Fungi: Sexual Reproduction Steps
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Published: Wed, 23 May 2018
Do you wonder why mold appears on a piece of bread after it was stored for a long time? The reason for this is fungi. Fungi are multi-cellular organisms, which have species up to 1.5 million species approximately. Fungi play important roles in the ecosystem. They decompose organic material and recycle nutrients, allowing other organisms to absorb those nutrients. Fungi are heterotrophs meaning that they can’t produce their own food. Unlike many animals, fungi absorb nutrients from their surroundings by secreting various types of enzymes. This wide variety of enzymes enables fungi to digest many different kinds of food sources. Many species of fungi can grow as filaments and yeasts, but they most grow as filaments. Fungi’s cell walls contain chitinm which provide the cell walls with strength and flexibility. The bodies of fungi are composed with network of filaments called hyphae. Fungi can reproduce either by sexual reproduction or by asexual reproduction.
Asexual reproduction in fungi occurs more frequently than sexual reproduction does. Asexual reproduction requires only one parent and the genetics of the offspring will be identical to that of the parents. The most important type of asexual reproductions is spore formation. Spores are produced by mitosis in fungi that grow as filament. They are informally known as molds when they become visible. After spores are released, they land on a favorable environment and form individuals. Another type of fungi reproduction is bud formation in yeasts. Fungi growing as yeast divide their cells and separate a bud cell to form a new individual. Fungi can also asexually reproduce by fragmentation. A mycelium is formed by the hyphae and is branches that infiltrate the material which the fungi are on. When the mycelium is damaged or cut into fragments, new fungi can form from the fragments. As mentions earlier, asexual reproduction happens from parts of a fungus develop into a new individual. Because the new individual develops from parts of fungus, the new individual will have the same genetic materials as their parents’.
Sexual reproduction requires two parents. Like many animals, fungi require different mycelia from each parent to reproduce sexually. Thus, it allows fungi to have genetic variation. Sexual reproduction in fungi is very complex. This type of reproduction requires two haploid nuclei. The two haploid nuclei have to fuse to form diploid nuclei.
The first stage of sexual reproduction begins when hyphae from two mycelia release signal molecules called pheromones. Pheromones will bind to the receptors and the hyphae will extend toward pheromones and fuse with another hyphae if the mycelia are different from each other. The reason for pheromones to combine with different mycelia is because it causes genetic variation. If the mycelia are identical, the hyphae will not combine with the other hyphae because it will not cause genetic variation. Plasmogamy is the union of cytoplasms from two mycelia. In many fungi, parts of the mycelia containing different nuclei fused. This type of mycelia is called heterokaryon. In other types of fungi, the haploid nuclei pairs up with another haploid nuclei to form a cell. This type of mycelia is called dikaryotic. The next stage of this reproduction is karyogamy. The haploid nuclei fuse to produce diploid cells. Zygote forms during karyogamy and meiosis produces spores that will be dispersed. Because of karyogamy and meiosis, fungi become more genetic varied.
As mentioned earlier, fungi can reproduce sexually and asexually. The ability to alternate between sexual reproduction and asexual reproduction is called heterogamy. Both asexual reproduction and sexual reproduction have pros and cons. If fungi are living in a suitable environment, they will reproduce asexually since the environment is favorable for them. Fungi do not need to evolve or develop diverse genetics. In this case, asexual reproduction will be more advantageous to fungi. In contrast, sexual reproduction will happen when fungi grow in an unsuitable environment. In an unfavorable environment, fungi are not able to function properly and may lead to the death of many fungi. Due to sexual reproduction, it provides fungi with genetic variation. Genetic variation may increase the chance of survival of the offspring. If fungi reproduce asexually in an unfavorable environment, the offspring will probably face the same problems as their parents have and may end up dying. Therefore, fungi choose to reproduce sexually in an unfavorable environment.
