Reproduction

Reproduction (Asexual reproduction)

Naturally, fungi reproduce by a variety of means. Asexual reproduction (also known as the somatic or vegetative reproduction) occurs without the involvement of the union of nuclei, sex cells, or sex organs. It may take place by the following means;

(1) fission of somatic cells producing two similar daughter cells.

(2) budding of somatic cells or spores; in which each bud consists of a small outgrowth of the parent cell that finally develops into a new individual.

(3) fragmentation or disjointing of the hyphal cells, in which each fragment itself develops into a new individual.

(4) spore formation.

Asexual spores, which function for the dissemination of the species, are generally produced in large numbers. There are various kinds of asexual spores.

1. Sporangiospore-formed within the sacscalled sporangia(singular, sporangium), These single-celled spores lie at the end of special hyphae (Sporangiospores). Non-motile sporangiospores are also called as Alpanospores whereas, Zoospores are the motile sporangiospores, their motility exist due to the presence of flagella.
2. Conidiospores or conidia (singular, conidium). microconidiaa are the Small, singled-celled conidia whereas, large, multicelled conidia are called macroconidia. These conidia are located at the tip or side of a hyphae.
3. Oidia (singular, oidium) or arthrospores. These are the single-celled spores that are formed by the disjointing of hyphal cells.
4. Formed from the cells of thevegetative hypha,These are thick-walled and single-celled spores that are highly resistant to adverse conditions.
5. These are spores that are formed by the process of budding.

Sexual production

The sexual reproduction occurs by fusion of the compatible nuclei of two parent cells. The sexual reproduction starts with the joining of two cells and fusion of their protoplasts;plasmogamy, thus allowing the two haploid nuclei of two mating types to fuse together;karyogamy, to form a diploid nucleus. Now, this is followed by meiosis, which finally reduces the number of chromosomes into the haploid number.

Gametangia which are the sex organelles of fungi(singular, gametangium) form differentiated sex cells (gametes) or may contain instead one or more gamete nuclei. In case the male and female gametangia are morphologically different, the male gametangium is named as the antheridium (plural, antheridia) and the female gametangium is called the oogonium (plural, oogonia).

The various processes of sexual reproduction by which compatible nuclei are brought together in plasmogamy may be summarized as follows:

1. Gametic copulation. includes the fusion of naked gametes, one or both of which are motile.
2. Gamete-gametangial copulation. Fusion does not occur although both the gametogonium come to the contact; the male nucleus migrates through a pore called the fertilization tube into the female gametangium.
3. Gametangial copulation. The protoplasts of two gametangia fuse and give rise to a zygote that further develops into a resting spore.
4. Somatic copulation. It includes the fusion of somatic or vegetative cells.
5. Spermatium (plural, spermatia) which can be better described as a special male structure unites with a female receptive structure. The spermatium empties its contents into the latter during plasmogamy.

Sexual spores are produced by the union of two nuclei and occur less frequently, later, and in smaller numbers than the asexual spores. There are various types of sexual spores:

1. These single-celled spores are generated within a sac called an ascus (plural, asci). Each ascus usually consists of eight ascospores
2. These single-celled spores are born on a club-like structure known as basidium (plural, basidia).
3. Zygospores are large, thick-walled spores originated when the tips of two sexually compatible hyphae, or gametangia, of certain fungi, combine together.
4. These are originated within a special female structure, called the oogonium. Fertilization of the eggs, or oospheres, by male gametes formed generally in an antheridium, produces oospores. There could be one or more oospheres in each oogonium.

A highly organized protective structures called fruiting bodies may surround asexual and sexual structures. Asexual fruiting bodies have names such as acervulus and pycnidium. Sexual fruiting bodies could have names such as perithecium and apothecium.

Even though a single fungus may produce both sexual and asexual spores by several methods at different times and under different conditions, the spores are always constant in their structures and in the process by which they are created to be used in identification and classification.

Physiology

Fungi are better able to stand against certain extreme environment conditions compared to most other microorganisms. For example, in the case of yeasts and molds, they can grow in a substrate or medium containing ahigh concentrate of sugars that inhibit most bacterial activities; this is why jams and jellies may be spoiled by molds but not by bacteria.As well as, yeasts and molds generally can withstand more acidic conditions than most other species of microbes.

Some yeasts are facultative; that is, they can grow under both aerobic as well as anaerobic conditions. Molds and many yeasts are generally aerobic microorganisms. Fungi can grow over a wide range of temperature, which is the optimum for most saprophytic species from 22 to 30-degree celsius; pathogenic species could have a higher optimum temperature, generally 30 to 37. Some fungi will easily withstand and grow at or near 0 and thus they can cause spoilage of meat and vegetables even in cold storage.

Fungi have the ability to use a wide variety of materials for their nutrition. However, they are heterotrophic. They cannot use inorganic carbon compounds, such as carbon dioxide, like some bacterial species as their sole carbon source. Carbon must evolve from an organic source, like glucose. Some species can even use the inorganic compounds of nitrogen, like as ammonium salts. However all fungi can use organic nitrogen; this is why culture media for fungi generally contain peptone, a hydrolyzed protein product. A summary of the physiological characteristics of fungi in comparison with those of bacteria.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.