Breeding for Resistance

1
Breeding for Resistance

• Control important crop diseases and pests
• Must have genetic variability
• Consider both genetic variability in the plant
  and in the pest
• Sources
• Fehr, ch. 21
• Slusarenko, Fraser, and van Loon, 2000

2
Types of Genetic Resistance

• Qualitative Resistance
• Distinct classes of resistance and susceptible
  plants
• Controlled by one or a few genes
• Also called Vertical resistance
• Quantitative Resistance
• Continuous variation among genotypes
• Many loci
• Also called Horizontal resistance

3
Qualitative Resistance

• Controlled by major genes (one or two)
• Genes are readily transferred from one genotype
  to another
• Presence of genes can be determined by exposing
  plants to particular races

4
Qualitative Resistance

• Advantages
• Easier screenings
• Easier transfer of genes all or nothing
• Disadvantages
• Vulnerability to new races
• Continuous use of particular cultivar may lead to
  development of new race or shift in pest
  population (shift from race 1 to race 2)

5
Qualitative Resistance
Examples Leaf Rust Resistance in Wheat
6
Quantitative Resistance

• Controlled by many genes each with minor effects
• Advantages
• Can control a broad range of races due to the
  fact that many loci are involved
• Disadvantages
• Difficult to transfer resistance from one
  genotype to another
• Individual genes harder to identify

7
Quantitative Resistance
Examples Fusarium Head Blight in Wheat
8
Sources of Resistance
Primary Gene Pool Other breeding
programs Landraces Germplasm collections
(GRIN) Wild Relatives Tomato Lycopersicon
genus Wheat Agropyron genus Aegilops
tauschii, Triticum monococum Mutant
Transformations powdery mildew in Barley
9
Vertical vs Horizontal Resistance
Resistance
Susceptibility
Vertical Resistance to Races 2, 5, and 6
10
Vertical vs Horizontal Resistance
Resistance
Susceptibility
Horizontal Resistance to all Races
11
Hypersensitive Response

• Stem Rust of Wheat
• Programmed cell death
• A lot of black boxes
• Pathogen recognition, signaling proteins,
  transcription factors,

12
Gene-for-Gene Hypothesis
Proposed by Flor (1956)-study on flax rust For
each resistance gene in the plant there is a gene
in the pathogen that determines if the pathogen
will be able to injure the plant.
13
(No Transcript)
14
Gene combinations and Disease Reaction Types in
the Gene for Gene concept
Virulence or avirulence genes in pathogen R (resistant, dominant) R (susceptible, recessive)
A dominant AR (-) Ar ()
a recessive aR () Ar ()
15
Complementary Interaction of Two Host Genes For
Resistance and the Corresponding Two Pathogen
Genes for Virulence
Resistance R or susceptibility r genes in the plant Resistance R or susceptibility r genes in the plant Resistance R or susceptibility r genes in the plant Resistance R or susceptibility r genes in the plant
R1R2 R1r2 r1R2 r1r2
A1A2 - - -
A1a2 - -
a1A2 - -
a1a2
Virulence a or Avirulence A genes In
the pathogen
16
Races of Pests

• Genetic variability within the pathogen
  population
• Different isolates cause different responses
  among host genotypes
• Example Soybean Cyst Nematode Races

17
Minimizing Changes in Races

• Alternation of Resistant and Susceptible
  Cultivars (soybean cyst nematode)
• Mixtures of Resistant and Susceptible Genotypes
• Prevention of New Races

18
Resistance vs Tolerance
Resistance no establishment of the pathogen
in/on the host, or a limited establishment Tolera
nce the host develops, continues to grow, and
produces well despite the pathogens presence
19
Breeding for Specific Resistance

• Individual Major Genes
• Multilines
• Pyramiding

20
Breeding for Specific Resistance

• Individual Major Genes

• Breed cultivars with major genes that control the
  prevalent pest races
• Select progeny from a segregating population or
• Transfer major genes from other sources
• Example Black Shank Tobacco Varieties

• Advantages and Disadvantages

21
Breeding for Specific Resistance

• Multilines

• Develop many individual lines each with
  individual major resistance genes and then
• Mix the seed of these lines together to get
• Protection against a broad spectrum of races
• Example wheat cultivar Miramer 63

• Advantages and Disadvantages

22
Breeding for Specific Resistance

• Pyramiding

• Put all known major genes into one line
• Advantages and Disadvantages
• Example Leaf Rust in Wheat

23
Comparative structural analysis of the Lr10
homologous regions from diploid wheat, rice 5
(japonica), indica rice and rice 1 (japonica).
Lr10
24
QTL and Resistance

• Quantitative Trait Loci
• Identify QTL that control variation in resistance
  observed in genotypes
• Use markers to screen genotypes (throw out the
  susceptibles)
• Applications in Marker Assisted Selection (MAS)

25
QTL and Resistance
Wheat chromosome 1A Leaf rust gene Lr10
Source J.C. Nelson et al. 1997 Crop Science
371928-1935
26
QTL for Fusarium Head Blight Resistance in Wheat
Ning 7840/Clark population 11 AFLP markers
associated with resistance 1 marker explained 53
of the variation
Bai, Kolb, Shaner, and Domier. Phytopathology
89343-348.
Sumai 3/Stoa population -- used RFLP markers 5
genomic regions were associated with resistance
(3 from Sumai 3, 2 from Stoa)
Waldron, Moreno-Sevilla, Anderson, Stack,
Frohberg. Crop Sci 39805-811.
27
Example of Quantitative Genetics

• 3 F23 populations, 40 families, 3 reps, 13
  individual observations per rep
• Separated variance into two parts
• Among families
• Within families
• Narrow sense heritabilities were estimated

28
ANOVA
Source df MS EMS
Among Families 39 5393.49
Within Families 1496 965.72
29
Example from Population 2
30
Genetic Parameters and h2 of FHB Severity
Pop 1 Pop 2 Pop 3
73.77 64.69 118.17
89.43 195.35 0.00
0.12 0.06 0.12
0.64 0.39 0.69
12.14 8.88 15.89