Chytrids are a division of the Fungi kingdom and are classified in the phylum Chytridiomycota. Chytrids are unicellular organism and are everywhere in lakes and soil. Most of Chytrids are decomposer; however they are important mutualists, like anaerobic chytrids. Anaerobic chytrids live in sheep and cattle’s digestive tracts and help digest plants, which greatly enhance sheep and cattle’s digestive system. According to many molecular evidences, chytrids are believed to be the most primitive fungi. Like other fungi, the cell walls of chytrids are composed with chitin. However, they have unique flagellated spores called zoospores. Similarly to other fungi, chytrids are able to produce asexually and sexually.
In asexual reproduction, chytrids reproduce by zoospores. After zoospores are released, they will travel until they find a suitable environment and attach themselves into the environment. The cytoplasm grows and meiotic division occurs. Then, the cell walls form around the zoospore. Protoplasm, the living part of cells that is surrounded by cell walls, increase as the cell grows. Lastly, the protoplasm divides, causing sporangia, structures in fungi that contain and produce spores, to produce zoospores.
Sexual reproduction in chytrids involves the production of diploid spores from the combination of gametes from different mycelia. The sexual reproduction in chytrids started from the haploid thallus, called the gametothallus, produces male and female gametes. A thallus is a plant structure that lacks specialized tissues, like stem and leaves. Male gametes are orange and small, while female gametes do not have color and are big. Female gametes attracted male gametes by producing serenin. Male gametes attracted female gametes by releasing parisin. The diploid thallus, sporothallus, produces two types of zoosporgia, zoosporgangium (meitosporangium) and resistant sporangium (meiosporangium). Zoosporgangium produces zoospores that function the same as those in asexual reproduction.
Ascomycetes are fungi in the phylum Ascomycota. Ascomycetes vary in different sizes and can range from unicellular organisms to cup fungi and morels. Ascomycetes are important decomposers. Many ascomycetes live with green algae or cyanobacteria. They live in beneficial associations called lichens.
In asexual reproduction, ascomycetes produce spores called conidia. However, not like other fungi, Conidia are formed at the tips of hyphae called conidiophores. Conidiophores are dispersed by the wind and traveled to a suitable environment. There, they land and form new individuals.
In sexual reproduction, Ascomycetes also involve the use of Conidia. After Conidia are released, conidia fused with a mycelium from a different individual. The combination of Conidia resulted in the formation of dikaryotic cells, which is called plasmogamy. Spores gathered around the dikaryotic cells in a process called free cell formation and because most cytoplasm surrounds the nucleus, the cell sap is left outside. The new hyphae are called asci. In each asci, there are two parental genomes and four different nuclei are formed by meiosis. After that, mitosis occurs and produces spores, which are called ascospores. The ascospores are released after they have been developed.
To sum up, fungi are organisms that can produce asexually and sexually. In the asexual way, fungi produce spores, undergo bud formation, and go through fragmentation. However, the genetic variation of an offspring by asexual reproduction is the same as its parent’s. As a result, fungi usually reproduce by asexual reproduction when they are in an ideal and a favorable environment. In sexual reproduction, fungi release pheromones and pheromones, then, bind with the receptor from different hyphae. The hyphae fuse with each other when they meet. After the hyphae combine, diploid cells are produced. Spores are produced by meiosis and are released. Sexual reproduction causes a diverse genetic variation because the offspring is produced by two parents. Therefore, fungi reproduce sexually when they are in an unsuitable environment because if the offspring have the same genetic variation as their parents’, their chance of dying is quite high. As mentioned above, different phylum of fungi also have similar techniques of reproducing. They release spores and let the spores land on a suitable environment to develop in asexual reproduction, and they also release spores, so they can fuse with spores from different mycelia. After they combine, they produce a new kind of spore and let the spore lands on a suitable environment. Fungi, believe it or not, are important organisms in the ecosystem. They can act as decomposers, parasites, and mutualists. As humans, we should appreciate the role of fungi in the ecosystem because without them, the ecosystem would not be balanced and would surely have an impact on us.
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