Sub-division : -Zygomycotina
- Most of the Zygomycotina are present in soil and dung, occurring mostly as saprophytes; few are parasitic on plants and animals. About 1000 fungal species belong to Zygomycotina.
- Vegetative ( somatic) body is Haploid .
- Thallus is usually mycelial, hyphae coenocytic.
- Cell wall is made up of chitin and chitosan.
- Asexual reproduction occurs most commonly by the formation of nonmotile, unicelled sporangiospores in uni- or multispored sporangia or merosporangia. In addition, arthrospores, chlamydospores, and yeast cells can be formed by some species.
- These characteristics are shared with the divisions of flagellated fungi (Mastigomycotina).
- Spores are dispersed either violently or passively by wind, rain or animals.
- Flagellated spores and gametes are absent in this division as well as in the remaining taxa of terrestrial fungi.
- Sexual reproduction occurs with the fusion of two multi-nucleate isogametangia or anisogametangia to produce a zygote.
- The zygote later develops into a thick-walled zygospore, the diagnostic feature of this division.
- Because of this the fungi of the class zygomycetes are also known as conjugation fungi.
- Two classes are recognized in this division :-
Zygomycetes :- The members of this class are mostly saprobic(saprophytes), though a few may become weakly parasite on plants.The common examples are black bread moulds or pin moulds (the genera Rhizopus and Mucor). In the class, Zygomycetes there are three orders:-
Order :- Mucorales
Rhizopus (common bread mold)
- This species is one of the most common members of this class.
- Vegetative thallus is eucarpic, branching, coenocytic (multinucleate) and aseptate (not divided by cross walls into cells or compartments) mycelium and haploid (n).
- The mycelium of R. stolonifera is differentiated into three different parts :-
- Stolons:- The mycelium develop aerially above the substratum is called "stolons".
- Rhizoids:- Stolons produce rhizoids in a cluster from below into substratum. Branching rhizoids behave as roots, anchoring the fungus into its substrate, releasing digestive enzymes, and absorbing nutrients for the fungus.
- Sporangiophores: -Sporangia form on the tips of sporangiophores, which are erect branches formed directly above the nodes of rhizoids.
Internal structure The hyphae cell wall are made up of chitin lined by a thin plasma membrane. The protoplasm includes typical cell organelles like nuclei, mitochondria, ribosomes, endoplasmic reticulum etc.
Life cycle of Rhizopus stolonifer
Rhizopus stolonifer Life cycle includes both asexual reproduction and sexual.
- Asexual reproduction :- Asexual reproduction during favourable condition.
- Asexual reproduction takes place by the formation of sporangiophores with terminal uni-to-multispored sporangia.
- Sporangiophores arise from mycelium.
- Each sporangium begins as a swelling into which a number of nuclei flow, and it is eventually cut off from the sporangiophores (Aplanospores) by the formation of a septum.
- The protoplasm within is cleaved, and a cell wall is in the sporangium. Which divides sporangium into an outer fertile sporiferous zone and a central sterile columella.The protoplasts of the sporiferous zone cleave mitotically to form dark-coloured multinucleate spores.
- In this process, nonmotile, single-celled, haploid sporangiospores are formed.
- The sporangium becomes black as it matures, giving the mold its characteristic colour. Each spore, when liberated, can germinate to produce a new mycelium.
- Sexual reproduction :- R. stolonifer is different from all other fungi because it reproduces by sexual reproduction via zygospores following gametangial fusion
- Sexual reproduction occurs only between different mating strains such fungi are known as heterothallic.
- Which have been traditionally labeled as + and - types. But the mating strains are morphologically indistinguishable,.
- Two hyphae of opposite strains come closer, hormones cause their hyphal tips to come together.These hyphae are called zygophores.
- The tip of zygophores develop into gametangia, which become separated from the rest of the fungal body by the formation of septa.
- The walls between the two touching gametangia dissolve, and the two multinucleate protoplasts come together.
- The + and - nuclei fuse in pairs to form a young zygospore with several diploid nuclei.
- The zygospore which can become dormant for several months.
- Meiosis occurs in the diploid nuclei of zygospore.It results in the segregation of separate '+' and '-' nuclei all the nuclei so formed disintegrate except one.
- After a long period of rest, the wall of the zygosporangium cracks. And produces a sporangium that produces spores same as the asexually produced sporangium, and the life cycle begins again.