Lecture - 8 Zygomycotina: Piloboulus
Sub-division: Zygomycotina:- It includes 2 classes -
a. Zygomycetes:-
> The class zygomycetes derives its name from the thick-walled resting spores, the zygospores formed as a result of the complete fusion of the protoplasts of two equal or unequal gametangia.
> It comprises 450 species which are grouped under 70 genera.
> They all are terrestrial molds which show a wide range in their habit.
> Most of them are saprobes. Among these some are soil saprophytes and others coprophilous (growing on dung).
> The parasitic species include pin molds which attack fungi, insects or protozoa. Some are facultative weak parasites (Rhizopus sexualis) of higher plants. A few are specialized parasites of animals. A very few occur as obligate parasites of other Zygomycetes.
> The members are saprobes or weak parasites on plants to specialized parasites on animals. A few occur on dung thus coprophilous in nature.
> The thallus usually consists of well developed, branched, filamentous, and coenocytic mycelium; some members possess very much reduced septate mycelium. In some cases, the coenocytic mycelium produces rhizoids and adheres to hard surfaces with their help.
> The cell wall is mainly composed of chitosan-chitin.
> The asexual reproduction takes place usually by means of non-motile sporangiospores, called aplanospores, but some also reproduce by chlamydospores or by oidia formation.
> The sexual reproduction takes place by means of gametangial copulation, resulting in the formation of thick-walled zygospores.
> The zygospore germinates by producing a germ sporangiophore that terminally bears a germ sporangium.
> Classification:-
5 Orders:-
a. Mucorales:-
3 Families:-
i. Mucoraceae:- Eg.- Mucor, Rhizopus
ii. Syncephalastraceae:- Eg.- Syncephalastrum
iii. Pilobolaceae:- Eg.- Pilobolus
b. Dimargaritales:- Eg.- Dimargaris
c. Endogonales:- Eg.- Endogone
d. Entomophthorales:- Eg.- Entomophthora
e. Zoopagales:- Eg.- Cytopage
b. Trichomycetes:-
> Trichomycetes, includes organisms that are clearly protozoans though they were mistakenly
classified as fungi for a long time, and are still called fungi by many people who should know
better.
> These extremely common organisms live only in the digestive tracts of insects and other
arthropods, generally as commensals, sometimes as pathogens or symbionts (mutualists), which
are associated with, although not penetrating, the cuticle lining the digestive tracts of the host
animal.
> Their hosts include terrestrial, marine and freshwater arthropods, most commonly midges
(Chironomidae), mosquitoes (Culicidae), black flies (Simuliidae), beetles (Coleoptera), stoneflies
(Plecoptera), and mayflies (Ephemeroptera), as well as several millipedes (Diplopoda) and
crustaceans.
> Classification:-
4 Orders:-
a. Amoebidiales:- Eg.- Amoebidium
b. Asellariales:- Eg.- Asellaria
c. Eccrinales:- Eg.- Enterobryus
d. Harpellales:- Eg.- Harpella, Smittium
Pilobolus:-
1. Classification:-
Kingdom:- Myceteae
Division:- Amastigomycota
Sub division:- Zygomycotina
Class:- Zygomycetes
Order:- Mucorales
Family:- Pilobolaceae
Genus:- Pilobolus
2. Habit and Habitat:-
> Pilobolus, also called hat-thrower fungus, a cosmopolitan genus of at least five species of fungi that are known for their explosive spore dispersal.
> It is also known as conjugation fungi.
> The Pilobolus is a terrestrial fungus and is very rarely parasitic in nature.
> Pilobolus species feed saprobically on the feces of grazing animals.
> These fungi are diminutive, usually less than 10 mm (0.4 inch) in height, and are characterized by a sparse mycelium (fungal body) that produces unbranched sporangiophores (fruiting bodies) capped with black sporangia (spore clusters).
> Pilobolus must pass through the digestive tracts of grazing animals as part of their life cycle.
> Because the animals avoid foraging near their excrement, the fungi use a phototropic (light-following) squirt-gun mechanism to disperse their sporangia up to 3 metres (10 feet) away from the parent fungus and onto uncontaminated vegetation.
3. Thallus structure:-
> The hyphal walls of these fungi contain chitin and fungal cellulose.
> Their mycelium is coenocytic in nature which means that it is multinucleate and aseptate in nature.
4. Reproduction:-
a. Asexual reproduction:-
> It takes place by means of sporangiospores.
> The modes of development of the sporangia, columella and the formation of spores agree closely with those of Mucor mucedo, but the structure of the sporangium represents a special modification of the Mucor type.
> The many-spored sporangium, instead of being spherical is somewhat flattened and the upper half of the sporangial wall is very much thickened.
> The columella is rather small and somewhat conical in shape.
> The part of the sporangiophore just below the sporangium enlarges considerably to form a sub-sporangial vesicle which may be three to four times the diameter of the flattened sporangium.
> Numerous glistening drops of water appear on the surface of the vesicle and the lower part of the sporangiosphore.
> When the sporangiophore is in a fully turgid condition, the sub-sporangial vesicle has been found to be sensitive to the directive influence of light. This stimulus causes curvature of the lower part of the sporangiophore, and the sporangium is directed towards the source of light.
> The neck, i.e. the constricted portion between the sporangium and the sub-sporangial vesicle, undergoes softening and stretching and finally the turgidity of the vesicle reaches to such a degree that the neck ruptures and the entire sporangium, with mature spore, is blown off along with the watery contents of the vesicle towards the sources of light, without much scattering of aim, to a distance of several centimeters. Hence, it is known as pilobous gun.
> The sporangia finally descend on the vegetation, adhere to it and are taken up by animals.
> The spores, only after passing out of their alimentary canals, are capable of germination and give rise to mycelia of this mould under favourable conditions.
b. Sexual reproduction:-
> It takes place by the union of two isogametangia and their coenocytic contents, and resembles essential features of the sexual reproduction of Mucor mucedo. But, in this case, the gametangia are somewhat club-shaped and they meet in such a way that they resemble a pair of tongs.
> A coenocytic zygospore is formed, by their union, at the point of contact, and it appears as a bud and occupies, as if, a terminal position.
> P. crystallius is heterothallic, and the formation of a zygospore is dependent upon the union of mycelia of two sexual strains, ( + ) and (-).
