduction introduces the possibility of variation into a population, and this is why most fungi have a sexual phase. To achieve sexual reproduction it is necessary to have two mating type haploid nuclei (n + n), or a diploid (2n) nucleus. In the case of the two haploid nuclei they must fuse to form a diploid first, but once fused the nuclei undergo meiosis, which is the reduction division that potentially brings about variation in the progeny. These event are followed by the formation of spores, which in most cases are resting spores that can withstand adverse conditions.
Sexual reproduction in the chytridiomycetes:
Sexual reproduction occurs in some members of the chytrids by the production of diploid spores after gametic or somatic fusion of two different mating types. The resulting spore may germinate to produce a diploid vegetative mycelium or it may undergo meiosis to produce a haploid mycelium. The diploid mycelium can also produces resting sporangia in which meiosis occurs, generating haploid zoospores that germinate to produce haploid vegetative mycelium (Fig 6):
Sexual reproduction in the Zygomycetes:
There are two possible nuclear states in the mycelia of this group of fungi. They can have a single type of nucleus in their mycelium, a condition termed termed homothallism, or they can contain the two mating type nuclei within their mycelium, termed heterothallism. If the fungus is homothallic the first event in the onset of sexual reproduction has to be somatic fusion. This is termed conjugation. To achieve such a mating it is necessary to attract each other and an elaborate sequence of cellular and biochemical events have been established for some of these fungi. This signalling involves the secretion of inducer molecules that are responsible for causing the formation of zygophores, modified hyphal tips, and these then grow towards each other long a gradient of hormone. The exact sequence is shown below. Sexual reproduction in the Zygomycetes:
Once in contact the two zygophores fuse, and then the nuclei fuse to form the diploid. Meiosis occurs, producing four haploid nuclei, but three may degenerate. The timing of fusion varies from species to species.
Sexual reproduction in the Ascomycetes:
In this group of fungi there are no specialized organs of hyphal fusion, different mating type mycelia merely fuse with each other to form transient dikaryons, mycelia with two mating type nuclei within it. The dikaryotic mycelium can differentiate to from varying amounts of sterile mycelium around what is to become the fertile tissue of the fruit body. In yeasts, a single, diploid yeast will undergo meiosis, producing four haploid progeny cells, but in more complex fungi there are a sequence of cellular and nucleic events that ensure an organized fertile layer. The events are illustrated below in figure Fig. 8.
Spores are delineated around these nuclei in a process called free cell formation, and as most of the cytoplasm is contained around the nucleus and within the spore wall, all that is left outside is cell sap. These modified hyphae are termed Asci, and the spores that are held within them are termed ascospores. The asci are often found packed tightly with other asci, and between a dense layer of supporting sterile tissue. Often the structure is large enough to be seen with the naked eye.
The asci can be aggregated together in various sorts of fruit body which we will see in the practical, including the, cup fungi (Discomycetes, apothecial), the flask fungi, (Pyrenomycetes, perithecial), the mildews (Plectomycetes cleistothecial) and the fungi with black, crusty stromata (Loculoascomycetes, pseudothecial fungi). There are also the yeasts, Hemiascomycetes,. Their ascospores are normally formed in loose asci and are not actively discharged. We have not looked at these. When they form ascospores in fruit pulps or liquids they are usually liberated by the disintegration of the ascus wall.
Sexual reproduction in the Basidiomycetes:
Basidiomycetes are characterised by the most complex and large structures found in the fungi. They are very rarely produce asexual spores. Much of their life cycle is spent as vegetative mycelium, exploiting complex substrates.
A preliminary requisite for the onset of sexual reproduction is the acquisition of two mating types of nuclei by the fusion of compatible mycelium. This creates a dikaryon where single copies of the two mating type nuclei are held within every hyphal compartment for extended periods of time. Maintenance of the dikaryon requires elaborate septum formation (clamp connections, Fig 10.) during growth and nuclear division.
Onset of sexual spore formation is triggered by environmental conditions and in the larger Basidiomycetes begins with the formation of a fruit body primordium. The primordium expands and differentiates to form the large fruit bodies of mushrooms and toadstools. The mycelium within this structure remains as a dikaryon, diploid formation only occurring within the modified hyphal tip called the basidium. Meiosis occurs within the basidium, and the four products are extruded from the tip of the basidium on sterigma (below). Usually this event occurs across a large area of basidia called a hymenium, or fertile layer. It is usually formed over an extensive sterile layer of tissue like a mushroom gill.
