Fundamentals of Microbiology

1.4.1.3 Taxonomic classification of fungi

Division	Common Name	Approx. No. of species
Zygomycota	Zygomycetes	600
Ascomycota	Sac fungi	35,000
Basidiomycota	Club fungi	30,000
Deuteromycota	Fungi imperfecti	30,000

The above classification is based primarily on variations in sexual reproduction.

Division Zygomycota

The zygomycetes are coenocytic. Most are saprophytic. A few are parasites of plants, insects, animals and humans. Eg. Rhizopus stolonifer, the bread mold. It grows on moist, carbohydrate rich foods such as breads, fruits and vegetables. Special hyphae called rhizoids extend into the substrate and absorb nutrients. Some hyphae produce asexual sporaniga at the tip. Each spore when released can start a new mycelium.

         

Rhizopus usually reproduces asexually, but if food becomes scarce, it begins sexual reproduction. Sexual reproduction requires compatible strains of opposite mating types (usually designated + and – strains). When two mating strains come close, hormones are produced and the hyphae form projections called progametangia which mature to gametangia. After fusion of the gametangia, the nuclei of the two gametes fuse to form Zygote. The zygote develops a thick, rough, black coat and becomes zygospore. Zygospore produces an asexual sporangium and the cycle begins.

 Division Ascomycota

The fungi of this division are called ascomycetes or sac fungi. Most of the red, brown and blue green molds that cause food spoilage belong to this division. This division contains several yeasts. Many ascomycetes are parasites of higher plants. Ergotism is a disease (toxic) condition in humans and animals who eat grains infected with the fungus, Claviceps purpurea. The ascomycetes are named for their characteristic reproductive structure, the club or sac-shaped ascus. The mycelium of ascomycetes is composed of septate hyphae. Asexual reproduction is common in the ascomycetes and takes place be way of conidiospores.

Sexual reproduction involves the formation of an ascus containing ascospores. Sexual reproduction starts with the development of special ascogenous hyphae. One nucleus from a male mycelium (antheridium) and the other from a female mycelium (ascogonium) migrate to form the ascogenous hyphae. The paired nuclei divide in such a way that there is one pair of nuclei in each cell. After the ascogenous hyphae have matured, nuclear fusion occurs at the hyphal tips in the ascus mother cells. The diploid zygote nucleus then undergoes meiosis, resulting in four haploid nuclei. These divide mototically to produce a row of eight nuclei in each developing ascus. These nuclei become ascospores. Once they mature, they are released from the asci with great force. Upon reaching suitable environments, each ascospore germinates. Some of the yeasts also reproduce sexually in this manner and are classified in ascomycota.

     

     

Division: Basidiomycota:

Basidiomycetes are commonly called club fungi. It includes mushrooms etc. Basidiomycetes are named for their characteristic structure, the basidium, involved in sexual reproduction. The members of basidiomycetes affect humans in many ways. Most are saprophytic and decompose plant debris, especially cellulose and lignin. Many mushrooms are used as food. Many mushrooms produce specific alkaloids that act either as poisons or hallucinogens. There are several other plant pathogens and human pathogens under badiomycetes.

The life cycle of a typical basidiomycete starts with a basidiospore germinating to produce a monokaryotic mycelium (single nucleus in each septate cell). The mycelium spreads through the soil and when this primary mycelium meets another monokaryotic, mycelium of a different making type, the two mycelia fuse to initiate a new dikaryotic secondary mycelium. The secondary mycelium consists of cells with two nuclei, one of each mating type. This mycelium is stimulated to produce a solid mass of hyphae, as a button that pushes through the soil, elongates and develops a cap. The entire structure is called basidiocarp. The cap contains plate like gills, each coated with basidia. The two nuclei in the tip of basidium fuse to form a diploid zygote nucleus. This zygote undergoes meiosis to form four haploid nuclei. These nuclei move into the developing basidiospores. These are released upon maturity.

Division Deuteromycota

When a fungus lacks the sexual phase or if this phase has not been observed, it is placed in the division Deuteromycota, commonly called Fungi Imperfeci. Most Fungi Imperfecti are terrestrial. They are either saprophytes or parasites of plants. A few are parasitic on other fungi. Several human pathogenic forms causing diseases like athlete’s foot, ringworm etc., belong to these group.

Slime molds and water molds

These molds resemble fungi in appearance and life-style. In their cellular organization, reproduction and life cycles, they are related most closely to the protists.