5.1 FUNGAL REPRODUCTION
Fungal reproduction is unique and distinct from those of other microbial cells such as bacteria. Generally, fungi exhibit two modes of reproduction which are sexual and asexual reproduction. In this section, the terms conidia and spores are synonymously used but with caution since conidia are generally used to describe asexual spores of fungi. Asexual reproduction in fungi is characterized by the formation of conidia (asexual spores) by the type of cell division known as mitosis. In asexual reproduction of fungal cells, budding yeast-like structures are formed and hyphae elements undergo fragmentation or they disintegrate into several components. Some of the spores formed in asexual reproduction aside conidia include arthrospores, blastospores (which are formed from vegetative mother cells by budding) and sporangiospores. Most fungi especially yeast cells such as the Saccharomyces species reproduce asexually by budding, a process in which new daughter cells originate from vegetative (parent) cells as buds or outgrowth (Figure 5.1).
The formation of spores by fungi form the basis for the dispersal of fungi in the environment, and their formation can also be used for fungal classification and identification in the laboratory. Generally, asexual reproduction in fungi includes the formation of conidia (asexual spores), budding of yeast cells and the fragmentation of hyphae. Fragmentation of fungal hyphae leads to the formation of arthrospores or arthroconidia (which are individual components of broken hyphae that behave like spores) and chlamydospores (which are round-thick walled and resistant hyphae cell components formed before separation of hyphae). When the spores develop within a sac (i.e., sporangiophore) they are known as sporangiospores but when they develop at the side of the hyphae without enclosure in a sac they are known as conidiospores. Conidiospores develop from conidiophores. Conidia formed asexually by fungi are usually borne or encased in sac-like structures known as sporangia (singular: sporangium). The sporangia contain numerous amounts of fungal spores, and they serve as route via which spores are released and dispersed in the natural environment especially when they rupture (Figure 5.2). The conidia released in this manner are known as sporangiospores. Sporangiophore and conidiophore are the two types of fungal structures from which fungal spores are normally released into the environment.
Figure 5.1: A sketch of budding in yeast cell. Yeast cells reproduce by budding, a type of asexual reproduction in which daughter cells develop from parent or vegetative cells by the formation of offshoots known as buds. Buds are outgrowth from the vegetative cells; and they eventually become the new cells formed after the whole budding process. MicroDok.
Figure 5.2: Bright field light micrograph showing the release of spores from a sporangium during asexual reproduction in fungi. Fungi can reproduce asexually by fragmentation, budding, or by spore formation. Asexual reproduction takes place in fungi by means of spore formation. Each spore formed may develop into a new individual fungus species. The spores may be produced asexually or sexually; and thus are named as either asexual spores or sexual spores depending on the type of reproduction taking place in the fungus at the time. Asexual spores are genetically identical to the parent fungus and may be released either outside or within a special reproductive sac called a sporangium (plural: sporangia). Sporangium is a case or capsule in which spores are produced in a fungus. In a sporangium, the spores are produced within a cell and are released when the cell breaks open. Mucor species and Rhizopus species are typical examples of fungi that form spores using this process.
In sexual reproduction, the reproductive components formed are generally known as sexual spores (conidia are formed in asexual reproduction). Sexual spores are formed by the type of cell division known as meiosis. In particular, sexual spores of fungi may include zygospores (produced by zygomycetes), ascospores (produced by ascomycetes) and basidiospores (produced by basidiomycetes). Zygospores, ascospores and basidiospores are examples of spores formed by fungi during sexual reproduction. Zygospores are sexual spores with thick walls commonly produced from a diploid zygote formed from the fusion of two haploid nuclei (known as the gametangia) or unicellular fungal gametes. They are commonly seen in the bread mould known as Rhizopus nigricans. Gametangia (singular: gametangium) are gamete producing structures of fungi. Ascospores are sexual spores formed by ascomycetes within a sac known as ascus. Saccharomyces cerevisiae (also known as baker’s yeast) is a typical example of fungi that form ascospores. Basidiospores are sexual spores formed at the end of the basidium (a club-shaped structure) by basidiomycetes.
