Monthly Archives: August 2020

by Piter Kehoma Boll

In my microbiology classes as an undergraduate student, I remember seeing only two genera of ascomycetes under the microscope: Aspergillus and Penicillium. However, there is another genus that is commonly used in biology classes, Sordaria, the dung fungi, whose most popular species is Sordaria fimicola, which I decided to call the lab dung fungus.

As its name suggest, the lab dung fungus is found growing on dung, more specifically on dung of herbivorous mammals. For a long time, it was thought that this species required dung to complete its life cycle. After growing on dung, the lab dung fungus releases its spores in the environment, they adhere to the surface of plants and are ingested by grazing mammals, restarting the cycle. However, it is now known that this species can grow and reproduce on plant matter without requiring dung, although more studies are needed to understand how the presence or absence of dung affects its fitness.

During most of its life, the lab dung fungus exists, just like other fungi, solely as a network of hyphae, the mycelium, growing inside the medium on which it feeds, in this case decaying plant matter, especially in dung. These hyphae are haploid (n), meaning that they have only one copy of each chromosome. When two hyphae touch, they can fuse and create a cell with two nuclei, the dykarion, each nucleus coming from one of the original hyphae. The dikaryotic cells divide through mitosis without fusing their nuclei, originating a set of dikaryotic hyphae that form a fruiting body, the perithecium, that grows inside the mycelium of haploid hyphae.

The perithecium is kind of pear-shaped and, inside of it, some dikaryotic cells allow their nuclei to fuse into a single, diploid nucleus, which now has two chromosomes of each type (2n), one from each parent hypha back then when the haploid hyphae meet. This newly formed diploid cell is a zygote but instead of growing into diploid hyphae by mitosis, it immediately undergoes meiosis to originate once again a set of haploid nuclei. The four resulting nuclei from meiosis each one undergoes mitosis, resulting in eight final nuclei, which remain lined up in the elongated cell. The cell then divides into eight individuals cells, each with one of the nucleus, and they turn into spores, ascospores, and remain inside an elongated sac, the ascus. When the ascospores are mature, they are released in the environment and can germinate to create a new set of haploid hyphae.

Lineages of the lab dung fungus found in nature often have very dark ascospores and this is called the wild type. However, one laboratory lineage has lighter, often gray ascospores, and is called the tan type. The color of the spore is determined by a single gene in one of the chromosomes. Thus, if you cross the wild and the tan types, the ascus of the hybrid will have four dark and four light spores, and this is how the lab dung fungus becomes a good species to understand meiosis and chromosome crossover in biology classes.

Before meiosis occurs, all chromosomes in a cell are duplicated, resulting in cell with four chromosomes of each type (4n) of which two are from one parent and two are from the other. When the nucleus divides for the first time, each daughter cell will be a special case of 2n, in which the two copies of each chromosome are originally from the same parent. In the lab dung fungus, considering the chromosome with the color gene, this would create a pattern like (AA)(AA) in these two nuclei. After the second division of meiosis, the pattern becomes (A)(A)(A)(A) and, after the mitosis that leads to the eight final spores, (A)(A)(A)(A)(A)(A)(A)(A).

However, if crossover occurs, one pair of chromosomes from different parents exchange pieces with each other, while the other pair remains unaffected. As a result, they exchange the gene responsible for the color and the final product, instead of being 4 of one color followed by 4 of another (4:4 patterns), shows a 2:4:2 or a 2:2:2:2 pattern.

Sometimes other weird patterns appear as well, such as 2:1:1:1:1:2 patterns, but I guess this happens because of some mechanical action where one spore can roll over another and end up outside of its original position inside the ascus, perhaps caused when they are squeezed out of the perithecium. Really stranged patterns are those in which there are not 4 spores of each color, which include very rare instances of 6:2 or 5:3 patterns, and those are explained as the result of errorSs during chromosome replication.

Isn’t the lab dung fungus indeed a very cool model to use in classes? I’m sad that I haven’t had the opportunity to see this in my genetics or microbiology classes. If you studied biology, were you lucky enough to see this? Let us know!

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References:

Dotson J (2019) Life cycle of Sordaria Fimicola. Sciencing. Available at < https://sciencing.com/life-cycle-sordaria-fimicola-6909851.html >. Acccess on 6 August 2020.

Kitani Y, Olive LS, E-Ani AS (1961) Transreplication and Crossing Over in Sordaria fimicola. Science 134: 668-669. https://doi.org/10.1126/science.134.3480.668

Newcombe G, Campbell J, Griffith D, Baynes M, Launchbaugh K, Pendleton R (2016) Revisiting the Life Cycle of Dung Fungi, Including Sordaria fimicola. PLoS ONE 11(2): e0147425. https://doi.org/10.1371/journal.pone.0147425

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