Cowger Lecture PP790 2008 Refuges & Pyramids

7/31/2019 Cowger Lecture PP790 2008 Refuges & Pyramids

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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?

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Cowger Lecture PP790 2008 Refuges & Pyramids