Fungi are classified into four groups. These are primarily classified on the basis of the type of their sexual reproductive structures and method of reproduction. These groups also have different types of hyphae and some other characters.
- A zygote is formed directly by the fusion of hyphae during their sexual reproduction. This zygote forms a temporary, dormant, thick-walled resistant structure called Zygospore. So they are named Zygomycetes.
- Zygospore undergoes meiosis to form a haploid spore. These spores germinate to produce new mycelium.
- Asexual reproduction by spore formation is common.
- Their hyphae are coenocytic, i.e. their hyphae are non-septate and multinucleate.
- Example: Rhizopus, it grows on spoiling moist bread, fruit, etc.
Life Cycle of Zygomycota – MUCOR or RHIZOPUS (Bread Mold)
Mucor and Rhizopus are saprophytic fungi. They grow on dead organic food substances, such as beads, rotten fruits, etc.
The body of Mucor and Rhizopus is called Mycelium. It consists of many threads-like structures, called Hyphae. The hyphae contain cytoplasm, many nuclei, and oil globules. Due to the presence of many nuclei-and un septate conditions the mycelium is called coenocytic mycelium.
In Mucor and Rhizopus there are two types of reproduction;
- Asexual reproduction.
- Sexual reproduction.
When the conditions are favorable, the Mucor and Rhizopus start asexual reproduction. In this process, many erect branches arise from, the hyphae, called sporangiophores. At the tip of each sporangiophore, a rounded body is developed.
In this rounded body cytoplasm, nuclei and oil globules are transferred from the hyphae; the oil globules form a wall in the rounded body. So, two portions of the arc formed. The upper larger portion is called Sporangium and the lower smaller portion is known as Columella.
In the sporangium, many spores are formed. The Columella increases in size and exerts pressure on the wall of the sporangium, as a result of which it ruptures and all the spores become free. These spores remain in the air. Whenever they find any organic food substance, they fall down and start germination, to form new mycelium.
The process of sexual reproduction is as follows:
- At the time of reproduction, two different hyphae come parallel to each other.
- From each hypha and outgrowth, the papilla is developed.
- Each papilla divides into two cells, the basal cell is called the suspensor, and the other cell is known as gametangium.
- In each gametangium, a gamete is formed. These gametes are similar in their external structure, so they are called isogametes.
- The two isogametes are fused together to form a zygospore. The fusion of two isogametes is known as conjugation.
The zygospore germinates into a tube-like structure, called Sporangiophare or promycelium. At the tip of, promycelium, a sporangium is developed; in which numerous spores are formed.
This sporangium is without columella. After the maturation of spores, the wall of the sporangium ruptures, and all the spores come out. When they fall on any organic matter, they germinate into a new mycelium of the fungus.
Division Ascomycota: (Ascomycota or Sac-Fungus)
- It is the largest group of fungi. It has over 60,000 species. 50 % of these species occur in lichens and Mycorrhizae.
- Most of these fungi are terrestrial. Some are marine or freshwater.
- This group shows a diversity of organisms. Some are unicellular like yeasts. Some are the large cups of fungi and morels.
- They produce haploid sexual spores called ascospores by meiosis. Ascospores are produced inside the characteristic sac-like structure called asci (singular ascus). Each ascus has 8 ascospores.
- Mostly, the asci are present inside the macroscopic fruiting bodies called ascocarps, e.g. in morels.
- Their hyphae are septate.
- The hyphae have a lengthy dikaryotic phase that forms ascocarps.
- They reproduce asexually by conidia. These conidia are dispersed by wind.
- Examples: yeasts, morels, truffles, molds.
Life-Cycle of Ascomycota:
In the life cycle of Ascomycota, there are two types of reproduction:
- Asexual Reproduction
- Sexual reproduction
Asexual reproduction takes place by different methods but conidia formation is the most common reproduction. Other methods are budding and fission etc.
Conidia formation is a very rapid and common method of reproduction. In this process the mycelium, some erect branches arise from the mycelium known as Conidiophore. These conidiophores produce many branches again. At the tip of each Branch, some rounded bodies are developed in a chain-like manner known as Conidia. The Conidia are separated from the branches, and then they develop into new mycelium.
In Ascomycota sexual reproduction takes place by the formation of male and female reproductive organs. Male is known as antheridia, and females are known as ascogonia.
The ascogonium produces a special structure, called Trichogyne, through which it is connected with antheridium. Through the trichogyne, the male nucleus is transferred into the female organ, where fertilization takes place and the zygote is formed.
