This document discusses sources of variability in plant pathogens. It explains that variability arises from processes like mutation, recombination, heterokaryosis, and parasexualism in fungi. In bacteria, the main sources of variability are conjugation, transformation, and transduction, which allow for horizontal gene transfer. Viruses also evolve new variants through mutation and recombination during replication when multiple strains infect the same host. The document provides examples of each type of variability and how it contributes to genetic changes in populations of plant pathogens over time.

2. Variability in
Plant
Pathogens
By
Kota Sunil Varma
NAM/2020-06
M.Sc.(Ag) Entomology
Course No: PL.PATH 513
Course Title: Disease Resistance in Plants

3. Variability of Plant Pathogen
 Variability: it is the property of an
organism to change its characters from
one generation to the other
 When progenies show variations in
characterstics from parents it is called as a
Variant
 With in species or f.sp. There are further
subgroups of individual that infect
different varietes of the host– such
subgroups are called Races/Strains.

4. • Dr.E.C.Stakman (U.S.Dept. Of
Agriculture) defined need and use of
variability in fungi.
• Pathogen population mutates with time
and environmental conditions like
stress.
• In nature variability in organisms may
be due to:
• Hybridization
• Mutation
Variability of Plant Pathogen

5. Variability in Fungi
Variability in Fungi can be seen due to
Following Processes:
 Heterokaryosis
 Parasexualism
 Cytoplasmic Adaptation
 Recombination
 Mutation

6. Heterokaryosis
• Hyphae or parts of hyphae contain
nuclei, which are genetically different,
generally of two different kinds. This
condition is known as heterokaryosis
.
• The phenomenon is commonly
brought about by hyphal anastomosis
between mycelia of two parental
genotypes.
• In Ascomycotina and Basidiomycotina

7. Parasexualism
• First demonstrated by Pontecorvo
(1956) in Aspergillus nidulans.
• Parasexualism is the process by
which genetic recombination can
occur with in fungal heterokaryon :
• Stage 1: Heterokaryosis
• Stage 2: Recombination

8. Parasexualism
• Dissimilar nuclei to fuse and produce
diploids known as mitotic
recombination.
• Recombination without sexual cycle
• There is no fine coordination
between plasmogamy, karyogamy and
haplodisation
• This sequence of events has been
described in the parasexual cycle

9. Recombination
• When two haploid nuclei (N)
containing different gnentic maeterial
unite to form diploid (2N) nucleus
(Zygote,) when under go meiotic
division produce new haploid .
• Recombination of genes 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

11. Mutation
 Sudden heritable change in genetic
material of an organism
 Mutation represent change in
sequences of the bases in DNA –
Either by substitution or by Deletion or
by Addition
 Mutations are Spontaneous.
Mutations occur due to:
1. Improper Cell division
2. Abnormalities during division
3. Physical Radiations
4. Mutagenic Chemicals

12. Cytoplasmic Adaptation
• Pathogens develop capicity to perform
biochemical reactions which were not
present in them earlier.
• Can utilize protoplasm of unfavourable
host.
• Adaptation to new cytoplasm

13. Types of Cytoplasmic
Adaptation
I. Pathogen may aquire tolerance to
toxic materials.
II. Utilization of new types of cytoplasm.
III. Change in virulence

14. Variability in Bacteria
Variability in Bacteria can be seen due
to Following Processes:
 Conjugation
 Transformation
 Transduction

15. Conjugation
• Transfer of DNA from one bacterial cell
to another Donor cell (F+) transfers
DNA to recipient cell (F- )
• In this two compatible bacteria come in
contact and exchange the portion of
plasmid or chromosome through
conjugation bridge or pilus

17. Transformation
 Bacterial transformation is a process of
horizontal gene transfer by which some
bacteria take up foreign genetic material
(naked DNA) from the environment.
 It was first reported in Streptococcus
pneumoniae by Griffith in 1928.
 DNA as the transforming principle was
demonstrated by Avery et al in 1944.

19. Transduction
 Transduction, a process of genetic
recombination in bacteria in which
genes from a host cell (a bacterium)
are incorporated into the genome of
a bacterial virus (bacteriophage) and
then carried to another host cell when
the bacteriophage initiates another
cycle of infection.

22. Variability in Virus
Recombination:
 May results from mixed infection of
two strains of the virus
 Occurs mostly during replication
Mutation:
 Results from nucleotide changes in
the coding regions due to addition or
deletion or replacement. – Ultimately
leads to functional changes in the
genes.

23. References
• R.S. Singh, INTRODUCTION TO
PRINCIPLES OF PLANT
PATHOLOGY, Oxford & IBH
Publishing CO. pg.200-208
• http://agriinfo.in/default.aspx?page=top
ic&su perid=3&topicid=2131
• http://www.columbia.edu/itc/hs/medica
l/pat hophys/id/2008/Notes.pdf