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Ascomycota: Life Cycle & Classification

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

We need fungi for everything from producing food to creating antibiotics to cleaning up decayed organisms. We are going to discuss the life cycle and classification of Ascomycota.

Ascomycota

Fungi are unicellular or multicellular organisms such as yeasts, molds, and mushrooms that break down organic matter. Although that sounds innocent enough, fungi sometimes get a bad reputation because most fungi are decomposers. They break down organic matter that was once part of living organisms, which can include feces and dead bodies. While this decomposition plays an essential role in the environment, it's easy to associate fungi with death and decay.

Many of us have managed to look past that association though. While some fungi are toxic to consume, there are many more that we use and enjoy on a regular basis. The soft, fluffy rolls that you like to eat at dinner are made using fungi in the form of yeasts. Along the same lines, the occasional beer that you may have is also produced using yeasts.

Despite the mixed associations, almost 75 percent of fungi are classified under a single phylum called Ascomycota. Ascomycota are fungi that contain a sac that holds spores during the sexual stage of the life cycle. Since the sac is one of the most prominent features used to identify members of Ascomycota, you'll also hear them called sac fungi.

Life Cycle

Ascomycetes change in form depending on which part of their life cycle they are in and whether they are undergoing asexual or sexual reproduction. Remember to keep in mind that asexual reproduction means reproduction without the joining of genetic information from two parents. Sexual reproduction, on the other hand, is the reproduction that occurs from joining genetic information from two individuals of the opposite sex. The life cycle of ascomycetes is largely divided by the type of reproduction they are going to take part in. Let's look at the asexual life cycle first.

Asexual Cycle

This cycle starts with a mycelium , which is the fungi in the vegetative form that contains branched filaments called hyphae. There are specialized hyphae, called conidiophores, that branch off from the mycelia and are capable of producing spores.

In the next step of the cycle, the conidiophores will release their spores, called conidia or mitospores. These spores, like the fungi in every step of the asexual life cycle, are haploid. The conidia will undergo the process of mitosis. This is the process of cell reproduction that creates two cells that are genetically identical to the parent cell and each other.

When the spores complete mitosis, they will remain dormant to wait for ideal environmental conditions. Once conditions are favorable, the spores will germinate to produce a new mycelium and the cycle starts over again.

Sexual Cycle

Let's start the sexual reproductive cycle from the point of the mycelium as well. With sexual reproduction, there will be at least one female and one male mycelium. The male mycelium has an antheridium and the female has an ascogonium as their sex organs. When the ascogonium and antheridium come together, they undergo plasmogamy, which is the fusion of the cytoplasm of ascogonium and antheridium. The interesting thing about plasmogamy is that fusion of the nuclei does not take place at this point.

In the next step of the life cycle, the fused antheridium and ascogonium are now called an ascocarp. Sac-like cells called asci begin to grow within the ascocarp. As the asci are forming, ascospores, which are haploid spores, from the female and male come together in each ascus. Ascus is just the singular form of asci, so an ascus is one of those sac-like cells, the distinguishing feature that gives Ascomycota the nickname sac fungi. Within the asci, the ascospores undergo karyogamy, which is the fusion of their nuclei to form a diploid zygote.

The diploid zygotes will go through the process of meiosis, which is cell division that creates haploid daughter cells that are not identical to each other. At this point, the ascospores are haploid again. The cells now go through mitosis to create more ascospores.

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