The zygote is changed into the special structure, called an ascus. In the ascus after fertilization, the zygote nucleus divides by meiosis into 2 then, 4, and 8 nuclei. These nuclei are converted into ascospores. The organ in which ascospores are formed is called an ascus. The ascospores are dispersed by wind. They can develop into new mycelium, during the favorable conditions.
Yeasts are unicellular microscopic fungi. They are derived from all three groups but mostly from Ascomycetes. They reproduce asexually by budding. Yeasts reproduce sexually by forming asci and ascospores or basidia and basidiospores. They cause the fermentation of carbohydrates (glucose) and change it to ethanol and CO2. Yeasts have great economic importance due to this feature. Saccharomyces cerevisiae is the most commonly used yeast.
(Basidiomycetes or Club-Fungi)
- These are the most familiar fungi. These fungi include edible mushrooms, dangerous plant pathogens rust and smut, puffballs, and bracket or shelf fungi.
- They produce club-shaped sexual reproductive structures called basidium (plural basidia). Basidium is their characteristic feature and they are named Basidiomycetes. They are also called club fungi due to the club shape of the basidium.
- The nuclei in basidium fuse. After that, meiosis takes place and four haploid sexual basidiospores are formed on the basidium (not inside).
- The hyphae are septate during most part of their lives.
- There are two phases of life:
- During the first phase, the cells are uninucleated.
- During the second lengthy phase, the cells are binucleate (dikaryotic).
- They have characteristic fruiting bodies (visible mushrooms). These fruiting bodies are formed from dikaryotic mycelium.
- Examples: Puccinia are the most common rust fungi and Ustilago species are the most common smut fungi.
(loose smut of wheat)
The wind carries the spore (teliospore) from the infected wheat ears to the healthy flowers. These spores germinate on the flowers and their hyphae penetrate into the flower ovaries. The mycelium spreads inside the ovary and becomes dormant. They remain dormant in seed grain. When such seeds are sown next season, the dormant hyphae grow within the growing plant. It forms smut spores inside the kernel and destroys it completely.
Puccinia or Rust of wheat
It produces numerous rusty orange-yellow spots on the host surface (stem, leaves). Later, the fungus releases brick or rust red spores.
It is a heterogeneous group. It includes all such fungi in which sexual reproduction ¡s absent. Most of these fungi are related to the sexually reproducing Ascomycetes. Some are related to the other two groups, Zygomycota and Basidiomycota.
If sexual structures are discovered in any imperfect fungi, they will be placed in their appropriate phyla. Biologists now can classify most imperfect fungi on the basis of DNA sequences, though the sexual structures may not be discovered.
Examples: Penicillium (blue-green molds), Aspergillus (brown molds), Alternaria, Fusarium, and Helminthosporium are some of the economically important genera of Deutromycetes.
There is an absence of sexual reproduction in imperfect fungi. But these fungi show a special kind of genetic recombination called Parasexuality. The exchange of a portion of chromosomes of two nuclei lying in the same hypha is called parasexuality.
These are widespread saprotrophic fungi. These are commonly found on decaying fruits, bread, etc. Its hyphae are septate. Penicillium reproduces asexually by forming naked spores (without sporangia) called conidia. The conidia are present in chains at the tip of special branched hyphae called conidiophores.
The brush-like arrangement of the conidia is the characteristic feature of Penicillium. These conidia give blue-green color to the circular colonies of the mycelium. Mature conidia are easily dispersed.
Land Adaptation of Fungi
These are present wherever organic matter is present. They grow best in moist habitats. These are successful groups of land organisms. They show many terrestrial adaptations.
- Fungi develop an extensive system of fast-spreading hyphae. The hyphae penetrate the substrate and increase contact and the surface area for absorption.
- There is cytoplasmic flow throughout the hyphae. It is responsible for the rapid growth of hyphae and these hyphae spread everywhere.
- The hyphal cell wall is made up of chitin. It is more resistant to decay than cellulose and lignin found in the plant cell wall.
- They can break down cellulose and lignin for obtaining their nutrients.
- In saprobes (saprotrophs), there are certain modified hyphae called rhizoids. The rhizoids anchor (fix) the fungus to the substrate. It also digests and absorbs food.
- They lack flagellated cells. They develop non-motile spores and conidia. These can be easily dispersed by wind. They develop thick-walled zygotes and other resistant structures. There are certain modifications in the hyphae so that fungi do not depend on water for reproduction.
- Most fungi are more tolerant than bacteria. They are not damaged in hyperosmotic (more solute concentration) surroundings. Many fungi can tolerate extreme temperatures, — 5°C below freezing and 50°C or more. So Penicillium can grow on oranges and jelly kept in the refrigerator. While generally, bacteria cannot grow in it.