In earlier systems of classification, only habit or external morphological characters were considered the sole basis of classification. But later on, organisms were classified on the basis of their natural affinities and finally on the basis of phylogeny (evolutionary tendencies). Many systems of classification were devised from time to time. The five-kingdom system of classification proposed by R.H. Whittaker in 1969 is widely accepted and includes kingdoms Monera, Protista, Fungi, Plantae, and Animalia.
This kingdom includes eukaryotic, heterotrophic, generally multicellular, achlorophyllous organisms. Fungi possess a thalloid structure with no distinct body parts. However, they possess a spore-producing sac at the tip of branch-like hyphae which are collectively known as mycelium. The cell wall of fungi is distinct. It is made up of chitin, a polymer of NAG (N-Acetyl Glucosamine). Fungi are generally multicellular except the unicellular yeast. They are heterotrophic and mostly saprophytic. Fungi are cosmopolitan. They generally grow in warm and humid conditions. Reproduction in fungi occurs by various means. The various types of vegetative reproduction are fragmentation, budding, and fission. They reproduce asexually by various types of spores like conidia, zoospores, etc. Sexual spores include oospores, ascospores, and basidiospores. The sexual cycle has three distinct phases: Plasmogamy is the fusion of protoplasms, and Karyogamy means the fusion of gametes and Meiosis.
Kingdom Fungi have been further divided into four classes based on the morphology of their mycelium, mode of spore formation, and types of fruiting bodies:
- Ascomycetes (aka sac fungi)
- Basidiomycetes (aka club fungi)
- Deuteromycetes (aka imperfect fungi)
This phylum includes fungi which exhibit only an asexual mode of reproduction. Hence they are termed imperfect fungi. When the sexual mode of reproduction is discovered they are placed into other classes. They reproduce by asexual spores called conidia. Their mycelium is branched and septate unlike that of Phycomycetes. They are either saprophytic or parasitic. Deuteromycetes hold great economical importance. They act as decomposers of organic matter and play an important role in mineral recycling. They are also known as ‘second class’ fungi. Deuteromycetes are responsible for various plant diseases and cause the degradation of food.
Characteristic Features of Deuteromycetes
- Deuteromycetes are saprophytes on a wide range of substrates, but a large number of these fungi are parasites on plants and animals. It causes a variety of diseases.
- The mycelium has well-developed, profusely branched, and septate hyphae possessing multinucleate cells and simple pore septa.
- The hyphae may be intercellular or intracellular and their cell wall chiefly contains chitin-glucan.
- They exhibit only an asexual mode of reproduction in various ways like hyphal fragments, budding, arthrospores, chlamydospores, or most commonly conidia
- They do not exhibit sexual reproduction but a parasexual cycle generally operates in their life to fulfill the requirements of sexual reproduction.
- Deuteromycetes play a vital role in the decomposition of organic matter and bring about the recycling of nutrients in nature.
- The fermentation industry is highly dependent on Deuteromycetes because of the essential chemicals they produce.
- Some Deuteromycetes fungi are medicinally very important. They are used in the synthesis and conversion of steroids which are of great significance in pharmaceuticals.
- Deuteromycetes produce a variety of toxins in foods, feeds, and grains in storage. Some of these toxins are carcinogenic.
- These fungi are serious pathogens and are responsible for causing a variety of plant and animal diseases.
Classification of Deuteromycetes
On the basis of various characters, Deuteromycetes are divided into the following four form orders:
- Moniliales: This form order includes both saprophytes and parasites. Their conidia are generally borne freely on the mycelium or on free conidiophores proceeding from the mycelium. In some cases, conidiophores are assembled into sporodochia or synnemata. Reproduction by budding and oidia formation can also be seen.
- Melanconiales: This form order includes plant parasites. Their conidia are formed singly or in chains from conidiophores and are packed closely to form an asexual fructification called an acervulus.
- Sphaeropsidales: This form order includes plant parasites. Their conidia are formed within small, globose, or flask-shaped pycnidia or on the modifications of such structures.
- Mycelia sterilia: This form order includes 20 genera and 200 form species. These are called imperfect fungi in which special reproductive stages including the conidia are absent. Reproduction takes place by sclerotia, rhizomorphs, and other vegetative means.
Examples of Deuteromycetes
- Cercospora: The majority of the species in this genus produce plant diseases and leaf spots.
- Colletotrichum: These fungi live as endophytes or phytopathogens on plants. Some may have a mutualistic relationship with their hosts.
- Trichoderma: It is the most common culturable fungus and is found in all types of soils. Most species in this genus are avirulent plant symbionts.
- Tolypocladium inflatum: The immunosuppressant ciclosporin is obtained from them.
- Lecanicillium species: These produce conidia which may control certain species of insect pests.
- Pochonia species: They are under development for control of Nematode pests.
- Red Rot of Sugarcane: Caused by Colletotrichum falcatum. Seen on leaf midribs as well as in the stem as red stripes.
- Helminthosporium: Helminthosporium oryzae causes sesame or brown leaf spot in rice. It is responsible for the cause of the Bengal famine of 1942-43 and similar conditions in the Krishna-Godavari area. The conidia are 5-10 septate.
