Zygomycota

Sporangia of Pilobolus kleinii. (Picture taken by Malcom Storey and obtained from Tree of Life.)

Zygomycota is a Phylum of the Kingdom Fungi. This phylum's name is derived from the method of sexual reproduction used by its members, which involve the creation of zygosporangia and zygospores. Identification through asexual reproduction is possible, but more difficult, so the former is the preferred way to determine the classification of a Zygomycota. [1] There are approximately 900 known species that fall into this Phylum, which composes approximately one-hundredth of all true fungi. It is believed to be one of the earlier branches of fungi, thought to have diverged before plants colonized land about 600 - 1,400 million years ago. [2] The unique structures for this Phylum are an uncommon example of sexually-reproducing fungus. [3]

Identification

Most isolated specimens of Zygomycota do not have zygospores present (not in active sexual reproduction), so generally identification is based on sporangial morphology. Once the fungus matures and establishes itself on a medium, identification is easier, with emphasis on examination of sporangial morphology. In addition, tease mounts with a drop of 95% alcohol is stated to be quite effective. [4] Identification procedures may vary depending on what phase a Zygomycete is currently in for reproduction. [5] One particularly common example of a Zygomycete, however, is Black Bread Mold (Rhizobus stolonifer). Additionally, molds are often Zygomycetes. [6] Beyond their unique zygospores, Zygomycetes share many characteristics with their true fungi brethren, such as their chitin walls and hyphae, but their mycelia lack septa. [2] Overall, the defining traits of Zygomycota (their sexual reproduction structures and mycelia without septa) are what tie its members together, spread out over 124 Genera in 32 Families, all organized within 10 Orders. [6]

Reproduction

Members of Zygomycota reproduce both sexually and asexually, with differing life phases based on what type of reproduction occurs. [5] For sexual reproduction, gametangial fusion occurs and involves the formation of zygospores. Asexual reproduction involves sporangia and sporangiospores. [2] Besides the pros and cons that natively come with both sexual and asexual reproduction, for members of this phylum, zygospores appear better suited for preserving the fungus during times of hardship, while sporangia seem to be better suited for rapid establishment and colonization. [1] The ability to reproduce sexually is a trait that is uncommon among fungi, although it is not exclusive to Zygomycota. [6]

Zygospores and Sexual Reproduction

Sexual reproduction in Zygomycota is similar to the conjugation process that a microscopic organism such as a protozoan might use. From these similarities, zygospores can also be referred to as Conjugating Fungi. To initiate this form of reproduction, certain hyphae called gametangia form a connection and exchange genetic material in nuclei in the center of the connected area. After it accumulates, septa are created to seal off the cell, meiosis creates chromosomes, and the cell where this has taken place grows thick, resistant walls that eventually disconnects. This site becomes known as a zygosporangium, and when the outer layers wear away (allowing the genetic material to be released if ready), it becomes a zygospore. [3] The gametangia used in this process have different "strains", plus and minus. These strains are morphologically similar but differ physiologically and biochemically. [5]

Asexual Reproduction

Asexual reproduction in Zygomycetes is centered on the production of sporangia, which themselves form at the ends of specialized hyphae called sporangiophores. Sporangiospores are formed by internal cleaving of cellular cytoplasm, and eventually the outer walls of sporangia will degrade, allowing spores with the same genetic material of the parent to disperse, aided by natural factors, such as water or air. [2]

Impact on Other Organisms

An illustration of a bread mold and its associated sporangiophores. (Picture obtained from Imagination Station)

This Phylum is mostly terrestrial but does have aquatic members, and many parasitic members; Insects in particular can be ravaged by Zygomycetes, but many larger animals (including humans) can become infected by them, along with smaller mesofauna, for example Nematodes. Some plants and fungi are also susceptible to infection by parasitic Zygomycetes. [1] However, some invertebrates have Zygomycetes in their digestive tracts, indicating that some members are mutualistic. Zygomycetes are also frequently neutral decomposers. For humans, arguably the main concern that Zygomycetes pose is their ability to soil foods. This is not necessarily a bad thing, though, as many foods have been created in which molds play a major role. On the other hand, some species can cause (potentially life-threatening) disease. Those most at risk for this are those with poor immune systems and broken skin. [6] Generally, disease transmission is through the development of spores inside the host, which can range from minor infection (esecially in larger animals) to completely infesting the host and draining it of nutrients, leaving it to die and the Zygomycetes to prepare for further reproduction. However, due to the large range of species (due to Zygomycota being a major Phylum of fungi), the role of Zygomycetes in an ecosystem can vary drastically.

References

[1] "Zygomycota". New Brunswick Museum. Accessed 2018-05-05. http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/Zygomycota.html

[2] James, Timothy Y. and O'Donnell, Kerry. "Zygomycota". Tree of Life. Accessed 2018-05-06. http://tolweb.org/Zygomycota

[3] "Fungi Reproducing Sexually By Means Of Sporangio and Zygosporangia". New Brunswick Museum. Accessed 2018-05-05. http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/Sporangia.html#Zygosporangia

[4] "Zygomycota". The University of Adelaide. Accessed 2018-05-06. https://mycology.adelaide.edu.au/descriptions/zygomycetes/

[5] "Fungi - Zygomycota, Conjugating Fungi". Accessed 2018-05-06. http://science.jrank.org/pages/2892/Fungi-Zygomycota-conjugating-fungi.html

[6] "Examples of Zygomycetes." Your Dictionary. Accessed 2018-05-07. http://examples.yourdictionary.com/examples-of-zygomycetes.html