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15.3: Fungus-Like Organisms - Biology

15.3: Fungus-Like Organisms - Biology


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Myxogastria (formerly Myxomycota): Plasmodial Slime Molds

Myxomycetes (members of the Myxogastria) are fungus-like organisms called slime molds, but they are not members of Kingdom Fungi. When it dries out or runs out of food, it begins to make fruiting structures called sporangia (sporangium, singular). Inside these sporangia, the myxomycete will undergo meiosis, wall off individual nuclei and make haploid spores for aerial dispersal. The spores have cell walls made of cellulose, like plants. This means that, unlike true Fungi, they have a diplontic life cycle.

Observe the slime molds on display. If possible, make a wet mount of a small sample. If there are motile cells (cells that are actively moving around), look for the presence of 2 flagella. This also distinguishes myxomycetes from Kingdom Fungi, where only one flagellum is present. If a mature amoeba is present, can you see cytoplasmic streaming occurring?

Draw what you see and make observations below. Label the name, function, and ploidy of any identifying structures.

Other Slime Molds: Dictyostelia and Protostelia

Quite on theme for fungal classification, there are several different groups of unrelated organisms that are called slime molds. The two mentioned above, Dictyostelia and Protostelia, represent cellular slime molds and are relatively closely related to the plasmodial slime molds. Unlike the plasmodial slime molds, these groups exist as individual amoebae until it is time to leave the area. In the Dictyostelia, the amoebae assemble together to form elaborate fruiting structures where only some of the individuals will become spores and the rest will die. They are studied for this altruistic behavior.

Oomycota -- The Water Molds

Oomycetes are also fungus-like organisms with cell walls made of cellulose. Similar to myxomycetes, they have motile spores with 2 flagella. However, one of these flagella is "normal"-looking (called a whiplash flagellum) and the other is ornamented. This strange characteristic puts organisms into a group called the heterokonts (meaning "different flagella"). Like us, true Fungi are part of the opisthokonts (opisth- meaning rear, -kont meaning flagellum).

Also similar to myxomycetes, oomycetes have a diplontic life cycle. What does this mean?

If supplies are available, place a dead bug into a petri dish with some pond water. Observe the insect under a dissecting scope, then add a cover to your petri dish. Label “Saprolegnia Culture”, your initials, and the date. Set the covered dish aside to incubate until Lab Heterokonts, where you will be learning about heterokonts in more detail.


Animal-like, Fungus-like, and Plant-like Protists

Protists are a diverse group of eukaryotic organisms belonging to Kingdom Protista. There are few similarities between individual members of this Kingdom, as it includes all the eukaryotes that are not animals, plants, or fungi.

Most protists are microscopic and unicellular, though a few species are multicellular. Typically, protists reproduce asexually, though some are capable of sexual reproduction. Some protists are heterotrophs, and feed on other microscopic organisms and carbon-rich materials they find in their surrounding environment others are photosynthetic and make their own food using chloroplasts.


Fungus-like organisms in deep time and deep rock

Discoveries are sometimes driven by a string of lucky chances. This is how we found fungus-like fossils deeper in rock and time than we would have thought possible.

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For me it started in 2001, when I received an e-mail from Birger Rasmussen in Perth. I didn’t know him personally but had been impressed by his innovative work on radiometric dating of sedimentary rocks. Birger wrote that he had just had a manuscript about very early animal trace fossils rejected by the journal Nature. Three referees had trashed it, but one had at least tried to be constructive, and he had a hunch that it was me. So would I like to come to Australia and collaborate with him in the study of the fossils?

His hunch was correct: it was me. He also correctly guessed that I could not resist the challenge to work on the fossils. Thus began years of joyous collaboration on the Stirling Range biota, resulting in several publications (Rasmussen et al., 2002, 2004 Bengtson et al., 2007) and the main conclusion that there were motile multicellular organisms on the seafloor some two billion years ago, more than a billion years before the oldest known fossil animal. Few of our colleagues liked the conclusion, though we never claimed that the trace makers were animals, but so far no-one has offered any better interpretation of the fossils.

