Mucor & Rhizopus - Class 11 PDF Download

Systematic Position

Habit & Habitat      

1. Rhizopus and Mucor are saprophytic fungi and in laboratory can be obtained on moist bread.
2. Rhizopus and Mucor are also called as bread mold.

Structure Of Mycelium

It is branched, coenocytic, aseptate and vacuolated.

1. The hyphal wall is made up of chitin. 
2. Some amount of glucosamine, galactose, protein, lipid, Mg and Ca are also present in the cell wall.

In the cytoplasm, storage food is present in the form of oil droplets and glycogen bodies.

Reproduction

      (i)   Vegetative reproduction

      (ii)  Asexual reproduction

      (iii) Sexual reproduction

Vegetative Reproduction

1. It takes place through fragmentation.

Asexual Reproduction

1. By aplanospore formation -

• Sporangia are aseptate and aerial.
• Sporangium develops on sporangiophores.
• Sporangiophores are branched (Mucor mucedo) or unbranched (M.jenevensis).
• Sporangiophores come out from mycelium and their distal ends swell up.
• The swollen spices accumulate cytoplasm, nuclei and food. Now it is called sporangium (plural-sporangia).
• In this stage protoplast of sporangium is differentiated into a peripheral dense multinucleated region and central region with small flattened vacuole.
• Nucleus are arranged in peripheral region. The peripheral region of sporangium is fertile.
• Central vacuolated region is sterile. It is hemispherical or club shaped which is called columella.
• At the stage of spore formation protoplast of peripheral region of sporangium get divided into many multinucleated segments (2-10 nuclei in each segment).
• Each segment secretes a wall around itself and form aplanospores. 
• When they mature outer wall of sporangium ruptures to release the spores in air.
• These spores germinate in favourable conditions and form new mycelium.

1. By Chlamydospores - 

• Asexual reproduction also takes place by chlamydospores.
• When unfavourable condition comes, mycelium becomes septate.
• Protoplast of each cell aggregates, to become spherical and secretes a thick wall around itself.
• These are perennating structure which can survive in unfavourable conditions.

1. Oidia -

• When mycelium grows in a liquid medium then it becomes septate and thin-walled. These cells are called oidia.
• Oidia are similar to buds of yeast.
• Fermentation takes place by oidia in sugar solution and alcohol is formed.
• Oidia are formed in a chain. This stage is called torula stage.

Sexual Reproduction

It takes place during the absence of nutrient substances.

In this process, gametangial copulation takes place between two gametangia.

Both homothallic and heterothallic species are occur in these fungi. But most of the species are heterothallic.

Mucor mucedo (Dung mold)   –         heterothallic

R.stolonifer                              –         heterothallic

Rhizopus nigricans                 –         heterothallic

R. sexualis                               –         homothallic (species found as obligate parasite in higher plant)

M.hiemalis                               –         homothallic

When two fungal hyphae which are respectively (+) and (–) strains come close to each other, then these fungal hyphae produce some outgrowths. These outgrowths are called "progametangia".

Anterior parts of progametangia are filled with cytoplasm and nuclei. Now, these progametangia (which are different in strains), come in contact and a septum is formed just behind the contact place.

Now the terminal part is called gametangium & long basal part is called suspensor, which is vacuolated.

The multinucleate undifferentiated protoplast of each gametangium is termed as coenogamete or aplanogamete.

 In mature gametangium at the place of contact, walls of gametangia are dissolved and fuse together and then zygospore is formed.

In zygospore diploid nuclei are formed while haploid nuclei are degenerate.

1. A black coloured, thick and ornamented coat is formed around the zygospore [Dormant stage approximately 6 months (In Rhizopus)]. Which germinate in favourable condition.

During germination outer wall of zygospore get ruptured.

1. A germ tube protrudes out from this ruptured place, it is called promycelium/Promycelial Hypha.

At the apex of promycelium, a sporangium is formed which is called germsporangium or zygosporangium.

In Rhizopus, at the time of germination Meiotic division takes place in zygospore (delayed) whereas in Mucor, meiotic division takes place in zygospore soon after karyogamy.

Each nucleus is converted into spore.

These spores germinate to form fungal hypha.

Hetrothallism

1. "Blakeslee" discovered heterothallism first in Mucor mucedo.
2. According to Blakeslee two types of species are found in order mucorales which are called respectively homothallic and heterothallic species.
3. Homothallic e.g. M.hiemalis.
4. Hetrothallus species – e.g., M.mucedo and R.Stolonifer.

Diseases

1. Rhizopus stolonifer – causes soft rot disease of vegetable crops. Soft rot disease of sweat potato.
2. M.pusilus – infects genitalia of human beings
3. Rhizopus arrhizus – Causes fruit rot of apple.
4. R.artocarpi – fruit drop of Jack fruits.