cowger lecture pp790 2008 refuges & pyramids
TRANSCRIPT
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Pathogen evolution: Resistance gene pyramidingand its effects on pathogens and pests
How do pathogen and insectpopulations respond to R-gene pyramids?
Are pyramids effective becauseof the low probability ofmutations to virulence atmultiple loci?
Do pyramids of defeated Rgenes work?
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Example 1: Hessian fly on wheat
An important pest ofwheat in theeastern U.S., Africa
Use of resistantcultivars canmaintain yield lossdue to Hessian flyat about 1% in U.S.
Lack/breakdown ofresistance can
cause severedamageMorocco, 36% crop
loss in wheat, 1999State of Georgia,
$28 million loss in
wheat crop, 1989
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puparia
Fly damaged plants and/or tillers
Hessian fly eggs
adult
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Host resistance to Hessian fly
A gene-for-gene system HF injects avirulence products in saliva
Resistant plants recognize avr products, initiate antibiosisagainst first instars
HF R genes considered moderately dominant (60-75%
of plants in segregating families appear unstunted) R genes usually overcome in 8-10 yrs after release
Within 3-8 yrs when R gene on >50% of wheat acreage
HF population included 16 biotypes in 1977
Classic approach: plant a cultivar with a single R gene.When its overcome, backcross an additional R gene intothe cultivar. Repeat as HF population adapts.
31 named R genes (H1-H31) by 2003 -- only 11deployed commercially. Will there be enough?
Early first instarHessian fly larva
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How about gene pyramids?
Three basic strategies for deploying resistantgermplasm could improve on classic single-genedeployment (Simulation model in Gould, 1986, EnvironmentalEntomology, 15:11-23):
Sequential release of 2 pure cvs with one R geneeach
Pyramiding both R genes in one pure cultivar
Mixtures of 50% R1, 50% R2
Durability always less than 16 gens. = 8 yrs(depending on whether virulence is assumed tobe co-dominant, fully dominant, or in-between)
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How does adding susceptible plants affect
durability of resistance? If add 10-20% susceptible plants, durability is increased
for each strategyAssume AABB is genotype of unadapted fly, and their fitness
on R plants = 0.04 on S plants In 9R:1S mixture, = 0.9(0.04) + 0.1(1.0) = 0.136
In 8R:2S mixture, = 0.8(0.04) + 0.2(1.0) = 0.232
So: AABB fitness nearly doubles when S proportion goes from10% to 20%
Durability of two-gene pyramid increases to 400 gens (200 yrs)! Adding susceptible plants lowers selective pressure
against unadapted fly genotypes (AABB) ensures most very rare aabb flies will mate with AABB
increases durability of all deployment strategies
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Example 2: Pyramiding Bttoxins:effects on pest populations?
Plants are engineered to express Bacillusthuringiensistoxins to protect againstLepidopterans:
Moar and Anilkumar, 7 Dec 2007,
The Power of the Pyramid,
Science, 318:1561-1562
-Tobacco budworm (Heliothis
virescens)
-Pink bollworm (P.gossypiella)
-Corn earworm (Helicoverpa
zea)
Transgenic insecticidalcultivars (TICs) killcaterpillars (larvae)
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Btcotton, corn and potatoes first
planted in 1996; by 2006, Btcornand cotton on 32 million ha.worldwide
Bt strains produce related toxins,
each encoded by a single genewith a single target site in insect
Bollgard II (Monsanto): Cry 1Ac,Cry2Ac
WideStrikeTM (Dow): Cry1Ac, Cry1F
Bollgard II (Monsanto): Cry 1Ac,Cry 2Ab
Bt parasporalcrystal
Cotton bollworm
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Development ofBtresistance
Plutella xylostella(diamondback moth; pestof canola, crucifers):
>200-fold resistance toCryIAb
Trichoplusia ni(cabbagelooper; pest of crucifers,
many vegetable crops) Laboratory strains of
other pests
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Risk factors for pest populations
evolving Bt resistanceGreat genetic diversity in pest populations
Sexual recombination
Constitutive production of toxinsIntense selection pressure on pest population
One target species has lower sensitivity to Bt
than another, so Cry protein concentrationsadequate to kill SS and SR in one species arebarely adequate for other species
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Bt resistance
Bt works by binding to toxin receptor (cadherin),which triggers cleavage of Bt protein
Bt-resistant insects express mutated cadherinproteins that do not bind toxins.Modified toxins can make resistant cadherin-mutated
insects susceptible again (Soberon et al, Science, 7Dec. 07)
Multiple resistance = cross resistance: one toxincan bind to several sites
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Managing Bt resistance
Low doses vs. high doses like partialresistance
Stacking / pyramidingRotation of toxins in space and timeRestrict toxin to certain tissuesOther, non-Cry toxins (e.g., Vip3A = vegetative
toxin)Refugia or mixtures of toxic and non-toxic plantsSpaceTime
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Managing Bt resistance
Low doses vs. high doses like partialresistance
Stacking / pyramidingRotation of toxins in space and timeRestrict toxin to certain tissuesOther, non-Cry toxins (e.g., Vip3A = vegetative
toxin)Refugia or mixtures of toxic and non-toxic plantsSpaceTime
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EPA requirements forBtcorn farmers:
1. Growers may plant up to 80% of theircorn acres with Btcorn. At least 20%
must be planted with non-Btcorn (refugearea)