Stirling Range trace fossils found on the soles of sandstone beds. In the lower image, the traces have been outlined in blue. After Bengtson et al. 2007.

Birger’s and my paths didn’t cross again for a while after that adventure, though we kept in sporadic contact. In the meantime I teamed up with Magnus Ivarsson, who had just concluded a Ph.D. project on fossil microbes of the "deep biosphere”, the immense but poorly known biota hidden in pores and cracks hundreds to thousands of metres deep in rock. Magnus had found fossil mycelia of filamentous fungi in 48 million-year-old basalts in the northern Pacific Ocean (Ivarsson et al., 2012). Basalts are the remains of lava flows, and when lavas erupt on the seafloor they cool rapidly into an almost foamy texture. The bubbles remain open for millions of years and provide a haven for colonizing microbes. This is where Magnus’ fungi lived before they became fossilized. We could also show how the fungi lived in symbiotic association with bacteria-like organisms that were using chemically stored energy (in the absence of light for photosynthesis) to produce organic matter (Bengtson et al., 2014 Ivarsson et al., 2015).

Symbiosis-like association of fungi (filaments) with microbes ("cauliflower" and small bright grains) from 48 million-year-old subseafloor porous volcanic rocks. Specimen is 0.8 mm across. X-ray tomographic rendering.

Fossils may thus inspire searches for fungi in the modern deep biosphere, and biologists are now looking for genetic markers of fungi in deep rock. But we may also use the 48 million-year-old finds as search images for more ancient deep life, as bubbly lavas have formed on Earth since the beginning of rock formation more than four billion years ago. This might even be the right environment to look for the last common ancestor of all life. We decided to go look in the really old rocks.

At that moment my mailbox went pling. It was Birger: “I hope you are well and apologize for not keeping in touch. I met a colleague of yours at the Florence Goldschmidt - Magnus Ivarsson, who had an excellent poster on fossilized fungi in seafloor basalts. I have recently found filamentous structures in Proterozoic basalts which seem quite similar to those you have been studying. I will be in Stockholm on Thursday and was wondering whether you had time for a chat? Be great to see you again.

Thus began the new chapter in our cooperation, and the intitial result is what you see in the now published article. We used X-ray tomographic microscopy to disentangle the mycelium-like structure in three dimensions, and we applied X-ray microanalyses and Raman spectrometry to characterize the mineralization sequence within the filament-containing bubbles and to determine the temperatures of crystallization. We were able to demonstrate that the filaments were indeed fossils of cavity-dwelling organisms and that they had colonized the vesicles and cracks in the cooling lava not long after eruption 2.4 billion years ago. We concluded that fungus-like fossils in the deep biosphere go back to at least 2.4 billion years, much further back than fungi are conventionally thought to have existed. Porous crustal rocks seem to provide a safe haven for deep-living organisms over eons of time, and these environments provide excellent conditions for fossil preservation.

Bubble in 2.4 billion-year-old lava containing mycelial fossils. Bubble is 0.8 mm in diameter. X-ray tomographic rendering.

The Earth is 4.6 billion years old. How far back in time can we go with this kind of biota? We continue searching and hope to inspire others to do the same. Stay tuned.

The paper in Nature Ecology & Evolution is here: http://go.nature.com/2o2zElS

Bengtson, S., Ivarsson, M., Astolfo, A., Belivanova, V., Broman, C., Marone, F., and Stampanoni, M. 2014. Deep-biosphere consortium of fungi and prokaryotes in Eocene sub-seafloor basalts. Geobiology, 12:489–496.

Ivarsson, M., Bengtson, S., Belivanova, V., Stampanoni, M., Marone, F., and Tehler, A. 2012. Fossilized fungi in subseafloor Eocene basalts. Geology, 40:163–166.

