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Types of parasitism and variability.pptx
- 2. Fungi are essentially heterotrophic or dependant on other organisms as they are chlorophyll less in nature. Moreover, they require predigested food to live and therefore they obtain food from either living (Parasites) or non living sources (Saprophytes). 1) Parasite An organism that lives on or in some other living organism (host) and derive nutrition are called as parasites. The relationship between the parasite and the host is called as parasitism. The parasites are of different types.
- 3. i) Ectoparasite The parasite may live on the external surface of the host (eg) Erysiphe polygoni. ii) Endoparasite The parasite may grow inside the host cell (intracellular) or in between the cells (inter cellular) of the host plant and draw nutrition. (eg) Leveillula taurica. iii) Endobiotic parasite Parasite which produces their entire body within a host cell is called endobiotic Eg:- Synchytrium endobioticum Facultative parasites They usually live as saprophyte but can attack living tissue under certain condition. i.e., they normally grow as saprophyte but also have the ability to parasitise. These fungi are having saprophytic phase with a short parasitic phase under suitable condition (eg) Pythium sp., Rhizoctonia sp
- 4. Facultative saprophytes They usually live as parasite but can grow saprophytically i.e., they have long parasitic phase and with a short saprophytic phase in their active life cycle (eg) Phytophthora sp., Venturia inequalis Obligate parasite They invariably occur as parasites in nature and cannot be grown in axenic culture media under in vitro condition which are called as 'biotrophs’ (eg) Downy mildew, Powdery mildew, Rusts, Viruses, Viroids
- 5. Hemibiotrophs These pathogen attack living tissues and grows in or on them in the same way as that of biotrophs but continue to grow and reproduce after the tissue is dead (eg) leaf spotting fungi. Perthotrophs They are so called less specialised parasite which can cause immediate and severe damage to their host tissue. They usually obtain nutrients from cells killed in advance (eg) Rhizopus, Pythium, Rhizoctonia, Fusarium, Sclerotium rolfsii They kill the tissue before actually entering and feeding on them. Necrotrophs There are some intermediate parasites which invade and kill host cell (eg) Phytophthora, Claviceps, Venturia.
- 6. Synergism It means the ability of two kinds of organisms to grow better or produce greater effect conjointly than either one could alone (eg) Diplodia natelensis and Colletotrichum gleosporioides together produce much greater effect on citrus than either one of the pair alone can produce. Symbiosis :- In some cases of parasitism, as with the root nodule bacteria of legume plants and the mycorrhizal infection of feeder roots of most flowering plants, both the plant and the microorganism benefit from the association. This phenomenon is known as symbiosis. Mycorrhiza It is one kind of mutualism between the fungi and roots of higher plants. The name 'mycorrhiza' literally means "fungus root" and it was coined by Frank in 1885 Two types Ectomycorrhiza Endomycorrhiza
- 7. Variability in Plant Pathogens
- 8. Variability:- The property or ability of an organism to change its characteristics from one generation to the other. General Mechanisms of Variability Mutation and Recombination A mutation is a more or less abrupt change in the genetic material of an organism, which is then transmitted in a hereditary fashion to the progeny. Mutations represent changes in the sequence of bases in the DNA either through substitution of one base for another or through addition or deletion of one or many base pairs. On average, one mutation occurs for every million copies of a gene per generation
- 9. Recombination occurs primarily during the sexual reproduction of fungi, and nematodes whenever two haploid (1N) nuclei, containing genetic material that may differ in many loci, unite to form a diploid (2N) nucleus, called a zygote. The zygote, sooner or later, divides meiotically and produces new haploid cells (gametes, spores, mycelium). Recombination of genetic factors (different genes or alleles of the same genes) occurs during the meiotic division of the zygote as a result of genetic crossovers in which parts of chromatids (and the genes they carry) of one chromosome of a pair are exchanged with parts of chromatids of the other chromosome of the pair.
- 10. Specialized Mechanisms of Variability in Pathogens Fungi Heterokaryosis Heterokaryosis is the condition in which, as a result of fertilization or anastomosis, cells of fungal hyphae parts of hyphae contain two or more nuclei that are genetically different Eg:- P. graminis tritici, the fungus causing stem rust of wheat,
- 11. Specialized Mechanisms of Variability in Pathogens Variability in fungi Parasexualism Parasexualism is the process by which genetic recombinations can occur within fungal heterokaryons. This comes about by the occasional fusion of the two nuclei and formation of a diploid nucleus. During multiplication, crossing-over occurs in a few mitotic divisions and results in the appearance of genetic recombinants as the diploid nucleus progressively and rapidly loses individual chromosomes to revert to its haploid state
- 12. Heteroploidy Heteroploidy is the existence of cells tissues or whole organisms with numbers of chromosomes per nucleus. Heteroploids may be haploids, diploid, triploid or tetraploids i.e. have one or more extra chromosomes from normal euploid number e.g.N+1 o This represents a normal situation in eukaryotes Physiological race – individuals within the species of a pathogen that morphologically similar but differ with respect to their pathogenicity on particular set of host varieties. Formae specialis (f. sp.) – individuals within the species of a pathogen that morphologically similar but differ with respect to their pathogenicity on particular host genera. E.g. Puccinia graminis f.sp. tritici
- 13. Specialized Mechanisms of Variability in Pathogens Variability in bacteria (1) Conjugation occurs when two compatible bacteria come in contact with one another and a small portion of the chromosome or plasmid from one bacterium is transferred to the other through a conjugation bridge or pilus. (2) In transformation, bacterial cells are transformed genetically by absorbing and incorporating in their own cells genetic material secreted by, or released during rupture of, other bacteria.
- 14. (3) In transduction, a bacterial virus (phage) transfers genetic material from the bacterium in which the phage was produced to the bacterium it infects next.
- 15. Recombination • May results from mixed infection of two strains of the virus • Occurs mostly during replication Reassortment • It is defined as the exchange of intact genes within the entire segment, which occurs during coinfection. Reassortment has been observed in Bunyaviridae, Reoviruses Mutation • Results from nucleotide changes in the coding regions due to addition or deletion or replacement. • Ultimately leads to functional changes in the genes. Variability in virus