Fungi are diversified and large group of eukaryotes that digest food externally and absorb nutrients directly through their cell walls. Most fungi reproduce by spores and have a body (thallus) composed of microscopic tubular cells called hyphae. Fungi are heterotrophs and, like animals, obtain their carbon and energy from other organisms. Some fungi obtain their nutrients from a living host (plant or animal) and are called biotrophs; others obtain their nutrients from dead plants or animals and are called saprotrophs (saprophytes, saprobes). Some fungi infect a living host, but kill host cells in order to obtain their nutrients; these are called necrotrophs.

Fungi were once considered to be primitive members of the plant kingdom, just slightly more advanced than bacteria. We now know that fungi are not primitive at all. In fact, recent taxonomic treatments such as the three domains of life show that fungi and animals both belong to same group. Fungi may not be our next of kin, but they are more closely related to animals than they are to plants. We also recognize that organisms traditionally studied as "fungi" belong to three very different unrelated groups: the true fungi in Kingdom Fungi (Eumycota), the Oomycetes, and the slime molds.

Fungi are an ancient group—not as old as bacteria, which fossil evidence suggests may be 3. 5 billion years old—but the earliest fungal fossils are from the Ordovician, 460 to 455 million years old.

Fungi are a large, diverse, and widespread group of organisms, consisting of the molds, mushrooms, and yeasts. Approximately 100,000 species of fungi have been described, and as

many as 1.5 million species may exist. Fungi form a phylogenetic cluster distinct from other organisms and are most closely related to animals. Most fungi are terrestrial. They inhabit soil or dead plant matter and play crucial roles in the mineralization of organic carbon. A large number of fungi are plant parasites. Indeed, fungi cause many of the economically significant diseases of crop plants. A few fungi cause disease in animals, including humans, although in general fungi are less important as animal pathogens than are other microorganisms. Fungi also establish symbiotic associations with many plants, facilitating the plant’s acquisition of minerals from soil and many fungi benefit humans through fermentation and the synthesis of antibiotics.

Fungal morphology

Most fungi are multicellular, forming a network of filaments called hyphae (singular, hypha). Hyphae are tubular cell walls that surround the cytoplasmic membrane. Fungal hyphae are often septate, with cross-walls dividing each hypha into separate cells. In some cases, however, the vegetative cell of a fungal hypha contains more than one nucleus, and hundreds of nuclei can form due to repeated nuclear divisions without the formation of crosswalls, a condition called coenocytic. Each hyphal filament grows mainly at the tip by extension of the terminal cell. Hyphae typically grow together across a surface and form a compact, macroscopically visible tuft called a mycelium. The mycelium arises because the individual hyphae branch as they grow over and into the organic material on which the fungus is feeding, and these branches intertwine, forming a compact mat. From the mycelium, hyphal branches may reach up into the air above the surface, and spores called conidia are formed on these aerial. Conidia are asexual spores (their formation does not involve the fusion of gametes or meiosis), and they are often pigmented black, blue green, red, yellow, or brown. Conidia give the mycelium a dusty appearance and function to disperse the fungus to new habitats. Some fungi form macroscopic reproductive structures called fruiting bodies (mushrooms or puff balls, for example), in which spores are produced and from which they can be dispersed. The fruiting bodies can release millions of spores that are spread by

wind, water, or animals to new habitats where the spores can then germinate. Some fungi grow as single-celled forms; these are the yeasts.

Most fungal cell walls consist of chitin, a polymer of the glucose derivative N-acetylglucosamine. Chitin is arranged in the walls in microfibrillar bundles, as is cellulose in plant cell walls, to form a thick, tough wall structure. Other polysaccharides such as mannans and galactosans, or even cellulose itself, replace chitin in some fungal cell walls. Fungal cell walls are typically 80–90% polysaccharide, with proteins, lipids, polyphosphates, and inorganic ions making up the wall-cementing matrix.

Fungal reproduction

Fungal kingdom is known for its diversified reproduction methods. The fungi have vegetative, asexual and sexual methods of reproduction.

Vegetative Reproduction

1. Fragmentation - The hypha fragment to many cells and each cell will raise as new individual. Ex. Yeast
2. Fission – As bacteria, binary fission gives two new daughter cells. Ex. Yeast
3. Budding – In yeast, the cell forms a bud like appendage, in which the divided nucleus will move and new cell formed. The detached bud will raise to new individual.
4. Sclerotia – resting vegetative structure can give the new individual
5. Rhizomorph – also gives the new individual

Asexual reproduction

Arthospores/Oidiospores: The broken hypha behave as spores are referred as arthospores or oidiospores

1. Chlamydospores: The cells become resting body and separate from hyphae referred as chlamydospores. When favourable condition araises , the chlamydospore will form new mycelium Ex. Fusarium
2. Asexual spores: Sporangiospores -spores produce from sporangium (sac like organ) ; Conidia - Spore produced at the tip of the hyphae

Sexual reproduction

Sexual reproduction of fungi involves the following sequence of processes: Plasmogamy (Union of protoplasm of two thallus or gametes); Karyogamy (Union of two nuclei); Meiosis (Cell division to maintain the n level of chromosome). Generally, all the thallus are at ‘n’ level of chromosome, referred as haplophase and before meiosis, the chromosome level is ‘2n’, which is referred as diplophase. After meiosis spores were formed, which vary with different classes of fungi. The sexual spores are Oospores; Zygospores; Ascospores and Basidiospores. The sex organs of fungi are referred as Gametangia / gametangium. The Cell / nucleus involved in the sexual reproduction is gamete and the fusion of two gametes lead to sexual reproduction ie., plasmogamy, karyogamy and meiosis.

Depending on the group, different types of sexual spores are produced. Spores formed within an enclosed sac (ascus) are called ascospores. Many yeasts produce ascospores. Sexual spores produced on the ends of a club-shaped structure (basidium) are basidiospore. Zygospores, produced by zygomycetous fungi, such as the common bread mold Rhizopus, are macroscopically visible structures that result from the fusion of hyphae and genetic exchange. Eventually the zygospore matures and produces asexual spores that are dispersed by air and germinate to form new fungal mycelia.