Ivarsson, M., Bengtson, S., Skogby, H., Lazor, P., Broman, C., Belivanova, V., and Marone, F. 2015. A fungal-prokaryotic consortium at the basalt-zeolite interface in subseafloor igneous crust. PLoS One, 10 (e0140106):19 pp.

Rasmussen, B., Bengtson, S., Fletcher, I.R., and McNaughton, N. 2002. Discoidal impressions and trace-like fossils more than 1200 million years old. Science, 296:1112–1115.

Rasmussen, B., Fletcher, I.R., Bengtson, S., and McNaughton, N. 2004. SHRIMP U–Pb dating of diagenetic xenotime in the Stirling Range Formation, Western Australia: 1.8 billion year minimum age for the Stirling biota. Precambrian Research, 133:329–337.

Bengtson, S., Rasmussen, B., and Krapež, B. 2007. The Paleoproterozoic megascopic Stirling Biota. Paleobiology, 33:351–381.


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14.3 Introduction to Fungi

Do you see the organisms growing on the bread in Figure below? They belong to the Kingdom Fungi. Molds growing on foods are some of the most common fungi in our everyday lives. These organisms may seem useless, gross, and costly. But fungi play very important roles in almost every terrestrial ecosystem on Earth.

Naked Science Scrapbook: What is a fungus?

What Are Fungi?

Fungi (singular, fungus) are a kingdom in the domain Eukarya. The fungi kingdom may contain more than a million species, but fewer than 100,000 have been identified. As shown in Figure below, fungi include mushrooms and yeasts in addition to molds.

Besides mushrooms, fungi also include yeasts and molds

Bozeman Science: Fungi: (Note: some minor evolution is mentioned in the beginning of the

Most fungi are multicellular, but some exist as single cells. Fungi spend most of their life cycle in the haploid state. They form diploid cells only during sexual reproduction. Like the cells of protists and plants, the cells of fungi have cell walls. But fungi are unique in having cell walls made of chitin instead of cellulose. Chitin is a tough carbohydrate that also makes up the exoskeleton (outer skeleton) of insects and related organisms.

Habitats of Fungi

You probably already know where some species of fungi live. No doubt, you’ve seen them growing on rotting logs and moist soil. In fact, most fungi live on dead matter or soil. However, some fungi are aquatic. Others live in or on other organisms in symbiotic relationships.

Structure of Fungi

Except for yeasts, which grow as single cells, most fungi grow as thread-like filaments, like those shown in Figure below. The filaments are called hyphae(singular, hypha). Each hypha consists of one or more cells surrounded by a tubular cell wall. A mass of hyphae make up the body of a fungus, which is called a mycelium (plural, mycelia).

A mycelium may range in size from microscopic to very large. In fact, one of the largest living organisms on Earth is the mycelium of a single fungus. A small part of a similar fungus is pictured in Figure below. The giant fungus covers 8.9 square kilometers (3.4 square miles) in an Oregon forest. That’s about the size of a small city. The fungus didn’t grow that large over night. It’s estimated to be 2,400 years old—and it’s still growing!

Reproduction of Fungi

The majority of fungi can reproduce both asexually and sexually. This allows them to adjust to conditions in the environment. They can spread quickly through asexual reproduction when conditions are stable. They can increase their genetic variation through sexual reproduction when conditions are changing and variation may help them survive.

Asexual Reproduction

Almost all fungi reproduce asexually by producing spores. A fungi spore is a haploid cell produced by mitosis from a haploid parent cell. It is genetically identical to the parent cell. Fungi spores can develop into new haploid individuals without being fertilized.

Spores may be dispersed by moving water, wind, or other organisms. Some fungi even have “cannons” that “shoot” the spores far from the parent organism. This helps to ensure that the offspring will not have to compete with the parents for space or other resources. You are probably familiar with puffballs, like the one in Figure below. They release a cloud of spores when knocked or stepped on. Wherever the spores happen to land, they do not germinate until conditions are favorable for growth. Then they develop into new hyphae.