Mushrooms and Cryptococcus species are typical examples of fungi that form basidiospores. It is noteworthy that four or eight ascospores are usually found in fungal sac-like structures known as the ascus sac. Sexual reproduction occurs when fungi mate to form sexual spores. Normally, sexual fusion occurs between gametangia, haploid gametes or hyphae; and this fusion result in the formation of diploid zygote – which go on to release sexual spores that propagate and disperse the fungal cell in the environment. Sexual spores formed by fungi are generally used for fungal classification as earlier said. The other type of fungi is the deuteromycetes (for example, Candida albicans and Coccidioidis immitis) – for which no sexual reproduction forms have yet being identified. Deuteromycetes are imperfect fungi because they lack mechanisms for sexual reproduction as against the zygomycetes, ascomycetes and basidiomycetes which all have known mechanisms for sexual reproduction. However, most fungi reproduce asexually than they do sexually. In asexual reproduction, conidia or asexual spores are formed, and this reproductive elements as well as sexual spores are critical for the propagation, maintenance and dispersal of fungi in their natural environment.
5.2 MICROSCOPIC STRUCTURES OF SOME FUNGI
- Penicillium: Penicillium is a genus of ascomycetous fungi. It belongs to the ascomycota fungi division. Ascomycota is the largest division of fungi. This genus of fungi is of major importance to man; and they have application in drug production and even in food production. Penicillin is an antibiotic used to treat bacterial infection, and it is naturally sourced from a member of the Penicillium genus known as Penicillium chrysogenum (also known as notatum). Penicillium species are recognized by their dense brush-like spore-bearing structures called penicilli (singular: penicillus). Structurally, the conidiophores are simple or branched and are terminated by clusters of flask-shaped phialides. The spores (conidia) are produced in dry chains from the tips of the phialides, with the youngest spore at the base of the chain; and the conidia of Penicillium species usually assume a paint brush morphological pattern (Figure 5.3). Penicillium is found in the soil, decaying vegetation, air; and they are common contaminants on various substances. Penicillium causes food spoilage, and it colonizes leather objects and other household furniture. It is an indicator organism for dampness indoors. Some Penicillium species are known to produce toxic compounds known as mycotoxins, which have pathological effects on humans. In terms of their reproduction, some species of Penicillium reproduce sexually by means of asci and ascospores produced within small stony stromata. However, Penicillium reproduces both sexually and asexually.
Figure 5.3: A sketch of Penicillium. Observe the paint brush pattern that is characteristic of conidia formation by Penicillium species. MicroDok.
- Scopulariopsis: Fungi in the genus Scopulariopsis are common soil saprophytes. This genus contains fungal species that are anamorphic in nature; and they are also pathogenic to animals and humans. They have asexual forms or stages of reproduction – which allow them to form conidia. Morphologically, Scopulariopsis species form a distinct type of spore that appears as a spiky/rough round conidia, usually in chains (Figure 5.4). Scopulariopsis species are commonly found in soil, dry walls, food, cellulose board, wallpaper, wood, mattress dust, decaying wood, and various other plant and animal products. Examples of fungal species in the genus Scopulariopsis include asperula, S. brumptii and S. brevicaulis. Clinically, S. brevicaulis causes deep mycoses; and the organism is notably resistant to antifungal agents. Scopulariopsis species are dermatomycotic molds and they have been associated with onychomycosis (i.e., fungal infection of the nails). They are common fungal contaminants in both the indoors and outdoors, but they also cause mycosis in humans, particularly in immunocompromised patients. Generally, Scopulariopsis species are known as saprobes because they feed on dead or decayed organic matter in the environment. Therefore, they undergo a saprotrophic mode of nutrition.
Figure 5.4: A sketch of Scopulariopsis. MicroDok.
- Cladosporium: This is a genus of fungi that contain fungal organisms commonly found in indoor and outdoor environments. They cause the blackening of walls inside the house, especially during humid conditions. Cladosporium species are ubiquitously found worldwide; and the two common species are cladosporioides, C. oxysporum, and C. sphaerospermum and C. herbarum. They are commonly found in the air and on surfaces such as wallpaper or carpet indoors, especially where moisture is present. However, Cladosporium species can grow at low temperatures, and thus can cause contamination of refrigerated foods especially meat. Fungi in this genus parasitize plants, animals and other fungi. Cladosporium species rarely cause disease in humans but they can cause mycoses of the skin and toe nails occasionally. Morphologically, they form brown or black colonies on their substrates, and they have dark-pigmented conidia that are formed in simple or branching chains (Figure 5.5). Cladosporium species are significant allergens. In large amounts, spores of Cladosporium species can severely affect people with asthma and respiratory diseases who inhale these spores. Though they rarely cause human infections; Cladosporium species may occasionally cause mycoses of the skin, eye, sinus, and brain in immunocompromised individuals.
