Classification of Fungi

Classification of fungi into four main groups is based primarily on the type of their sexual reproductive structures and methods of reproduction. However, these groups also differ in the type of hyphae and some other characters.

 

Zygomycota (zygomycetes or conjugating Fungi)

The phylum or division zygomycota has about 600 species. They are called zygospore fungi, and mainly saprotrophs living off plant remains or bakery goods on vegetables and fruits. Some are parasites of small soil protists. Hyphae are non-septate, mycelium well developed and branching. Asexual reproduction takes place by conidia or spores e.g. Rhizopus nigricans. It is known as black or bread molds. It is a mass of mycelium. Asexual reproduction in Rhizopus takes place by the sporangia containing spores.

Sexual reproduction

It takes place by conjugation. Conjugation occurs only between a member of a plus (+) strain and one of a minus (-) strain.

      Sexual reproduction in Rhizopus 

When hyphae (stolon) of opposite mating types meet, hormones are produced that cause the tips of the hyphae to come together and to form gametangia, structures that produce gametes. These structures become separated from rest of the mycelium by the formation of septa and plus and minus nuclei then fuse to form a diploid nucleus, the zygote. The zygote develops into a zygospore. The wall of the zygospore is thick and resistant to unfavorable conditions.

Germination

Zygospores germinate under favorable conditions and divide by meiosis. The wall of the zygospore splits and hyphae grows upward. The tip of the hyphae develops into a sporangium. The sporangium contains many nuclei. The wall of the sporangium ruptures and the spores are liberated. Each spore grows into a new plus or minus strain of mycelium. Thus the life cycle of Rhizopus is continued. The division or phylum name refers to the zygospore seen during sexual reproduction. Zygospore fungi produce spores within sporangia. During sexual reproduction a zygospore forms prior to meiosis and production of spores.

Diseases caused by Zygomycota

Albugo Candida is a most common species causing white “rust” of cruciferous plants e.g. mustard plant throughout the world. It forms white shining patches on stem and leaves and causes much deformation of the inflorescences and fruit. Peronospora causes common plant diseases generally known as downy mildews i.e. form grayish white downy patches on the undersurface of the leaves of cabbage, cauliflower, radish and turnip.

 

Ascomycota (Ascomycetes or Sac Fungi)

Ascomycetes are the members of phylum or division ascomycota. It is a large group. It has about 30,000 described species.

Sac Fungi

Ascomycetes are also known as sac fungi because their sexual spores are produced in little sacs called asci (sing: ascus). Their hyphae usually have septa but the cross walls are perforated so that cytoplasm can move from one compartment to other.

Asci and Ascospore 

Reproduction: Ascomycetes reproduce both asexually and sexually.

Asexual reproduction

It involves production of spores called conidia (sing: conidium or conidiospores (Greek: konis means dust, and spora means seed). Conidia vary in shape, size and may be multicellular. There are no sporangia in Ascomycetes. The conidia develop directly on the tips of modified aerial hyphae called conidiophores. When released conidia are windblown. Conidia occur in various shapes, sizes and colors in different species. The color of conidia is what gives the characteristic brown, blue, pink or other tint to many of these molds.

Budding

In unicellular yeasts, asexual reproduction takes place by budding in this process a small protuberance (bud) grows and eventually separates from the parent cell. Each bud can grow into a new yeast cell. Yeast also reproduces asexually bi fission.

Sexual Reproduction

It takes place after two hyphae grow together and their cytoplasm mingles. Within this fused structure, nuclei from the parent hyphae pair but do not fuse. New hyphae develop from the fused structure and the cells of these hyphae are dikaryotic. The n +n hyphae form a fruiting body known as ascocarp.

The asci develop in the ascocarp. The asci are usually surrounded by sterile hyphae. An ascocarp is a fruiting body. It is a reproductive structure where spores are produced and released. Ascocarps can have different shapes, in cup fungi they are cup shaped, in molds they are flask shaped and in the morels they are stalked and crowned by bell shaped.

Within an ascus the two nuclei fuse and form a diploid nucleus the zygote which undergoes meiosis to form four haploid nuclei. This process is usually followed by one mitotic division of each of the four nuclei, resulting in eight haploid nuclei. Each haploid nucleus develops into an ascospore.

So there are usually eight haploid ascospores within the ascus. In most Ascomycetes the asci become swollen as they mature and then they burst liberating the ascospores, which are then windblown if lands in a suitable location and germinates to form a new mycelium e.g. in Yeasts, Neurospora etc.

Sac fungi produce sexual conidiospores. During sexual reproduction, asci within a fruiting body produce spores. Example: Yeasts, Neurospora, Morels, Truffles.

Diseases caused by Ascomycota

A large number of ascomycetes are parasitic on plants, powdery mildews grow on leaves, and chestnut blight and Dutch elm disease destroy these trees.