Smashing a Giant Puffball Mushroom

Yeasts do not produce spores. Instead, they reproduce asexually by budding. Budding is the pinching off of an offspring from the parent cell. The offspring cell is genetically identical to the parent. Budding in yeast is pictured in Figure below.

Sexual Reproduction

Sexual reproduction also occurs in virtually all fungi. This involves mating between two haploid hyphae. During mating, two haploid parent cells fuse, forming a diploid spore called a zygospore. The zygospore is genetically different from the parents. After the zygospore germinates, it can undergo meiosis, forming haploid cells that develop into new hyphae.

Classification of Fungi

For a long time, scientists considered fungi to be members of the plant kingdom because they have obvious similarities with plants. Both fungi and plants are immobile, have cell walls, and grow in soil. Some fungi, such as lichens, even look like plants (see Figure below).

The Kingdom Fungi

Today, fungi are no longer classified as plants. We now know that they have unique physical, chemical, and genetic traits that set them apart from plants (and other eukaryotes). For example, the cell walls of fungi are made of chitin, not cellulose. Also, fungi absorb nutrients from other organisms, whereas plants make their own food. These are just a few of the reasons fungi are now placed in their own kingdom.

Fungal Phyla

Classification of fungi below the level of the kingdom is controversial. There is no single, widely-accepted system of fungal classification. Most classifications include several phyla (the next major taxon below the kingdom). Three of the most common phyla are compared in Table below.

Lesson Summary

  • Fungi are a kingdom in the domain Eukarya that includes molds, mushrooms, and yeasts. Most fungi are multicellular. They are unique in having cell walls made of chitin.
  • Most fungi live on dead matter or soil. Some live in aquatic habitats. Many are involved in symbiotic relationships.
  • Most fungi grow as thread-like filaments called hyphae. A mass of hyphae make up the body of a fungus, called a mycelium.
  • The majority of fungi can reproduce both asexually and sexually. This allows them to adjust to conditions in the environment. Yeast reproduce asexually by budding. Other fungi reproduce asexually by producing spores. Sexual reproduction occurs when spores from two parents fuse and form a zygospore.
  • Fungi used to be classified as plants. Now, they are known to have unique traits that set them apart from plants. For example, their cell walls contain chitin, not cellulose, and fungi absorb food rather than make their own. Below the level of the kingdom, fungi classification is controversial.

Lesson Review Questions

Recall

2. List several habitats where fungi live.

3. Describe the general structure of multicellular fungi.

4. Identify ways that fungi spores may be dispersed.

5. State why fungi were once classified as plants.

Apply Concepts

6. Create a diagram to show the life cycle of a multicellular fungus.

Think Critically

7. Explain the significance of the chitin cell wall of fungi.

8. Compare and contrast a fungi spore and zygospore.

Points to Consider

In this lesson, you read that fungi differ from plants in major ways. For example, unlike plants, fungi do not make their own food by photosynthesis.


Examples of Organism

Bees are an example of organisms that live socially. Many bees work to collect sugary nectar from flowers, which they store in their hive. They protect the hive and work cooperatively to build and repair it. The hive is usually attached to another organism, a tree. This is an example of a mutualistic relationship between organisms. The bees are provided a place off the ground, away from bears and other animals that want to eat their honey. The tree is provided with a source of pollination for reproduction. Bees are a major pollinator of agricultural crops as well. In fact, it has been estimated that without the bees, billions of dollars of crops would not be able to pollenate. That is a scary fact, considering bees have been in decline globally for decades.

