Strategies for Major GenesOne gene at a timeGene rotationGene “pyramids”MixturesRegional 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 materialcan 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 Number of QTL50 3 100 4 150 5 200 5.5 300 6 409 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
ResistanceComponents 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
VIDAL L., INRA
Stripe Rust Stripe Rust Puccinia striiformisPuccinia striiformis
Use of Top Crosses to Maintain Minor Genes
for Stripe Rust Resistance While Improving Other
Traits
Stuff Happens
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Effects of Interplot Interference (uredia/tiller) Isolated Adjacent t
Cultivar 3x4 m 2x2 m 1/4x1 mL98 1000 500 2300Sultan 750 250 1700Volla 110 40 700Julia 17 12 450Vada 1 15 130Range 1000X 42X 18X
Test with Single Race or Multiple Races?
No. races No. of to which lesions Cultivar resistant per cm2 Zenith 64 2Kataktara 55 14Kano 51 40 39Lacrosse 19 66KTH 4 94
Vertifolia Effect
Percent of Willamette Valley Winter Wheat Acreage Planted to ‘Foote’
Year Percent2001 222002 502003 502004 512005 322006 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 GenesOne gene at a timeGene rotationGene “pyramids”MixturesRegional deployment
Working With Minor Genes
Single Gene at a Time“Use It Till You Lose it”
• May be needed as a stop-gap 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
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