This document discusses the concept and mechanisms of pathogen variability. It explains that pathogens can vary at the genus, species, variety/forma specialis, race, and variant levels. Variability can occur through mutation, mixed inoculation, conjugation, transformation, transduction, recombination, heterokaryosis, parasexuality, heteroploidy, cytoplasmic adaptation, and hybridization in viruses, bacteria, and fungi. The document provides examples of each mechanism and concludes by emphasizing that further research is still needed to fully understand even well-known plant diseases.

1. PATHOGEN
VARIABILITY,CONCEPT AND
MECHANISM
Submitted To- Submitted By-
Dr. Sandeep Jain Arvind M (L-2016-A-124-M)
Shridhar Hiremath (L-2016-A-130-M)
Wazir Ahmad (L-2015-A-340-M)

3. What is Pathogen Variability
• The property or ability of an organism to change its characteristics from
one generation to other is called Variability(Agrios)
• Individuals have different characteristics, not fixed i.e. phenomenon of
Variation
• Individuals produced by sexual process including Sexual spores,higher
Parasitic plants, Nematodes,Bacteria,Cultivated Plants differ among
themselves and from their parents
• Low in asexually reproducing ones, however, some have much higher
variability e.g C. lindemuthianum

4. Stages of variation in pathogens
• Genus- A group of species having common characters
• Species- Organisms with common morphological and
phenotypical characters
• Variety or forma specialis(f.sp.)- Individuals attacking only one
crop e.g only wheat or barley or oats
• Race- Individuals infecting only some varieties of host
• Variant- Offspring of race attacking a new variety
• Biotype- The identical individuals produced asexually by the
variant

6. Variability in Virus
• Mutation
• Mixed Inoculation
a)Pseudo-recombination
b)Heterologous Encapsidation or Genome Masking

7. Mutation
• Sudden, Large, hereditable variations
• Represents changes in sequences of nucleotide bases
due to Deletion, Substitution and Addition
• When a virus maintained continuously on same susceptible host is
passed on through resistant cultivars of host ,it induces mutation
• Virus having different characters from original are known as virus
mutants

8. Mixed Inoculations
Presence of many strains or genetic material in the same environment
may lead to following consequences-:
a) Pseudo-recombination: Fragments of genome aggregate , re-
assemble forming new type of progeny in viruses with split genome.
b) Heterologous encapsidation or genome masking : Direct
interaction between viruses in mixed inoculation in which genome of
one virus is encapsidated in protein of other strain of virus.

10. Variability in Bacteria
• Conjugation
• Transformation
• Transduction

11. Conjugation
• Transfer of DNA from one bacterial cell to other bacterial cell(F+ or
Hfr F-)

12. Transformation
• The process in which bacterial cell absorbs genetic material from
external environment,exuded by a compatible cell or freed by lysis of
cell wall

13. Griffith Experiment(1928)

14. Transduction
• Transfer of bacterial gene with a bacteriophage

16. Variability in Fungi
• Mutation
• Recombination
• Heterokaryosis
• Parasexualism
• Heteroploidy
• Cytoplasmic adaptation
• Hybridisation

17. Recombination
• Most changes in the characteristics of pathogens are the result of
recombination occurs during sexual processes.
• When two haploid nuclei (1N) containing different genetic material
unite to form diploid (2N) nucleus called a Zygote, when under go
meiotic division produce new haploid . Recombination of genetic
factor occurs during meiotic division of zygote as a result of cross
over in which part of chromatid of one chromosome of a pair are
expressed with that of the other
• Recombination can also occur during mitotic division of cell in the
course of growth of the individual and is important in fungi
Puccinia graminis

18. Heterokaryosis
• Hyphal anastomosis between mycelia of two parental genotypes.
• Heterokaryons can arise in following ways
a) Mutation in any bi or multi-nucleate cell
b) Formation of bi-nucleate sexual spores
• Reported in Colletotrichum lagenarium, Helminthosporium
sativaum, Alternaria solani, Fusarium solani, Fusarium oxysporum,
Rhizoctonia solani, Verticillium albo-atrum, Phytophthora infestans,
Botrytis cinerea

19. Parasexuality
• Process by which genetic recombination can occur within fungal
heterokaryons
• Occasional fusion of two nuclei and formation of diploid nucleus
results in parasexuality
• During multiplication,crossing over occurs in a few mitotic divisions
and results in appearance of genetic recombinants
• Steps in parasexual cycle-:
a) Heterokaryosis
b)Fusion of unlike nucleus to form heterozygotes
c) Segregation and recombination at mitosis

20. First demonstrated by Pontecorvo (1956) in
Aspergillus nidulans.

22. Heteroploidy
• Existence of cell,tissues or whole organism with chromosome
number different from 1N or 2N complement for the particular
organism
• Heteroploids may be haploids, diploids, triploids, tetraploids or
aneuploids
For eg. Verticilium albo-atrum, cause of wilt in cotton,lose ability to
infect cotton plant even when derived from highly virulent haploids

23. Cytoplasmic Adaptation
• Capacity of pathogen to perform biochemical reactions not present
earlier, by utilising protoplasm of unfavourable host
• Types of adaptability
a) Pathogen acquire tolerance to toxic materials
b) Utilisation of new type of cytoplasm
c) Change in virulence
• Common in all the pathogens except viruses and viroids

24. Hybridization
• Mating of different strains, varieties or species results in new
recombination of genes
• New biotypes or races are produced by combination of two haploid
nuclei to form a zygote
• Transfer of genetic material to the progeny takes place when diploid
nucleus undergoes meiosis

27. References
• Agrios G N (1970) Plant Pathology Pp: 128-30 Academic press New York.5
th edition
• Singh R S () Introduction to The Principles of Plant Pathology. Oxford and I
BH publishing company.
• Alice D and Jeyalakshmi (2012) Plant Pathology Pp: 58-60 A. E.
Publications, Coimbatore.
• Rossinck M J (1997) Mechanisms of plant virus evolution. Annu. Revi. Of
Phytopathol 35: 191-09.
• Rajas S E, Dixon P M, Batzer J C and Gleason M L (2013) Genetic and
Virulence Variability Among Erwinia trachiephila strains Recovered
from different Cucurbit Hosts. Phytopathology 9: 900-05.

28. • Buxton E W (1956) Heterokaryosis and Parasexuality Recombination in
Pathogenic Strains of Fusarium oxysporam. J. gen. Microbiol 15: 133-
39.
• Manardo F, Praz R, Wyder S and Bourras (2015) Hybridization of powdery
mildew strains gives rise to pathogens on novel agricultural crop
species. Nature genetics. 48: 201-05.
• Brasier C M, Cooke D E L and Duncan J M (1999) Origin of a new
phytopthora pathogen through interspecific hybridization. Pop Biol 96:
5878-83.

29. "There remain many important problems to be solved, and there is much to be learned
about even the best-known diseases. May the oncoming years be fruitful ones and each
one of you have some share in the joy of the harvest!“
Erwin Frink Smith 1929
"