Most fungi contain hyphae that form a network called mycelium. Except yeast which is unicellular and filamentous, there are two types of hyphae - coenocytic hyphae and septate hyphae. In fungi, cell walls are made of chitin and polysaccharides. Fungi are saprophytic or parasitic or symbiotic in lichens and mycorrhiza of plant roots. Fungi reproduce vegetatively by fragmentation, fission and budding. They reproduce asexually by eitherconidia, zoospores or sporangiospores, sexually by either oospores, ascospores or basidiospores. There are three steps in the sexual cycle: plasmogamy, karyogamy and meiosis. Plasmogamy is the fusion of protoplasm between two gametes, followed by the fusion of nuclei, called karyogamy, followed by meiosis to form haploid spores.
The fruiting bodies, the mode of spore formation and the morphology of the mycelium together divided the Kingdom Fungi into various classes namely Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes. Phycomycetes include Mucor, Rhizopus and Albugo, and have aseptate and coenocytic mycelium. They reproduce asexually by zoospores in the case of motile gametes and by aplanospores in the case of non-motile gametes. Sexual reproduction leads to the formation of zygospores after the fusion of gametes.
Ascomycetes are commonly called sac-fungi and include Aspergillus, Claviceps and Neurospora. They can be saprophytic, decomposers, parasitic or coprophilous. They have branched and septate mycelium. They reproduce asexually by conidia and sexually through ascospores arranged in fruiting bodies called ascocarps.
Basidiomycetes include mushrooms, bracket fungi and puff balls. They have branched and septate mycelium. Vegetative reproduction is by fragmentation. Sexual reproduction occurs when two somatic cells of different genotypes fuse to form a dikaryote, which gives rise to the fruiting body called the basidiocarp, on which basidia arise and produce basidiospores.
Deuteromycetes, are known as ‘imperfect fungi’ because only asexual or vegetative phases are seen. They reproduce asexually through conidia. They have septate and branched mycelium and are saprophytic or parasitic while a large number of them are decomposers. Alternaria, Colletotrichum and Trichoderma are examples of Deuteromycetes.