Reproduction
Reproductive Process of Fungi or Fungus Reproduction we have to know that Fungi reproduce by three methods vegetative, asexual and sexual.
1. Asexual reproduction.
It occurs practically in all fungi. It usually takes place during unfavourable conditions by means of a wide variety of unicellular or multicellular spores. They are either motile or non-motile. There is also much variation in the form of spore bearing structures. Some common types of asexual spores are briefly described below.
(a) Zoospores.
These are unicellular motile, uninucleate and naked flagellate structures. Zoospores are formed endogenously within a sac-like structure called zoosporangium; e.g., Albugo, Synchytrium.
(b) Conidium (pl.= conidia).
Conidia are non-motile spores borne either singly (e.g., Phyto-phthora) or in chains (e.g., Penicillium Aspergillus) at the tip of the hyphae. The hypha bearing conidia at its tip is called conidiophore. Conidia are usually unicellular but septate conidia are found in some fungi; e.g., Alternaria.
(c) Oidium (pl. = oidia).
These are small, thin walled free cells formed due to the fragmentation of a somatic hyphae into cells. The hyphal branch forming oidia is called conidiophore; it cuts off oidia apically in succession. Each oidium behaves as a spore; e.g.. Coprinus, Mucor.
(d) Chlamydospores.
These are unicellular,thick walled, non-motile resting spores formed by the rounding off of the contents of the hyphal cells. Chlamydospores may be terminal or intercalary in position; e.g., Mucor, Peziza,
Ustilago.
(e) Aplanospores or sporangiospores.
These are non motile unicellular spores which are formed within a sac-like sporangium. The hypha bearing terminal sporangium is called sporangiophore; e.g., Mucor.
In some fungi the entire thallus is converted into one or more reproductive structures (sexual or asexual). Therefore, the vegetative and reproductive phases do not occur together at the same time in the same individual. Such fungi are called holocarpic, e.g., Synchytrium. However, in the majority of fungi only a part of the thallus produces the reproductive structures and the rest of the thallus remains vegetative. Such fungi are called eucarpic; e.g., Mucor
2. Sexual reproduction.
Sexual reproduction in fungi, as in other organisms, also involves fungi fusion of two compatible nuclei. Some fungi develop distinct male and female sex organs whereas others (the advanced forms) do not develop them. In such forms the somatic hyphae take part in sexual reproduction.
The sexual process involves following three steps.
(a) Plasmogamy.
This is the fusion of the protoplasm of two sexually compatible elements such as the spores, gametes or vegetative cells. In this process the haploid nuclei come to lie in a single cell but they do not fuse.
(b) Karyogamy.
This is the fusion of two nuclei brought together by plasmogamy. These nuclei are haploid and fuse to form a diploid nucleus. Karyogamy may take place either immediately after plasmogamy or there is sufficient interval between these two steps.
(C) Meiosis.
Karyogamy, sooner or later, is followed by the reduction division of the diploid nucleus. In fungi the plasmogamy and subsequent stages of sexual process may be brought about by any one of the following types
(i) Gametic copulation.
It may be any one of the following three types
(1) Isogamy.
It involves fusion of two morphologically similar and motile gametes
(2) Anisogamy.
Here both the fusing gametes are motile but the male gamete is smaller than the female gamete.
(3) Oogamy.
In this type of copulation, female gamete 1S larger and non-motile while the male gamete is smaller and motile due to flagella Primitive fungi usually have isogamous plano gametes whereas in advanced forms the male anisogamous Fusing gametes is motile and small, and female is non-motile Synchytrium, large, e.g and relatively Monoblepharella.
(ii) Gametangial contact.
In this process gametes are not released from the gametangia, instead the two gametangia of opposite sex come in contact and the gametes are directly transferred from one gametangium to the other through a tube called fertilization tube ; e.g., Albugo, Pythium.
(iii) Gametangial copulation.
It involves fusion of the entire contents of two gametangia. Two gametangia of different sex come close together and the common wall between the two gametangia dissolves. As a result a common cell is formed where the protoplasm and the nuclei of the two gametangia fuse; e.g., Mucor.
(iv) Spermatization.
Some fungi (Basidiomycetes and Ascomycetes) produce numerous minute, uninucleate, spore-like male gametes called spermatia (sing spermatium). These are non-motile and are carried by wind, insects or water to respective female hyphae. A pore develops at the point of contact of spermatium and receptive hyphae through which nucleus of the spermatium is transferred to the receptive hypha; e.g Puccinia, Podospora
(v) Somatogamy.
In many higher fungi sex organs are not produced and instead somatic cells directly function as gametes; e.g., Peniophora.
