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Strategies for Major Genes
One gene at a time
Gene rotation
Gene “pyramids”
Mixtures
Regional deployment
Working With Minor Genes
Minor Gene Resistance
• Often multiple gene inheritance
• Often recessive
• Usually gives incomplete
resistance
• Can be more complicated to
work with
• Not highly specific
• Tends to be stable over time
Minor Gene Resistance
• Often more available than
we think
• Simply purging the most
susceptible material
can be very useful
• Inheritance is much less
complex than once thought
Estimating Number of
Genes and Hertitability
• Statistical, quantitative
genetic approaches
• QTL approaches
• Both give same answer
Minor Gene Resistance
Commonly only 2-4 genes identified
Often 1 or 2 that contribute
majority of effect
Heritability usually moderate to
high
Estimating Number of
Genes and Hertitability
• Statistical, quantitative
genetic approaches
• QTL approaches
• Both underestimate gene
number
• Both depend on population
size
Barley Stripe Rust
Pop. Size
50
100
150
200
300
409
Number of QTL
3
4
5
5.5
6
8
Barley Stripe Rust
• Number of QTL increased
with population size
• ID QTL with biggest effect
with small population size
• ID additional QTL with
smaller effects with larger
population size
Components of Resistance
Length of latent period
Infection efficiency
Lesion size
Sporulation rate
Length of infectious period
Selection Based on
Components of Resistance
Components tend to be highly
correlated
More recent evidence suggests
that components are
pleiotropically controlled
Example of barley stripe rust
Can be relatively easy to
maintain minor gene
resistance once you
have it
Stripe Rust
Puccinia striiformis
VIDAL L., INRA
Use of Top Crosses to
Maintain Minor Genes
for Stripe Rust
Resistance While
Improving Other Traits
Stuff Happens
XXXX
Effects of Interplot
Interference (uredia/tiller)
Isolated
Adjacent
Cultivar 3x4 m 2x2 m 1/4x1 m
L98
1000
500
2300
Sultan
750
250
1700
Volla
110
40
700
Julia
17
12
450
Vada
1
15
130
Range
1000X
42X
18X
t
Test with Single Race or
Multiple Races?
No. races No. of
to which lesions
Cultivar
resistant per cm2
Zenith
64
2
Kataktara
55
14
Kano 51
40
39
Lacrosse
19
66
KTH
4
94
Vertifolia Effect
Percent of Willamette Valley Winter
Wheat Acreage Planted to ‘Foote’
Year
2001
2002
2003
2004
2005
2006
Percent
22
50
50
51
32
4
Lessons:
The Vertifolia Effect can be
real
“Perfect Storms” do happen
QTL Analysis and MAS
Source: CAST
QTL Analysis and MAS
Molecular Breeding
• Biology and engineering are very
different
• High-quality phenotyping is still
the most important ingredient of a
resistance breeding program, and
this likely will always be so
Strategies for Major Genes
One gene at a time
Gene rotation
Gene “pyramids”
Mixtures
Regional deployment
Working With Minor Genes
Single Gene at a Time
“Use It Till You Lose it”
• May be needed as a stopgap measure
• In general, don’t go there
- Puts growers at risk
- Disruptive to breeding
programs
Rotating Genes
• Likely a disaster
• Virulence unlikely to
decline to previous levels
Pyramids or Combinations
of Major Genes
• Useful in increasing durability
of major genes
• More complex than using
single genes
• Takes time to determine
which combinations work
best
Mixtures or Blends of
Major Genes
• Can be highly effective
• Should be considered more
often
• Results variable, empirical
testing critical
Regional Deployment
• Likely very useful if done
• Is it feasible?
Minor Gene Resistance
• It’s hard to go wrong with
this approach
• May take a bit more effort
up front
• Likely to eliminate a lot of
headaches in the longer
term
Tolerance
Don’t Forget About IPM
Potato late blight severity:
Susceptible, no fungicide = 100%
Weekly fungicide =
56%
Minor gene resistance =
46%
Resistance + fungicide =
9%
Stuff Happens
XXXX