2. Refuge area must be within, adjacentto or near the Btcornfields. it must beplaced within 1/2 mile of the Btfield.
3. If refuge are strips within a file, thestrips should be at least 4 rows.
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High dose plus refugia
Plants express enough Btprotein to killall except rare homozygous recessives(RR)
Refugia dilute out heterozygousresistant individuals (RS)
Assumption: initially, resistant RS mutantsare very rare
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High-dose plus refugia
Non-Bt cotton fieldSS SS SS
SS SS SS
SS RSSS SS
SS SS SS
Bt cotton field
RR
RR
Initial population: 99.9% SS, 0.1% RS
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h reflects dominance
h = 1.0: Bt resistance (R) is dominant (like GfG)h = 0.5: Bt resistance is additive
h = 0.1: Bt resistance is partially recessiveMany studies show this is the case
Refuge has more of an effect
h = 0.0: Bt resistance is fully recessive
Refuge is more effective the less dominant thatBt resistance is.
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Why does adding susceptible plants (refuges) slowevolution of Bt resistance? (from Gould, 1998):
Assume resistance trait has additive inheritance (h =0.5) and toxicity of TIC is high (t = 0.9)
Fitness () of insect feeding on pure TIC is
RR (homozygous resistant): 1.0
RS (heterozygote): 0.55(heterozygotes have fitness of 1- [(1-h)(t)]
SS (homozygous susceptible): 0.10
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Fitness ()
Pure TIC 1:1 Mixture (TIC andnon-toxic)
RS SS RS SS
0.55 0.10 0.775(0.5 x 0.55) + (0.5
x 1.0)
0.55(0.5 x 0.10) + (0.5
x 1.0)
5.5x more fit 1.4x more fit
Evolution of resistance is expected to be about 4x slower
in 1:1 mixture than in pure TIC (until frequency of R = 0.1)
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What does this mean?
Speed of evolution to resistance dependson selective differential between RS and
SS In real life, refuge usually 4-10%, not 50%
Plantings that minimize the differential infitness between the more and lessresistant genotypes will slow evolution ofresistance
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Bt pyramids should have different modes ofaction to minimize cross-resistance
E.g., Bollgard II cotton (released 2003):
Cry1Ac and Cry2Ab bind to different receptorsin midgut
Finding new pyramid candidates requiresknowing how insects develop resistance to
specific toxins, and then modifying thosetoxins so resistance must occur in anothermanner.
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Common ideas HF and Bt
To prolong effectiveness of available resistances/toxins Key is to reduce fitness differential between virulent and
avirulent types
Need to reduce selective presure against nonadapted strains
While maintaining economically practical levels of control Pyramids are especially effective if pyramided genes
have different modes of action (low potential for cross-resistance) (Soberon et al, Science, 7 Dec. 07)
Pyramids in combination with refuges of some kind maybe the most effective strategy at slowing evolution ofBtresistance/virulence
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Dogma: gene pyramids work because oflow probability of mutation to multiplevirulence
Probabilities hypothesis: cultivarspossessing multiple race-specific R
genes owe their durability to a lowprobability of the pathogen mutating tovirulence independently at avrlocicorresponding to those R genes.
A debate inPhytopathologyMundt, 1990, 80:221-223 (required)
Kolmer et al, 1991, 81:237-239
Mundt, 1991, 81:240-242
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Mundts critique:No correspondence between durability of spring
wheat cultivars resistance to stem rust and thenumber of R genes they possess.
Many pyramided R genes have been previouslydeployed, selecting for virulence to them. Ifpyramids including these genes are durable, it isbecause of some other factor.
It seems that certain genes are durable, e.g.,
Sr6, rather than more genes conferring greaterdurability.Maybe mutation to virulence against these
genes entails fitness costs to pathogen.
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Probabilities hypothesis requires the assumptionthat virulence mutations at different loci are
independent, yet there are several mechanismsfor attaining simultaneous changes to virulenceat different loci
A deletion of several linked avrloci
A locus that simultaneously inhibits expression ofmultiple avirulence genes
Alternative mRNA splicing: a single avr gene codesfor different products, depending on host genotype
So which genes are pyramided may be at leastas important as whetherand how many
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Is it worthwhile pyramiding resistances thatare already partially or completely defeated?
Residual resistance or ghost effects
Bacterial blight of rice (Ahmed et al, 1997, Phytopathology87:66-70.)
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Kousik and Ritchie, Phytopathology, 89:1066-
1072:6 races ofXanthomonas campestrisinoculated on 8isolines of bell pepper with three R genes in differentcombinations
Races 4 and 6 caused less disease on isolinescarrying 2 or 3 defeated major genes than on isolines
with those genes individuallyDefeated major resistance genes deployed in pyramids
were associated with lower AUDPC than when theywere deployed individually
Conclusion on pyramiding defeated genes:
some evidence that it has some effect. Whywould it work?