Tapeworms

Tapeworms are an example of a parasitic organism, or an organism that feeds off of other organisms to survive. The tapeworm lives in the intestines of mammals, and feeds off the dissolved nutrients that the mammal has worked so hard to gather. Tapeworms reproduce in the gut, lay eggs in the feces, and new animals are exposed when they come into contact with the eggs, which can lay dormant in the soil for years. Parasitism is a type of relationship between organisms in which one organism benefits and one organism suffers. Single parasites do not often kill their host, because in doing so they would lose a home. However, large infestations of parasites can lead to malnourishment and even death if not treated.

Great White Shark

Considered the top of the food chain in the ocean, the great white shark is the ultimate predatory organism. The shark’s keen sense of smell allows it to track the scent of blood for miles underwater, leading it to wounded animals and corpses it can devour. The great white is one of only a few sharks ever documented leaping from the water in a strike on prey. Great whites often feed on seals, which are very agile and can outturn the shark. However, the sharks usually strike from below, honing in on the seal and hitting it at great speeds. Cells around the shark’s mouth are sensitive to small electrical impulses given off by prey, and the shark can literally feel its prey before it touches it. This makes the great white one an ultimate predatory organism.


List of 3 Common Saprophytic Fungus (With Diagram)

List of three common saprophytic fungus: 1. Mucor 2. Yeast 3. Penicillium.

Saprophytic Fungus # 1. Mucor:

Mucor, also called mould, is a very common saprophytic fungus growing abundantly on decayed organic matters, parti­cularly on those rich in carbohydrates—starch and sugar. Soft white cottony patches of Mucor are frequently found on rotten bread, vegetables and dung.

Plant Body:

The plant body is a copiously branched mycelium, which is a collection of slender non-septate threads called hyphae (sing, hypha). The wall, made, of fungus-cellulose, enclose cytoplasm with many vacuoles and innu­merable nuclei (Fig. 192).

So they are coenocytic. Glycogen and oil globules are present to serve as reserve food. The hyphae become thinner and thinner, the more they penetrate into the sub­stratum for absorbing nourishment. Non-septate mycelium becomes septate on attaining old age and during reproduction.

Reproduction:

Mucor is reproduced by asexual and sexual methods. During asexual reproduction a number of stout aerial hyphae shoot up from the superficial mycelium. After growing to a certain extent, the tip of each of them swells, and some protoplasm with reserve food flows to the enlargement from the adjoining region. Proto­plasm collects more densely towards periphery, the central portion remaining comparatively thin and vacuolated.

A good number of vacuoles arrange themselves between the outer denser protoplasm, called sporoplasm, and the central thinner protoplasm, known as columella-plasm.

The flattened vacuoles coalesce, and as a result, a distinct cleft is formed separating the two regions now a wall is constructed along the cleft delimiting the central sterile dome-shaped region, called columella, which projects into the enlarge­ment. By progressive cleavage or furrowing the sporoplasm now breaks up into a good number of angular masses, each with cytoplasm and many nuclei.

They round off, tough black walls are secreted and final­ly become spores. Mucor spores are also called gonidia (sing, gonidium). The en­largement containing the spores is the sporangium or gonidangium, and the hypha bearing the sporangium at the tip is called sporangiophore or gonidangiophore (Fig. 193).

The outer wall of the sporangium dis­solves in water and the spores are libera­ted. Often a number of spores remain held in suspension. On suitable medium each spore germinates forming one or more germ tube which gives rise to new mycelium.

Sexual reproduction in Mucor takes place by conjugation. Usually hyphae of two sexually different strains, designated as + strain and — strain, send out club-shaped branches called pro-gametangia, which touch at the tips.

Terminal portion of each branch swells and is cut off by a transverse septum. The compart­ments thus formed, function as gametangia, and their protoplasmic contents as gametes. The gametes here are multinucleate and hence called coenogametes.

The remaining portion of hyphal branch is known as suspensor. On dissolution of the wall between them, the gametes fuse, cytoplasm with cytoplasm and nuclei with nuclei in pairs. The nuclei which do not fuse in pairs ultimately disintegrate.

