The fungus Sordaria fimicola is an easily produced fungus with a unique form of reproduction. It is one of many kinds of sac fungi. This fungus provides a model organism for studying genetics. S. fimicola is an especially useful tool for teaching students about meiosis.
TL;DR (Too Long; Didn't Read)
The life cycle of the dung fungus Sordaria fimicola provides an ideal model for studying genetics and meiosis.
What Type of Fungus Is Sordaria Fimicola?
The source for Sordaria fimicola is not glamorous. In fact, it often grows in decaying organic matter, and most notably on the dung of plant-eating animals. S. fimicola is therefore also called dung fungus.
It is classified as an ascomycete fungus. The phylum name for these types of fungi is Ascomycota.
Properties of Ascomycota
The fungi species belonging to Ascomycota are called ascomycetes. So far, mycologists have discovered at least 30,000 species of ascomycetes.
Many of these ascomycetes are known as sac fungi because of their asci shape and characteristics. These asci hold eight haploid spores or ascospores. Ascomycete fungi are known for their projection of spores, sometimes at significant distance.
Ascomycetes are considered dikaryon fungi because of their nuclear phase as dikaryons, or having two haploid nuclei.
Ascomycetes are broadly different from each other. Some species are considered pathogens and can cause illness in animals as well as plants. Others are beneficial. Common yeast is an ascomycete that is used in fermentation for alcoholic beverages like beer.
As for Sordaria fimicola, it is considered a fairly typical ascomycete in its life cycle and reproductive methods.
The Sordaria Fimicola Life Cycle
The fungus S. fimicola starts its life cycle as an ascospore. This ascospore was stored in an ascus until enough pressure built up to eject the spore into the air. This ascospore exists in haploid form. It then germinates and forms long haploid cell filaments called hyphae.
These grow in their environment, such as dung or decaying plants, digesting as they go. Asexual reproduction in these fungi is referred to as their anamorph life cycle.
Sexual Reproduction and Meiosis
Sexual reproduction does not occur unless these haploid hyphae encounter others. Eventually, some of these haploid hyphae meet up and join together into one cell with two nuclei. It undergoes mitosis, continually dividing into new cells. This new cell, a dikaryon, is not a true diploid cell, despite the fact that two haploid cells have joined up; the two nuclei remain separate and do not fuse.
The dikaryotic hyphae keep growing inside a mass of haploid cells, forming the fruiting body or ascoma. Eventually, after the cells have been through a few rounds of mitosis, some of the dikaryon cells can fuse and form zygotes with a single diploid nucleus. This sexual reproductive portion of the Sordaria life cycle is called the telomorph life cycle.
Through the process of meiosis, a recombination of the genome from “crossing over,” those diploid zygotes develop four haploid nuclei. Meiosis yields greater genetic diversity for the fungus.
These nuclei then undergo their own mitosis. Eight haploid nuclei result from this. At that point, cells form around the nuclei. These new cells are ascospores.
An Explosive Distribution
The eight ascospores reside in a sac called an ascus. Asci are held in the perithecium, or fruiting body (also sometimes called ascoma). This is the sac that bursts in nature and sends the ascospores out into the air, so the process can start over.
Using ejection from the fruiting body is necessary to distribute the ascospores, because they are not mobile otherwise. The fungus relies on kinetic energy to get the job done. The explosive eruption of spores results from pressure building up in tip of the ascus.
In order to ensure the ascospores distribute into the air, the ascus must help them shoot toward the sky. Glycerol and other components lead to the pressure buildup. Sometimes the pressure can reach three atmospheres.
Does Sordaria Fimicola Require Dung to Reproduce?
For many years, scientists used the presence of dung fungi to infer the behavior of herbivore mammals in ancient times. Because S. fimicola ascospores burst out of mammalian dung, scientists assumed the life cycle of the dung fungi depended on the presence of dung. However, recent research suggests a lack of such correlation.
It is true that the ejection of S. fimicola ascospores from dung allows them to adhere to the surfaces of plants. Herbivores would eat the plants with the fungus on them, and begin a cycle of reintroducing spores to the animal’s gastrointestinal tract.
In fact, S. fimicola does not require mammalian herbivore dung to subsist. Scientists found that the fungus can also grow on plant tissues. The fungus can also affect different plants in different ways; it can inhibit growth in maize, for example. But other plants receive a benefit from the fungus.
So despite the prevalence of dung fungi in mammalian dung, the species does not require dung as a substrate for reproduction. More research is needed to compare the prevalence of Sordaria fimicola on dung versus plant residues.
Why Sordaria Fimicola Is Ideal for Teaching
This fungus is appealing to teachers for its ease of culture and its elegant and interesting reproduction methods. Straightforward experiments with S. fimicola can be carried out in a laboratory without much effort.
Sordaria can produce fruiting bodies within a week’s time, allowing students to witness and record genetic processes.
S. fimicola provides an orderly arrangement for students to view the first and second divisions of meiosis. Students can, within a short period of time, gain hands-on knowledge about “crossing over” or chromosome exchange.
One helpful feature of Sordaria is its ascospore color. The color represents phenotypes in genetic variants of the fungus. For example, black ascospores are the wild-type color. There are also other colors such as red, pink, tan and gray that represent differences in their alleles, separating them from the wild-type.
Students can have plated cultures of S. fimicola to observe the asci and their ascospore colors. Those with mixed colors reveal mating between different strains.
Types of Asci
There are many distinct qualities in sac fungi; one is their variation of asci. There are different types of asci that can occur. Some of them are unitunicate-operculate asci. This kind of asci has a sort of lid that opens to eject the spores. Only apothecial ascomata use these kinds of asci.
Another type of asci that can occur are the unitunicate-inoperculate asci. These do not have lids, but rather a little elastic-like mechanism at their tip that stretches and allows spores to eject. These types of asci can be found primarily in perithecial ascomata.
Prototunicate asci work via oozing spores, rather than ejecting them out. Prototunicate asci have a rounded shape, and their walls dissolve at maturity.
Another type of asci that can occur are the bitunicate asci. These are double-walled asci. The outer wall ruptures at maturity and the inner wall expands up with the ascospores within it. This structure stretches up and launches the spores.
It is clear that members of the phylum Ascomycota possess unique and interesting ways to reproduce and to spread their spores in nature. The life cycle of Sordaria fimicola provides the ideal model to learn about these kinds of fungi, how they reproduce and how they can serve as models to educate students about genetics and meiosis.