There are three major divisions in the basidiomycetes.
There are two major functions of fungal spores, dispersal and survival. Often these two requirements are met by two different spores formed at different points in a fungus life cycle. Some are survival spores formed in response to adverse abiotic conditions that can include desiccation, high UV, high/low temperatures or starvation. Biotic factors can also induce sporulation including competition, antagonism, and pathogens presence. These spores have thick cell walls, and lots of reserves.
Dissemination spores are spores that are smaller, with thin cell walls, and limited reserves, and will germinate readily when on a suitable substrate. They are formed as part of the active life cycle of the fungus and are often concerned with epidemic spread of a pathogenic species from plant to plant, or with rapid colonization of a substrate.
Spores in general:
By wet weight spores generally contain 25% protein and 20% fat, and they have a low water content relative to vegetative mycelium. Cell walls of spores are generally not fibrillar, but they are multi-layered and often contain melanin and have ornamentations.
Spores contain all normal mycelial organelles. Respiratory reserves include lipids, glycogen, phospholipids and polysaccharides that can include sugar alcohols like Trehalose). Respiration rates in spores are only 1-4% those of vegetative mycelium, but obviously the more reserves a spore has, the longer it will survive.
Dormancy occurs when spores do not immediately germinate after formation. Dormancy is a break in the life cycle. There are two types, endogenous (constitutive) and exogenous (induced). Endogenous dormancy is due to some internal quality of the spore, a barrier to water or nutrient entry, a metabolic block, or an inhibitor. Self inhibition prevents spores from germinating in dense suspensions. It can be by excessive sensitivity to oxygen or carbon dioxide levels, nutrient competition, or most usually due to the presence of inhibitors. These molecules are often active in the 1-10 nanomolar range. These inhibitors have to be leached away before germination takes place.
There can also be physical barriers to germination. In one of the athlete foot fungi, Microsporium gypseum, there is a protein layer around the spore which prevents the uptake of water. This layer is removed by the action of a fungal acid phosphatase enzyme. This enzyme is inhibited by high levels of phosphate, and until phosphate levels in the environment drop the fungus spore does not germinate.
Endogenous (induced) dormancy occurs because of some external condition, and whilst these conditions prevail the spore will not germinate. As soon as the limiting factor is removed the spore germinates.
Table 1. A summary of the characteristics of fungal spores with endogenous and exogenous dormancy:
Displaced from point of origin
Remain at point of origin
Definite launch mechanism
Released by autolysis
Small and thin walled spores
Large and thick walled
Short survival time
Survive for a long time
Germinate readily under suitable conditions
Germinate after a specific stimulus or removal of an inhibitor
Optimal environmental signals trigger the end of dormancy and the onset of germination. Chemical stimuli can trigger germination. This is frequently seen in pathogens where host compounds can act as germination stimulants.
Germination begins with imbibition, the uptake of water, which can cause a 3 to 20 fold increase in size. Spherical growth also accounts for some of the swelling. Eventually polarized growth starts, with the emergence of a germ tube from the spore. The spore wall may be ruptured and a new cell wall covered germ tube emerges, or the spore wall may be softened and the germ tube then emerges.
In between formation of spores and their eventual germination is a phase where spores are disseminated from their point of origin. Many fungi have elaborate mechanisms for getting their spores into the atmosphere which the best medium through which to spread spores. Many spores are very dry and friable, which means they are light enough to be lifted by air currents into the turbulent air above the boundary layer. Others have active spore guns that fire spores up into the atmosphere.
The end result is that over both countryside and towns there is a characteristic air spora. In the countryside this is very typically full of spores from pathogens of agricultural crops, from saprophytes of plant structures and from decaying matter. Species like Penicillium and Cladosporium tend to predominate. In the towns there are fewer agricultural pathogens but there are still hundreds of spores per cubic litre of air. Within homes and workplaces spore numbers can be even higher, and the species distribution tends to differ. In warm, dry areas Aspergillus spp. can become significant members of the air spora.
In normal circumstances these fungal spores pose little or no hazard, our immune systems have evolved with these spores and we are at no risk from them. However, in a population that contains a significant number of individuals that are for whatever reason immuno-compromised, these spores can represent a hazard that as yet has not been quantified. Furthermore, there is increasing evidence to show that inhalation of fungal spores can cause significant allergic responses in atopic individuals. This is an area of very active research.
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