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 (table 8.1).
Table 8.1 c=Classification of fungi
| Phylum (group) | Typical examples | Sexual reproduction | Asexual reproduction | Hyphae |
| Zygomycota (zygomycetes) | Rhizopus, (black bread mold) pilobolus (spitting. Fungus) | zygospores | Non-motile spores form in sporangia | Non-septate, multi mucleate |
| Ascomycota (Ascomycetes of sac- fungi | Yeasts, morels, truffles, powdery mildews, molds | Ascospores inside sac-like asci | Conidia cut off from tips of conidiophores | Septate, lengthy dikaryotic phase. |
| Basidiomycota (basidiomycetes or club-fungi | Mushrooms, rusts, smuts, puff balls, bracket fungi | Basidiospores borns on club shaped basidia | Uncommon | Septate, lengthy dikaryotic phase |
| Deuteromycota (Deuteromycetes/Imperfect fungi | Aspergillus, Penicillium, Alternaria | Sexual phase has not been observed | Conidia | varied |
Zygomycota (zygomycetes or conjugating fungi)
During their sexual reproduction, zygote formed directly by the fusion of hyphae forms temporary, dormant, thick walled resistant structure called zygospore, hence the name zygomycetes. Meiosis takes place when zygospore germinates and haploid spores are produced. Spores on germination produce new mycelium. Asexual reproduction by spores is common. Hyphae are coenocytic.
Example: Rhizopus, found growing on spoiling moist bread, fruit etc.
Ascomycota (Ascomycetes or Sac – Fungi)
It is the largest group of fungi, including over 60,000 species, 50% or so occurring in lichens and some, such as morels, are mycorrhizal. Most are terrestrial, though some are marine or fresh water. The group shows diversity from unicellular yeasts to large cup fungi and morels. They produce haploid sexual spores called ascospores by meiosis inside their characteristic sac like structures called asci (sing. Ascus). Meiosis follows nuclear fusion inside the ascus, commonly 8 ascospores are produced inside each ascus. Most sac – fruiting bodies called ascocarps-the visible morels etc. their hyphae are septate. They have lengthy dikaryotic phase that forms ascocarps. They reproduce asexually by conidia that are often dispersed by wind.
Yeasts are unicellular microscopic fungi, derived from all the three different groups of fungi but mostly ascomycetes, and reproducing mostly asexually by budding (Fig. 8.7). however yeasts reproduce sexually by forming asci/ascospores or basidia/basidiospores. They ferment carbohydrate ( glucose) to ethanol and carbondioxide. Because of this feature and many other reasons, these are of great economic importance (see economic importance of fungi). Saccharomyces cerevisiae is the most commonly exploited yeast.
Basidiomycota (Basidiomycetes or Club – Fungi)
These are among the most familiar fungi: edible mushrooms, devastating plant pathogens rusts and smuts, puffballs, and bracket/shelf fungi are all club fungi. Basidiomycetes are named so for their characteristics, club-shaped (hence also called club fungi) sexual reproductive structure, the basidium (plural basidia). Nuclear fusion in the basidium is followed by meiosis.
Fig. 8.10 Basidiomycetes. A meshroom’s fruiting structures. The gills on underside of mushroom’s cap are lined with basidia, on which basidiospores are produced.
Four haploid sexual spores, called the basidiospores, are born on, not inside, each basidium. During most part of their life cycle the hyphae are septate; the cells are uni-nucleate during one phase, and binucleate (dikaryotic) during the remaining, lengthy phase. Their characteristic fruiting bodies, or visible mushrooms, are formed entirely of dikaryotic mycelium. Puccinia species are most common rust fungi, and Ustilago species most common smut fungi.
| Rusts are called so because of numerous rusty, orange-yellow coloured 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. (Fig. 8.22, 8.15) |
Spores (teliospores) of Ustilago tritici (loose smut of wheat) are carried by wind from infected wheat ears to healthy flowers, where they germinate, the resulting hyphae penetrate flower ovaries. Inside the ovary mycelium spreads and becomes dormant and remains so in the seed (grain). When such infected seeds are sown next season, the hyphae also grow within the growing plant and form smut spores inside the kernel, thus destroying them completely. The covering of the grain braks exposing the black spores mass, that may be dispersed by wind (fig. 8.11)
Fig. 8.11:disease cycle of loose smut of wheat caused by a club- fungus (ustilago tritici)
Deuteromycota (Deuteromycetes or Imperfect Fungi)
This heterogenous grou includes all such fungi in which sexual phase has not been observed. Most of them are related to their sexually reproducing relatives of ascomycetes; however some are related to other two phyla (Zygomycota, Basidiomycota) as well. If sexual structures are found on an imperfect fungus, it is then reassigned to the appropriate phylum. Biologists now can classify most imperfect fungi on the basis of DNA sequences, though sexual structures may not be found.
| Despite absence of sexual reproduction, imperfect fungi show special kind of genetic recombination, called parasexuality, in which portions of chromosomes of two nuclei lying in the same hypha are exchanged. |
Penicillium (blue, green molds), Aspergillus (brown molds), Alternaria, fusarium, helminthosporium are some of the economically important genera of Deuteromycetes (see economic importance of fungi).
Penicillium sp. (blue, green molds) are wide spread saprotrophic species common on decaying fruit, bread etc. its hyphae are septate. Penicillium reproduces asexually by means of naked spores called conidia. These are found in chains at the tips of special hyphae called conidiophores, which are branched. Brush-like arrangement of its conidia is characteristic of penicillium (fig. 8.12). these conidia give colour to the mycelia colony, which is circular. Mature conidia are easily and readily dispersed.
Fig. 8.12penicillium (a) a moldy orage: the blue mold is caused by saprobic species of penicillium. (b) penicillium showing asexual reproduction, characteristic brush-like arrangement of conidia.