FUNGAL REPRODUCTION

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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 1).

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Figure 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. Photo courtesy: https://www.microbiologyclass.com

The formation of spores by fungi also forms 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 through which spores are released and dispersed in the natural environment especially when they rupture (Figure 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 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 isa 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. Photo courtesy: https://www.microbiologyclass.com

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 ascosporesare 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 (i.e., conidia) as well as sexual spores are critical for the propagation, maintenance and dispersal of fungi in their natural environment.

Further reading

Anaissie E.J, McGinnis M.R, Pfaller M.A (2009). Clinical Mycology. 2nd ed. Philadelphia, PA: Churchill Livingstone Elsevier. London.

Baumgardner D.J (2012). Soil-related bacterial and fungal infections. J Am Board Fam Med, 25:734-744.

Calderone R.A and Cihlar R.L (eds). Fungal Pathogenesis: Principles and Clinical Applications. New York: Marcel Dekker; 2002.

Champoux J.J, Neidhardt F.C, Drew W.L and Plorde J.J (2004). Sherris Medical Microbiology: An Introduction to Infectious Diseases. 4th edition. McGraw Hill Companies Inc, USA.       

Gladwin M and Trattler B (2006). Clinical Microbiology Made Ridiculously Simple. 3rd edition. MedMaster, Inc., Miami, USA.

Larone D.H (2011). Medically Important Fungi: A Guide to Identification. Fifth edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Stephenson S.L (2010). The Kingdom Fungi: The Biology of Mushrooms, Molds and Lichens. First edition. Timber Press.

Sullivan D.J and Moran G.P (2014). Human Pathogenic Fungi: Molecular Biology and Pathogenic Mechanisms. Second edition. American Society of Microbiology Press, USA.

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