3. Vegetative reproduction.
Here are some common methods of vegetative multiplication are as follows.
(a) Fragmentation.
Thallus or hypha breaks into a number of segments. Each segment grows into a new individual.
(b) Fission.
In fission a cell simply breaks into two daughter cells by constriction. It is common in unicellular fungi such as yeasts.
(c) Budding.
In this process a ‘bud’ or an outgrowth is produced by the parent cell. Meanwhile the nucleus divides and one of the two daughter nuclei migrates into the bud. Eventually the bud gets separated from the parent cell and grows into a new individual; e.g., yeast.
Reproduction System Of Fungi
Here it’s time to know about the reproduction of fungus or fungi so before we know the reproduction of fungi we have to know one thing that Saccharomyces reproduce by vegetative and sexual methods.
Vegetative Reproduction of fungi
Vegetative reproduction takes place by budding and fission. Accordingly, yeasts have been grouped into budding yeasts (e.g., S. cerevisiae) and fission yeasts (e.g.. S. octosporus).
1. Budding. It is the most common method of multiplication in Saccharomyces. Budding usually occurs during favourable conditions when food material is available in plenty. During the formation of a bud one or more outgrowths are formed at or near one pole of the cell. The bud enlarges in size.
In the meantime the nucleus divides mitotically, one daughter nucleus passing into the bud and the other remaining in the parent cell.
The bud eventually separates from the parent cell by a constriction. It leaves a circular scar with a convex surface on the parent cell wall. This is called bud scar.
The process of budding is very rapid. Usually a bud starts forming new buds before its separation from the parent cell. This frequently results in the formation of branched or unbranched chains of buds, called pseudomycelium
2. Fission. Fission involves division of a cell into two by transverse wall. During this process the parent cell elongates and the nucleus divides mitotically into two daughter nuclei. A transverse wall now divides the mother cell into two daughter cells each containing a daughter nucleus Later both the cells separate, cach leading an independent life.
Sexual Reproduction of Fungi
Sexual reproduction in yeasts takes place during unfavourable conditions, particularly when there is less amount of food. Yeasts are homothallic (e.g..S. octosporus) or heterothallic (e.g., S. cerevisiae). They lack definite sex organs, instead somatic cells or ascospores function as copulating gametangia. Their fusion results in the formation of a diploid zygote cell.
The following three life cycle patterns are found in different species of Saccharomyces.
(1) Haplobiontic life cycle (e.g., Saccharomyces octosporus)
(2) Diplobiontic life cycle (e.g., S. ludwigii)
(3) Laplo Diplobiontic life cycle (e.g., S. cerevisiae).
1. Haplobiontic life cycle.
This type of life cycle is shown by S.octosporus, a homothalliC species. Here the somatic cells are haploid. Two such haploid cells come together and form conjugation tubes. The common wall where the two conjugation tubes meet dissolves and the nuclei of two conjugating cells fuse.
Diploid cell, thus formed, now functions as an ascus. The diploid nucleus undergoes a meiotic division followed by a mitotic division forming eight haploid nuclei. Each of these eventually Thus eight transforms into an ascospore. ascospores are formed within the ascus. The wall of the ascus ruptures at maturity and ascospores are set free. Each ascospore now develops into a somatic cell.
In this type of life cycle the zygote is the only diploid cell. Other stages, forming the major part of the life cycle, are haploid. On this basis the life cycle is called haplobiontic.
2. Diplobiontic life cycle.
This type of life cycle is shown by S. ludwigii.The somatic cells are diploid. Each cell directly functions as ascus. The diploid nucleus of ascus divides meiotically forming four haploid ascospores. These spores now behave as gametes and fuse in pairs within the ascus. Thus two diploid cells are formed from two pairs. subsequently, these cells germinate by a germ tube which pushes through
the ascus wall. It is multicellular and functions as diploid sprout-mycelium. The cells of the mycelium form many diploid cells by budding.
In this type of life-cycle the haploid stage is represented only by ascospores and all other stages
are diploid. Hence it is called diplobiontic.
3. Laplo Diplobiontic life cycle.
It is shown by S. cerevisiae, a heterothallic species. Here both haploid and diploid phases are equally developed. Diploid somatic cells multiply by budding, forming a large number of diploid cells. Some of the diploid cells function as ascii. They divide mechanically, forming four haploid ascospores. These upon liberation, form a number of haploid somatic cells by budding which are smaller than the diploid cells. Fusion also takes place between haploid cells to form diploid somatic cells.
In this type of life cycle both haploid and diploid phases are capable of reproducing independently, thus both are equally important. Hence, it is called haplodiplobionte.