The zygote (zygospore) thus formed enlarges and secretes a tough wall around it which is often warty or spiny. After a period of rest it germinates, when the outer wall bursts and the inner wall with the protoplasmic contents comes out as an un-branched tube (Fig. 194). This is called promycelium.

Tip O promycelium enlarges and produces spores, each of which can give rise to a new mycelium. The spores produced in the sporan­gium resulting from the germination of a zygospore are either all + or all —, never both.

Though Mucor produces isogametes, it shows distinct differen­tiation of sex. American botanist Blakslee showed in 1904 that zygote formation in Mucor is possible only when gametes pro­duced by two different strains meet.

He termed them as + strain and — strain, rather than male and female for the two mycelial types- Morphologically, there is not much difference between the two, only + mycelium grows a bit more vigorously than its — counter­part.

Such species are called heterothallic. Parthenogenesis is not uncommon in this fungus. One of the gametes may behave like a zygospore without actual pairing. This is called azygospore or parthenospore.

Yeast condition or Torula stage of Mucor:

If a portion of non-septate mycelium is put in sugar solution, it readily develops septa and finally breaks up into many one- celled parts called oidia. Like yeast, oidia multiply by budding and can also excite alcoholic fermentation in sugar solution. This is yeast condition or torula stage of Mucor.

Saprophytic Fungus # 2. Yeast (Saccharomyces):

Yeast is a common saprophytic fungus growing in sugary substances. They are abundantly present in grape juice, vine­-yards, nectaries of flowers and sugary exudates of plants like date- palm, juice and palmyra-palm juice.

Plant Body:

The plant body is very simple. Yeasts are unicellular organ­isms. Each cell is elliptical or round in shape having a distinct cell wall. There is granular cytoplasm, a single nucleus and granules of glycogen and protein and oil globules as reserve food. It was believed that the nucleus is a degenerate one due to occurrence of a vacuole, but now it has been found to be a true nucleus.

Reproduction:

Under favourable conditions, i.e., when there is sufficient food, yeast cells reproduce vegetatively by budding. This is the most common method of reproduction in this fungus. A bud or protuberance arises at one end of the cell and gradually enlarges.

The nucleus divides into two by mitosis. One nucleus with some cytoplasm and food flow from the mother cell to the bud. By constriction the bud is separated from the mother cell. By this process a large number of buds are produced which may often remain in the form of short chains (Fig. 196). The idea that the nuclei divide directly by amitosis during the process of budding has been found to be incorrect.

A few of them are called fission yeasts, in which the proto­plast of the mother cell is separated into two parts by the forma­tion of a septum, rather than by constriction as in budding. When food supply is exhausted yeast cells produce spores.

Individual cells enlarge and the nuclei divide once, twice, or thrice, forming 2,4, or 8 nuclei in each cell. Cytoplasm collects round each nucleus and ultimately resistant spores are formed.

The spores are called ascospores and the mother cell (sporangium) is known as ascus. The ascus wall breaks to liberate the asco­spores. Each of them goes on reproducing by budding in suitable medium. This process, described as asexual, is really parthenogenetic (Fig. 197).

Sexual reproduction takes place in some species of yeast by conjugation. Two yeast cells approach each other and touch, where short protuberances are formed.

Dissolution of the wall results in the formation of a short conjugation tube connecting the two cells. The two nuclei move to the tube which broadens considerably and, as a result, the whole thing takes up more or less barrel-shaped appearance. The nuclei fuse in the tube forming zygote (diploid) nucleus.

The zygote nucleus usually divides thrice, of which the first division is reduction division. Cytoplasm collects round each nucleus and usually eight spores are delimited. The spores are the ascospores and the barrel-shaped sporangium is the ascus (Fig. 198). Ascospores come out of the ascus and reproduce by budding is suitable medium.