Ergot, a parasitic sac fungus infects rye. When ground with the rye and made into bread the fungus releases toxic alkaloids that cause the disease ergotism. In human, vomiting, feelings of intense heat or cold, muscle pain, yellow feces etc. are the symptoms of ergotism.

Basidiomycota (Basidiomycetes or Club-Fungi)

Basidiomycetes re included in the phylum Basidiomycota. There are 25,000 or more species in this phylum. Included in this phylum are mushrooms, bracket fungi, rust, smut and puffballs. These structures are all fruiting bodies called basidiocarp. Basidiocarp contains the basidia. Each basidium is a club shaped structure. It is a hyphal cell on the tip of which develops four basidiospores, from which this phylum takes its name.

Each individual fungus produces millions of basidiospores arid each basidiospore has the potential to give rise to a new primary mycelium. Hyphae of primary mycelium are composed of monokaryotic (n) cells. The mycelium of a basidiomycete e.g. Mushroom -- Agaricus consists of mass of white, branched, thread like hyphae that occur mostly below ground. The hyphae are divided into cells by septa. The septa are perforated and allow cytoplasmic streaming between cells.

Reproduction

Although club fungi occasionally do produce conidiospores asexually, they usually reproduce sexually. Hyphae of a primary mycelium encounter other monokaryotic (n) hyphae of a different mating type and the two hyphae fuse. However the two haploid nuclei remain separated from each other. In this way a secondary mycelium with dikaryotic (n + n) hyphae is produced, in which each cell contains two haploid nuclei. The n + n hyphae of the secondary mycelium grow and forms compact mass, called buttons, along the mycelium. Each button grows into a fruiting body known as mushroom. A mushroom, which consists of a stalk and a cap, is more formally referred to as basidiocarp. Each basidiocarp actually consists of intertwined hyphae that are matted together

The walled off ends of the tightly packed hyphae become the club shaped basidia. The lower surface of the cap usually consists of many thin perpendicular plates called gill6 that radiate from the stalk to the edge of the cap. On the gills of the mushroom, haploid nuclei of the dikaryotic cells fuse to form diploid zygotes. Meiosis then takes place forming four haploid nuclei that move into finger like projections forming basidiospore, which are released later.

                       Life cycle of a Mushroom 

Disease caused by Basidiomycota

Smut and rusts are club fungi that parasitize cereal crops such as corn, wheat, oats and rye. These cause great loss every year. Smut and rusts do not form basidiocarp. Their spores are small and numerous, resembling soot. Some smuts enter seeds and exist inside the plants, becoming visible only near maturity. Other smuts externally infect plants. In corn smut, the mycelia grow between the corn kernels and secrete substances that cause the development of tumors on the ears of corn.

Rusts are called so because of numerous rusty and orange-yellow colored disease spots on their host surface (mostly stem, leaves), later revealing brick/rust-red spores of the fungus. Smuts are called so because of their black, dusty spore masses that resemble soot or smut; these spore masses replace the grain kernels such as those of wheat, corn etc.

Rust

Deuteromycota (Deuteromycetes or Imperfect Fungi)

There are about 25,000 species in this phylum. These fungi are called “imperfect” fungi because of the absence of the sexual stage in their life cycle. Imperfect fungi always reproduce asexually by forming conidiospores. Usually cellular morphology and biochemistry indicate that these fungi are sac fungi which have lost the ability to reproduce sexually. These fungi live either saprophytically or parasitically on plants. Several imperfect fungi have economic importance. Examples: Penicillium, Aspergillus, Alternaria and Fusarium.

Penicillium

Penicillium is commonly known as blue green mold. These are widely spread saprophytes, which grow on decaying fruit, vegetables, bread etc. It reproduces sexually by condia. They are present at the tips of hyphae called conidiophores, which are branched. The conidia give color to the mycelial colony which is circular in shape. Mature condia are easily and readily dispersed.

                                 Penicillium

Diseases Caused By Deuteromycetes

Some imperfect fungi cause diseases in human. Certain dust borne spores can cause infections of the respiratory tract, while athletes foot and ring worm are spread by direct contact. Candida Albicans is yeast like organism that causes thrush - an inflammation of the mouth and throat.

 

Adaptations in Fungi for Terrestrial Mode of Life

1. Absence of flagellated cells.

2. Evolution of protective layers around spores and in some cases around Zygospores.

3. Evolution of hyphae with thickened supporting wall. Spores are produced on upward growing hyphae. So that spores can be dispersed easily.

4. Hyphae are also modified for sexual reproduction.

5. Evolution of new methods of reproduction: asexual by spores and sexual by conjugation e.g. Rhizopus.

6. Independence of external water for reproduction. Many fungi are more tolerant than bacteria to damage in hyper-osmotic surroundings. Many can tolerate temperature extremes up to 5°C below freezing and 50°C or more.