Q.27. Write short notes on -
(a) Clamp Connection (2005)
(b) Endospore (2012)
(c) Apothecium (2010)
(d) Ascus (2009, 13)
(e) The Amphigynous Antheridium (2009)
Ans. (a) Clamp Connection: -
A type of connection found within a single hyphal strand of a Basidiomycete fungus. It ensures that two adjacent hyphal cells (divided by septa) each have 2 different nuclei from mating with hyphae of another sexual type. It is used in the “nuclear shuffle” similar to that found in croziers during sexual reproduction.
(b) Endospore: -
An endospore is a dormant, tough, and non-reproductive structure produced by bacteria from the Firmicute phylum which forms when a bacterium produces a thick internal wall that encloses its DNA and part of its cytoplasm. Examples include Bacillus and Clostridium.
The primary function of most endospores is to ensure the survival of a bacterium through periods of environmental stress. They are therefore resistant to ultraviolet and gamma radiation, desiccation, lysozyme, temperature, starvation, and chemical disinfectants. Endospores are commonly found in soil and water, where they may survive for long periods of time. Some bacteria produce exospores or cysts instead.
Structure: -
In contrast to eukaryotic spores, which are produced by many eukaryotes for reproductive purposes, bacteria will produce a single endospore internally. The spore is often surrounded by a thin covering known as the exosporium, which overlies the spore coat. The spore coat is impermeable to many toxic molecules and may also contain enzymes that are involved in germination. The cortex lies beneath the spore coat and consists of peptidoglycan. The core wall lies beneath the cortex and surrounds the protoplast or core of the endospore. The core has normal cell structures, such as DNA and ribosomes, but is metabolically inactive.
Up to 15% of the dry weight of the endospore consists of calcium dipicolinate within the core, which is thought to stabilize the DNA. Dipicolinic acid could be responsible for the heat resistance of the spore, and calcium may aid in resistance to heat and oxidizing agents. However, mutants resistant to heat but lacking dipicolinic acid have been isolated, suggesting other mechanisms contributing to heat resistance are at work.
(c) Apothecium: -
In lichens and in fungi of the class Ascomycetes, such as mildews and yeasts, a cuplike structure that produces spores. In the common orange lichen Xanthoria parietina, apothecia may be seen attached to the surface of the leaf like thallus of a darker colour than the rest of the lichen.
(d) Ascus: -
An ascus (plural asci) is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain 8 ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number 1 (e.g. Monosporascus cannonballus), 2, 4, or multiples of four. In a few cases, the ascospores can bud off conidia that may fill the asci (e.g. Tympanis) with hundreds of conidia, or the ascospores may fragment, e.g. some Cordyceps, also filling the asci with smaller cells. Ascospores are nonmotile, usually single celled, but not infrequently may be septate, and in some cases septate in multiple planes. Mitotic divisions within the developing spores populate each resulting cell in septate ascospores with nuclei.
In many cases the asci are formed in a regular layer, the hymenium, in a fruiting body which is visible to the naked eye, here called an ascocarp or ascoma. In other cases, such as single-celled yeasts, no such structures are found. In rare cases asci of some genera can regularly develop inside older discharged asci one after another, e.g. Dipodascus.
Asci normally release their spores by bursting at the tip, but they may also digest themselves passively releasing the ascospores either in a liquid or as a dry powder. Typically, actively discharging asci have a specially differentiated tip, either a pore or an operculum. In some hymenium forming genera, when one ascus bursts, it can trigger the bursting of many other asci in the ascocarp resulting in a massive discharge visible as a cloud of spores - the phenomenon called “puffing”. This is an example of positive feedback. A faint hissing sound can also be heard for species of Peziza and other cup fungi.
Ascus Classification: -
The form of the ascus, the capsule which contains the sexual spores, is important for classification of the Ascomycota. There are four basic types of ascus.
· A unitunicate-operculate ascus has a “lid”, the Operculum, which breaks open when the spores ripen and in this way sets them free. Unitunicate-operculate asci only occur in those ascocarps which have apothecia, for instance the morels. ‘Unitunicate’ means ‘single-walled’.
· Instead of an operculum, a unitunicate-inoperculate ascus has an elastic ring that functions like a pressure valve. On ripening it briefly expands and so lets the spores shoot out. This type appears both in apothecia and in perithecia
· A bitunicate ascus is enclosed in a double wall. This consists of a thin brittle outer shell and a thick elastic inner wall. When the spores are ripe the shell splits open so that the inner wall can take up water. As a consequence this begins to extend with its spores until it protrudes above the rest of the ascocarp so that the spores can escape into free air without being obstructed by the bulk of the fruiting body.
· Prototunicate asci are mostly spherical in shape and they have no active dispersal mechanism at all. The ripe ascus wall simply dissolves so that the spores can escape, or it is broken open by other influences such as animals. Asci of this type can be found both in perithecia and in cleistothecia,
(e) The Amphigynous Antheridium: -
In this the antheredium is first formed at the tip of a hypha then the oogonial incept penetrates grows entirely through it and swells up to form the spherical oogonium above the antheredium. The mature antheredium thus forms. This is the common example in Phytopthora infestans.
Q.28. Write short note on Aecidiospore. (2011)
Ans. The dikaryotized (binucleate) basal cells are popularly called aecidiospore mother cells or sprophores and from them are cut off terminally the chains of binucleate spores, the acidiospores, alternating with sterile cells or disjunctor cells. As a result, closely packed chains of cells are formed at the tip of aecidiospore mother cells. In each case, the oldest aecidiospore is present at the top and the youngest at the base or near to the tip of aecidiospore mother cell.
The aecidiospores are unicellular, binucleate, thin walled and polyhedral in shape. The aecidiospores are unable to reinfect barberry, but they can infect only the wheat plants, thus repeating the life cycle.
Q.29. Write short note on the following.
(a) Facultative parasite
(b) Mycorrhiza (2011)
Ans. (a) Facultative parasite: -
Among the parasites different degrees of parasitism occur. Those Which live entirely on the living protoplasm of their hosts and can never grow on the dead tissues or organic matters, are termed as obligate parasites such as rusts powdery mildews and downy mildews. These are some true fungi which are usually parasitic in their mode of life but later on according to the circumstance, may pass their mode of life as saprophytes. Such fungi arc called facultative saprophytes, e.g., some smuts.
(b) Mycorrhiza: -
In true fungi live in symbiotic association with the members of bryophytes pteridophtes and with the roots of higher plants, the symbiotic association is mycorrhiza.
Mycorrhiza may be of two types: (i) ectotrophic mycorrhizas,and (ii) endotrophic mycorrhizas
In ectotrophic mycorrhiza, the fungus form an external sheath of pseudo parenchyma around the surface of the roots and rootlets of forest trees. In endotrophic mycorrhiza, the fungus develops within the roots.