Mucor:
Kingdom: Plantae
Sub - kingdom: Cryptogams
Division: Thallophyta
Sub -Division : Fungi
Class : Phycomycetes
Sub - class: Zygomycetes
Cohort Mucorales
Family: Mucoraceae
Genus: Mucor
Occurance:
Mucor is a very common fungus which can be seen on decaying organic materials such as fruits, prickles old wet shoes etc. Mucor stolonifer grow on damp bread while Mucor mucedo grow on cow dung or horse dung. In the laboratory it can be cultured by keeping moist bread in the bell jar for the period of two to three days. During this time bread gets covered by fine cottony white filaments which are nothing but hyphae of mucor forming mycelium. This genus is optimum temperature and moisture are two factors essential for the rapid growth of mucor.
Thallus of mucor is known as mycelium which is white delicate highly branched, unicellular and achlorophylous. Hypahal wall is made up of fungus cellulose while cytoplasm is granular and vacuolated with many nuclei. Reserve food is in the form of sugar, glycogen and droplets of lipids. Hyphae forming mycelium are tubular and aseptate but at the time of formation of reproductive organs the hyphae become septate. In beginning mycelium spreads on substratum. Some hyphae are spread on the substratum and some enter the substratum and absorb nutritive substances. Sporangiphores are essential for the asexual reproduction and they arise solitary in genus mucor.
Reproduction:
Mucor shows three types of reproduction
(1) Vegetative reproduction,
( 2) Asexual reproduction and
(3) Sexual reproduction.
(1) Vegetative Reproduction (Fragmentation):
Mucor filaments may break down by mechanical injury and such fragments develop into a new mucor plant. This method of reproduction is known as "reproduction by fragmentation method".
(a) Reproduction by stolons:
In this mode of reproduction mycelium develops some aerial hyphae which bend down and touch the substratum. Later on it gives out rhizoidal hyphae and sporangiophore. Thus, gradually it covers the entire region of substratum. Stolon breaks and results into two separate plants. Such type of reproduction is met with only Rhizopus genus.
(2)Asexual Reproduction:
In mucor asexual reproduction takes place by Aplanospores, Chlamedospores and oidiospores.
(a) By aplanospore:
This type of asexual reproduction is very common and such spores develop in humid environment. When substratum is nutritive in nature.
(i) Development of Sporangium and spores: From the thallus of mucor, some aerial hyphae develop which are known as spornagiophores. The apical region becomes rounded in which several nuclei; cytoplasm and food particles enter in. This round dome shape region is known as sporangium which appears white in the beginning but later on due to development of spores, it appears black. Wall of the sporangium is thin and spiny. The peripheral cytoplasm which touches the wall of sporangium is dense and shows more nuclei while inner cytoplasm is thin and with few nuclei. Dome shape structure develops in the center with thin cytoplasm. Peripheral and inner cytoplasm separated by a layer of small vacuoles. As a result dome shaped columela and spore producing region get well defined. Now, peripheral protoplast shows cleavage furrows and divides the protoplasm into several small coenocytic groups. Each group gets covered by a wall and then it is known as spore. Each spore is minute and coenocytic i.e. multinucleate.
(ii) Dehiscence of sporangium and dispersal of spores: During development of sporangium, columela and sporangiophore receive more thin protoplast in liquid form and hence it exerts more pressure on columela. As a result sporangium dehisces and spores are dispersed. After the dehiscence, remnants of sporangial wall appears as rugged collar. Mature spores are very minute, light in weight and are dispersed by the wind. In favourable condition, they germinate on the substratum and produce new mycelium.
(b) By Chlamydospores, Oidiospores and torula stage: When mucor is immersed in sugar solution, its mycelium gets septate and becomes multicellular. Some of the cells swells up become enlarged in size and become thick walled. These cells are as chlamydospores. These chlamydospore behave as resting spores.
Some times, the septate mycelium gets fragmented. Each small fragment consists of thin walled cells called oidial cells. These cells shows budding like yeast cell in sugar solution. This stage is referred as yeast stage of mucor. This stage shows anaerobic respiration and produces alcohol in sugar solution. Hence, this stage is named as anaerobic stage or alcoholic stage or alcoholic fermentation stage of mucor.
Torula can be defined as a small swollen region. During immersion of mucor in sugar solution, hyphae become septate and form small swollen regions as chlamydospores. This stage is therefore called torula condition.
(2) Sexual Reproduction:
Mucor shows isogamous type of reproduction. Some species are homothallic while some are heterothallic. In heterothallic species hyphae are similar externally while physiologically they are differ from each other. Dumbel shaped lateral branches develop from heterothallic filaments as shown in the diagram. Apical region is called progametangium which shows development of a septum. Terminal portion is known as gametangium and remaining portion is called suspensor. Gametangium shows denser granular protoplast with many nuclei while suspensor shows vacuolated protoplast with few nuclei. As gametangia are similar in appearance, instead of referring them as male and female gametangia, they are referred as +ve strain and -ve strain gametangia. These gametangia possess gametes which are referred as coenogemets or aplanogametes. When two gametangia meet and unite with each other, the intervening walls between them dissolve. At this time, +ve strains and -ve strains nuclei fuse in pairs. The combined protoplast first results into a cell called zygote which increases in size, its wall becomes thick, black and warty and it is now known as zygospore. This zygospore is isodiametric having a diameter 70-80 µ. Here, glycogen and lipid droplets are seen as reserve food material. After the detachment from suspensor, zygospore passes 6-9 months in resting phase.
- Germination of Zygospore:
The zygospore germinates under suitable conditions. The zygospore protoplast absorbs water and swells up. As a result, outer wall splits open and a small, stout vertical hypha develops form the inner wall which is known as promycellium or germ tube which grows to a limited extent. Now, apical end swells and assumes round shape which develops a transverse wall and thus sporangium develops which includes several nuclei and protoplast. Nuclei undergo meiosis and resultant haploid daughter nuclei are of +ve and -ve strain. Each nucleus shows protoplast and results into a round haploid spore which is referred as aplanospores. It may be of +ve or -ve stain. On germination, it results into +ve stain or -ve stain filaments. On maturity, wall of sporangium breaks open. Aplanospores are light in weight and hence, dispersed by the wind. On getting suitable substratum and condition, they germinate. As there are two types of aplanospores (+ve and -ve) +ve and -ve strain mycelium are produced.
Parthenogenesis: Some times, the gametangia fails to fuse. Each such gamete functions as parthenospore which develops into a thick walled structure called azygospore. If conditions are favourable, it germinates and results into a mycelium without any meiotic division.
Mucor express gametophyte phase and sporophytic phase by mycelium and by zygospore respectively. They alternate with each other. Here, haploid mycelium (n) is the main phase and while zygospore (2n) is the subsidiary phase of life cycle.
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