- Early Blight: Alternaria salani causes early blight of Potato and Tomato. The leaves develop small oval brown spots with concentric rings and wither off. The fruits also develop lesions.
- Tikka Disease: Circular necrotic dark brown leaf spots develop in groundnut due to Cercospora (eg. C. personata). The conidia are septate and filamentous. The perfect stage has been found to be Mycosphaerella (e.g., M. berkeleyii).
- Wilts: Many economically important plants (e.g. Potato, Tomato, Cotton, Banana (Panama disease), Flax, (Pigeon Pea) show sudden signs of wilting due to blockage of the xylem by the growth of the fungus Fusarium oxysporum. The fungus shows three kinds of spores – chlamydospores, microconidia, and macroconidia.
- Gibberellins: First discovered in the extracts of Fusarium moniliforme growing on rice (bakanae disease of rice). The perfect stage of fungus is gibberella fujikori. Gibberellins are natural plant growth hormones.
- Arthrobotrys is known as a predator fungus that forms traps for capturing, paralyzing, and digesting nematodes. Fungi feeding on nematodes are called nematophagous fungi. Digestive enzymes are poured over the nematode. The digested products are absorbed.
Reproduction in Deuteromycetes
The fungi which lack sexual reproduction are classified under Deuteromycetes. They exhibit only an asexual mode of reproduction and some parasexual modes as well. Commonly this group of fungi is known as the “imperfect fungi” as they are unable to produce spores through sexual reproduction. There are a variety of ways of reproduction in Deuteromycetes. Fission, budding, and conidia formation are some common methods of asexual reproduction in these fungi.
Fission is a type of asexual reproduction in which a single cell divides into two or more daughter cells. This process happens either through binary fission, where the cell divides into two equal parts, or by multiple fission, where the cell divides into multiple parts. Budding is a type of asexual reproduction where a small outgrowth i.e. the bud forms on the surface of a parent cell. The bud then separates from the parent cell and grows into a new, independent cell.
Conidia formation is a type of asexual reproduction where a small, spore-like structure called a conidium is produced. Conidia can be spread by wind or water and can germinate to form new fungal colonies. These fungi do not exhibit sexual reproduction but a parasexual cycle generally operates in their life to fulfill the requirements of sexual reproduction.
Deuteromycetes as Pathogens
Deuteromycetes fungi are a diverse group of fungi that can be found in a variety of habitats which includes soil, water, and plants. This group of fungi is also called to be a prime cause of human and animal infections. Some of the most common human infections caused by Deuteromycetes fungi are candidiasis (thrush), aspergillosis (fungal infection of the lungs), and histoplasmosis (fungal infection of the lungs). Deuteromycetes fungi are able to cause infections by invading the body and colonizing the tissues. They can also produce toxins that can damage the tissues.
Some of the common symptoms of Deuteromycetes fungal infections are fever, coughing, chest pain, and shortness of breath. The treatment of Deuteromycetes fungi is with a range of antifungal medications. These medications work by either killing the fungi or stopping them from multiplying. A large number of Deuteromycetes fungi are serious plant pathogens and are responsible for causing a variety of plant diseases resulting in great economic losses. Deuteromycetes produce a variety of toxins in foods, feeds, and grains in storage. Some of these toxins are carcinogenic.
Reason for Deuteromycetes being called the Imperfect Fungi
Deuteromycetes are called imperfect fungi because only their vegetative or asexual reproductive methods are known. When their sexual stages were discovered, they were rightly moved to different classes such as ascomycetes or basidiomycetes. When linkage was discovered these fungi were correctly identified and moved out of Deuteromycetes. They are known as the fungi imperfecti, because of their “imperfecti” lack of sex.
Key Points on Deuteromycetes
- Deurteromycetes is an artificial class of fungi that includes all those fungi in which the sexual stage is not known.
- Some of the Deuteromycetes are unicellular like yeasts and are often studied along with the latter.
- Asexual reproduction mainly occurs by conidia along with some other types of spores (like oidia and chlamydospores).
- It is believed that most members of Deuteromycetes are actually ascomycetes in which sexual reproduction is absent.
- Some members are saprophytes or parasites while a large number of them are decomposers of litter and help in mineral cycling.
FAQs on Deuteromycetes
Question 1: Which disease is caused by Trichoderma?
The disease named Ringworms is caused by Trichoderma.
Question 2: What are the problems faced during classifying Deuteromycetes?
Deuteromycetes exclusively exhibit asexual reproduction. However many organisms which were initially placed in Deuteromycetes have to be moved out when their sexual stages are discovered.
Question 3: Why are Deuteromycetes known as imperfect fungi?
As they exhibit only asexual reproduction, Deuteromycetes are known as imperfect fungi.
Question 4: The imperfect fungi which help in mineral recycling?
Deuteromycetes help in mineral recycling.
Question 5: What do you mean by mineral recycling?
Mineral recycling is the way in which elements are continuously broken down and exchanged for reuse between the living and non-living components of an ecosystem.
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