variability in plant pathogens › admin › upload › 1588253849.pdf · often infect the same...
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VARIABILITY IN PLANT PATHOGENS
PRESENTED BY
Arushi Arora
A-2016-30-050
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GENETICS AND VARIABILITY OF PLANT PATHOGENS
• Dr. E.C. Stakman (U.S. Deptt. Of Agriculture)defined need and use of variability in fungi
• Pathogen population mutates with time andenvironmental conditions like stress
• In nature variability in organisms may be dueto:
HYBRIDIZATION MUTATION
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• When progenies show variations incharacterstics from parents it is called as aVARIANT.
• Physiological specialization: – with in thespecies of a pathogen there exist certainindividuals that are morphologically similarbut differs w.r.t their physiology, biochemicalcharacters and pathogenicity and aredifferentiated on the basis of their reaction oncertain host genera or cultivars.
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• Physiological race: – a group of populationwithin a species which have ability of infectinga particular genotype and do not differ in theirmorphology but have physiological differencessuch as a specific host or food type orpathogenicity
• Variability: it is the property of an organism tochange its characters from one generation tothe other
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• With in species or f.sp. There are furthersubgroups of individual that infect differentvarietes of the host– such subgroups arecalled RACES/ STRAINS
• A sub-group within a species or race,characterized by a common possesion ofsingle of few new characters and differ only infew minor characters from parents are calledBIOTYPES
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BREAKDOWN OF RESISTANCE:
• This term is used when a previously resistantvariety suddenly develops disease.
• It implies that the host has changed, that theresistance mechanisms no longer work.
• New pathogen races have developed becauseof selection pressure that was put onpopulation by the host resistance mechanism
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Races are defined by ability to develop on specific host genotypes (varieties, cultivars, hybrids, etc.).
Races are identified by ability to cause disease on members of a set of 10 differential varieties that contain specific
resistance genes.
V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 Race
x x x 14
x x x 27
x x 31
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VARIABILITY IN FUNGI
MUTATION
HERTEROKARYOSIS
PARASEXUALISM
RECOMBINATION
CYTOPLASMIC ADAPTATION
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HYBRIDIZATION
• Mating of dissimilar strains or species of fungi
• 2 haploid nuclei (1N) with different geneticmaterial combine to form a diploidnucleus(zygote)
• Eg. Basidiospores( haploid) from different racesoften infect the same leaf or plant
Chances of dikaryotization high
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HERTEROKARYOSIS
Hyphae or parts of hyphae contain nuclei,which are genetically different, generally oftwo different kinds. This condition is known asheterokaryosis .
The phenomenon is commonly brought aboutby hyphal anastomosis between mycelia oftwo parental genotypes.
In Ascomycotina and Basidiomycotina
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PARASEXUALISM
• First demonstrated by Pontecorvo (1956) inAspergillus nidulans.
Parasexualism is the process by which genetic recombination can occur with in fungal heterokaryon :
• Stage 1: heterokaryosis
• Stage 2: recombination
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PARASEXUALISM
• Dissimilar nuclei to fuse and produce diploidsknown as mitotic recombination.
• Recombination without sexual cycle
• There is no fine coordination betweenplasmogamy, karyogamy and haplodisation
• This sequence of events has been described inthe parasexual cycle
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PARASEXUALISM IMPORTANCE
• In rust fungi as P. graminis tritici, mitoticrecombination may represent a mostimportant method of generating new racesespecially in countries such as India wheresexual stage of the fungus is rare due toscarcity of the alternate host, the barberry.
• Common in deutromycotina
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RECOMBINATION
• When two haploid nuclei (1N) containingdifferent gnentic maeterial unite to form diploid(2N) nucleus (Zygote,) when under go meioticdivision produce new haploid .
• Recombination of genes occurs during meioticdivision of zygote as a result of cross over inwhich part of chromatid of one chromosome of apair are expressed with that of the other
• Important in fungi Puccinia graminis.
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Cereal rust fungus variability
• Races of rust pathogen Puccinia graminisdiffer from each other in shape and size ofuredospores but principle diffrence betweenthem is
• Preference for groups of host in differentmembers of graminae
Eg. Race tritici, secalis, avenae etc
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MUTATIONSudden heritable change in genetic material of an
organism
Mutation represent change in sequences of thebases in DNA either by – substitution or bydeletion or addition
Mutations are spontaneous
May occur due to:
1. Improper cell division
2. Abnormalities during division
3. Physical radiations
4. Mutagenic chemicals
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CYTOPLASMIC ADAPTATION
• Pathogens develop capicity to performbiochemical reactions which were not presentin them earlier
• Can utilize protoplasm of unfavourable host.
• Adaptation to new cytoplasm
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Three types of cytoplasmicadaptatibility
I. Pathogen may aquire tolerance to toxic materials
II. Utilization of new types of cytoplasm
III. Change in virulence
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VARIABILITY IN BACTERIA
Transformation transduction
Conjugation
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CONJUGATION
• Transfer of DNA from one bacterial cell toanother Donor cell (F+) transfers DNA torecipient cell (F- )
• In this two compatible bacteria come incontact and exchange the portion of plasmidor chromosome through conjugation bridge orpilus
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TRANSFORMATION
DNA taken up from external environment byabsorption
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TRANSDUCTION
Transfer of bacterial genes with a bacteriophage
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VARIABILITY IN VIRUSES
Recombination
• May results from mixed infection of twostrains of the virus
• Occurs mostly during replication
Mutation – Results from nucleotide changesin the coding regions due to addition ordeletion or replacement. – Ultimately leads tofunctional changes in the genes.
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REFRENCES
• R.S. Singh, INTRODUCTION TO PRINCIPLES OF PLANT PATHOLOGY, Oxford & IBH Publishing CO. pg.200-208
• http://agriinfo.in/default.aspx?page=topic&superid=3&topicid=2131
• http://www.columbia.edu/itc/hs/medical/pathophys/id/2008/Notes.pdf
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