Alcoholic Fermentation:

Yeast has the property of setting up alcoholic fermentation in sugary solution. We know that alcoholic fermentation is an energy- releasing process brought about by micro-organisms under anaero­bic conditions. Yeast cells secrete an enzyme, zymase, which de­composes sugar into alcohol and carbon dioxide with liberation of energy. CO2 comes out as bubbles forming froth.

It may be represented thus:

For this particular property yeasts are used commercially in breweries for the manufacture of alcoholic beverages like wines, beers, etc. The same principle applies in the preparation of indi­genous liquor, toddy, from sugary exudates. They are also used in bakeries or ‘raising’ of breads.

Yeast cells excite alcoholic fer­mentation in bread paste (dough), and carbon dioxide bubbles, while escaping on application of heat, raise the bread. Yeasts are rich in vitamin B complex. So they have nutritional value as well.

Saprophytic Fungus # 3. Penicillium:

Penicillium is a common saprophytic fungus growing on decayed organic matters like bread, jam, jelly, vegetables and fruits and even on damp shoes and leather. It is known as green or blue mold. The spores -of this fungus are abundantly present everywhere and often cause considerable damage to fruits and vegetables.

Some species are also used in industries. Sir Alexander Flemming isolated the wonder drug, penicillin from Penicillium notatum in 1929. The antibiotic peni­cillin had revolutionised medical science and proved to be a real boon to humanity.

The mycelium is composed of much branched septate hyphae occurring in tangled masses. They ramify extensively on the subs­tratum and many of them penetrate into it to serve as rhizoids. The hyphal cells are multi-nucleate.

Reproduction

This fungus reproduces mainly by asexual method, through the spores called conidia, which are formed in very large number. Sexual method of reproduction has also been reported in some species, though the stages are not very clearly known.

Asexual:

Some stout hyphae come out erect from the mycelium and function as conidiophores. Smaller branches develop from the tip of the conidiophore, which again divide to form a row of closely- packed branches called sterigmata. Un-branched chains of asexual spores—conidia, are cut off from the tip of the sterigmata in basipetal order (Fig. 198A).

The terminal portion of conidiophores with branches and chains of conidia together looks like a broom and is called penicillus—meaning broom. The conidia are globose, ovoid or elliptical with smooth or spiny surface and usually green in colour.

They are uninucleate at the early stage but may become multinucleate in some cases. The conidia are easily dispersed by wind, and germinate on a suitable substratum. A germ tube is first formed which ultimately develops into a new mycelium.

Sexual:

Sexual reproduction, though not clearly known, is oogamous. A hypha comes out erect, enlarges, becomes club-shaped and develops into the ascogonium. The nucleus divides again and again to form 32-64 nuclei dispersed in the cytoplasm of the ascogo­nium. In the meantime another branch comes out of a neighbouring hypha which twines round the ascogonium.

The terminal part of that branch is cut off by a septum, swells and forms the antheridium. It comes in contact with the ascogonium where a pore is formed for movement of the protoplast of the antheridium to the ascogonium. Though gametic union has not been established it may be assumed that the process takes place. By formation of septa the multinucleate ascogonium gives rise to a row of bi-nucleate cells.

Many hyphae with bi-nucleate cells now develop from these cells—which are called ascogeneous hyphae. They become septate, each cell having two nuclei, and the terminal cell develops into an ascus. The two nuclei of the ascus fuse to form the zygote nucleus, which then divides thrice, the first division being reductional, and ultimately produces eight ascospores.

Meanwhile sterile vegetative hyphae send out many branches around the sex organs forming a closed fruit body—the ascocarp. This cover made of hyphal cells is known as cleitothecium, the inner layer of which is nutritive in function. With maturity of the Ascospores the asci dissolve leaving them free and scattered in the cleistothecium. The periderm now decays liberating the ascospores which are easily blown off by wind.


Watch the video: Lecture 10: Fungi like Organisms and True Fungi. Dr. Rana Samara (January 2023).