Figure 5.5: A sketch of Cladosporium species. MicroDok.
- Fusarium: Fusarium species are globally-distributed filamentous fungi that are abundant in the soil, and they form both large conidia (macroconidia) and small conidia known as microconidia (Figure 5.6). Some fungi form phialides, from which their conidia develop, and this is applicable in fungi that undergo asexual reproduction such as solani. Phialides are bottle-shaped structures within which or from which conidia develop (Figure 5.7). Many Fusarium species are plant pathogens. They are major plant pathogens that affect agricultural plants in temperate regions, causing Fusarium infection known as Fusarium head blight in grains such as barley and wheat. Fungi in this group are known for their ability to produce toxins that have medical significance. Fusarium species are toxigenic in nature, and the mycotoxins produced by these fungi are often associated with animal and human diseases. When these mycotoxins enter the food chain, they can cause disease in human population. For example, Fusarium species cause onychomycosis and mycoses of the cornea (keratomycosis). They cause opportunistic infections in immunocompromised individuals. Many species of Fusarium found in the soil are harmless, and they undergo a saprotrophic mode of nutrition. F. moniliforme, F. solani, and F. oxysporum are examples of fungi in the genus Fusarium.
Figure 5.6: A sketch of microconidia and macroconidia of Fusarium species. MicroDok.
Figure 5.7: A sketch of phialide types of sporulation. Some fungi form phialides from which their conidia develop. Phialides are formed during asexual stages of reproduction of some fungi. MicroDok.
- Fonsecaea: Fonsecaea is a genus that contains fungi implicated in causing chromoblastomycosis. Chromoblastomycosis is a localized fungal infection of the skin and subcutaneous tissues. Human infection usually occurs through the traumatic implantation of conidia or hyphal forms of the organism. Fonsecaea species are pleomorphic and sporulating in nature. They form spores or conidia used for reproduction and dispersal (Figure 5.8). Fonsecaea pedrosoi is a typical example of fungus in the genus Fonsecaea; and the organism grows as a soil saprotroph and can be pathogenic in humans. Saprotrophs are organisms whose mode of nutrition or feeding are saprophytic in nature (i.e., they feed on decaying organic matter). Fusarium species can also be found on trees and plants. Other likely pathogenic fungal species in the genus Fonsecaea include nubica and F. monophora. People who work in occupation where dusts particles can become easily aerosolized (for example farmers) especially in endemic areas of the disease can easily develop chromoblastomycosis.
Figure 5.8: A sketch of various types of sporulation of Fonsecaea pedrosoi. MicroDok.
- Exophiala: Exophiala is a fungal genus that contains fungi known as black yeasts. Exophiala species are ubiquitous fungi found worldwide, and they are dematiaceous in nature. Typical example is Exophiala dermatitidis, formerly known as Wangiella dermatitidis. E. dermatitidis is an imperfect thermophilic black mold that rarely causes infection in humans. However, people with compromised immune system are mostly at risk of acquiring infection with Exophiala dermatitidis. Exophiala dermatitidis is found in low abundance in nature; and the fungus occurs in the soil in decaying matter. It can also be found in dead plant matter in the soil. Exophiala species are saprobes that derive their nutrients from the decomposition or breakdown of dead organic matter in the soil. Exophiala species can be found in wet and moist man-made environments such as in pools. Exophiala dermatitidis is an anamorphic fungus that forms many conidia. The hyphal forms of E. dermatitidis shows fertile sterigmates of hyphae and conidiophore which are surrounded by spores or conidia during growth (Figure 5.9). E. dermatitidis also form black yeast forms on substrates (Figure 5.10). Fungi in the Exophiala genus cause subcutaneous and cutaneous opportunistic infections in endemic regions, especially in immunocompromised individuals. Pulmonary infections, liver infections and infections of the bone or joints are some
Figure 5.9: A sketch of hyphal form of Exophiala dermatitidis
Figure 5.10: A sketch of black yeast forms of Exophiala dermatitidis. MicroDok.
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