backcrossing breeding

7/30/2019 Backcrossing Breeding

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Backcross Breeding


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History of Backcrossing

Harlan and Pope, 1922

Smooth vs. rough awn

Decided to backcross smooth awn

After 1 BC, progeny resembled Manchuria


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Terminology

Recurrent parent (RP) - parent you aretransferring trait to

Donor or nonrecurrent parent (DP) -

source of desirable trait Progeny test - when trait is recessive


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Single dominant gene for diseaseresistance- pre flowering

Cross recurrent parent (rr) with resistantdonor parent (RR) - all F1s are Rr

Cross F1 to RP to produce BC1 progeny

which are 1 Rr: 1 rr Evaluate BC1s before flowering and

discard rr plants; cross Rr plants to RP


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Single dominant gene for diseaseresistance- pre flowering

BC2 F1 plants evaluated, rr plantsdiscarded, Rr plants crossed to RP

. BC4 F1 plants evauated, rr plants

discarded, Rr plants selfed to produce BC4F2 seeds, which are 1RR: 2 Rr: 1rr

BC4 F2 plants evaluated before flowering,rr discarded, R_ selfed and harvested by

plant, then progeny tested. Segregatingrows discarded, homozygous RR rows keptand tested.


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Single dominant gene - postflowering

Cross susceptible RP (rr) with resistant DP (RR) -all F1s are Rr

Cross F1 to RP to produce BC1 progeny which are1 Rr: 1 rr

BC1F1 plants crossed to RP, trait evaluated beforeharvest, susceptible plants discarded

BC2F1 plants (1 Rr:1rr) are crossed to RP, traitevaluated before harvest, susceptible plants

discarded


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Single dominant gene - postflowering

Procedure followed through BC4 Seeds from each BC4 F2 individual are

harvested by plant and planted in rows

Segregating rows are discarded,homozygous RR rows are maintained,harvested and tested further


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Single recessive allele -progeny test in same season

Cross susceptible (RR) RP to resistant (rr) DP

F1 plants crossed to RP, BC 1 seeds are 1 RR:1Rr

All BC1 plants crossed to RP and selfed to provide

seeds for progeny test Screen BC1F2 plants before BC2F1 plants flower.

BC1 F1 plants that are RR will have only RRprogeny. BC1 F1 plants that are Rr will produceBC

1

F2

progeny that segregate for resistance.


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BC2 F1 plants from heterozygous (Rr) BC1plants are crossed to RP; those fromsusceptible (RR) BC1 plants are discarded

BC2 F2 selfed seed is harvested forprogeny testing

Progeny tests are conducted before BC3F1plants flower. Only plants from (Rr) BC

2

plants are crossed to RP


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Each BC4F1 plant is progeny tested.Progeny from susceptible BC3 plants areall susceptible and family is discarded

If progeny test completed beforeflowering, only homozygous resistant (rr)plants are selfed. Otherwise, all plantsselfed and only seed from (rr) plantsharvested.

Additional testing of resistant familiesrequired.


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Single recessive allele - progenytest in different season

Cross susceptible (RR) RP to resistant (rr)DP

F1 plants crossed to RP, seeds are 1

RR:1Rr BC1 plants selfed, seed harvested by plant

BC1F2 plants grown in progeny rows,

evaluated, seed from resistant (rr) rows isharvested. BC1F3 progeny crossed to RP toproduce BC2F1 seeds.


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BC2F1 plants crossed to RP to obtain BC3F1seeds which are 1Rr: 1 RR

BC3F1 plants are selfed, and progeny are

planted in rows BC3F2 seeds are harvested from resistant

(rr) progeny rows

Resistant BC3F3 plants crossed to RP toproduce BC4F1 seeds


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BC4 F1 plants selfed and produce1RR:2Rr:1rr progeny

BC4F2 plants selfed and resistant onesharvested by plant

Resistant families tested further


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Importance of cytoplasm

For certain traits (e.g. male sterility) it isimportant that a certain cytoplasm beretained

In wheat, to convert a line to a malesterile version the first cross should bemade as follows: Triticum timopheevi(male sterile) x male fertile wheat line.From that point on, the recurrent parentshould always be used as the male.


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Probability of transferringgenes

How many backcross progeny should beevaluated?

Consult table in Fehr, p. 367; for example

in backcrossing a recessive gene, to havea 95% probability of recovering at least 1Rr plant, you need to grow 5 backcrossprogeny.


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Probability of transferringgenes

To increase the probability to 99% and thenumber of Rr plants to 3, you must grow14 progeny

If germination is only 80%, you mustgrow 14/0.8 = 18 progeny


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Recovery of genes from RP

Ave. recovery of RP = 1-(1/2)n+1, where nis the number of backcrosses to RP

The percentage recovery of RP varies

among the backcross progeny For example, in the BC3, if the DP and RP

differ by 10 loci, 26% of the plants will behomozygous for the 10 alleles of the RP;remainder will vary.


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Recovery of genes from RP

Selection for the RP phenotype can hastenthe recovery of the RP

If the number of BC progeny is increased,

selection for RP can be effective


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Linkage Drag

Backcrossing provides opportunity forrecombination between the favorablegene(s) from the RP and the unfavorable

genes that may be linked Recombination fraction has a profound

impact: with c=0.5, P(undesirable genewill be eliminated) with 5 BC is 0.98

with c=0.02, P(undesirable gene will beeliminated) with 5 BC is 0.11


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Backcrossing for QuantitativeCharacters

Choose DPs that differ greatly from RP toincrease the likelihood of recovery ofdesired trait (earliness example)

Effect of environment on expression oftrait can be a problem in BC quantitativetraits


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Backcrossing for QuantitativeCharacters

Consider selfing after each BC

Expression of differences among plantswill be greater

May be possible to practice selection

Single plant progeny test will not beworthwhile; must use replicated plots


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Other Considerations

Marker assisted backcrossing

Assume that you have a saturated geneticmap

Make cross and backcross

To hasten the backcrossing process, selectagainst the donor genotype (except for

the marker(s) linked to the gene ofinterest) in backcross progeny


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Marker-Assisted Backcrossing

May improve efficiency in three ways:

1) If phenotyping is difficult

2) Markers can be used to select against the

donor parent in the region outside the target

3) Markers can be used to select rare progeny

that result from recombinations near the

target gene


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Model

Two alleles at marker locus M1 and M2

Two alleles at target gene, Q1 and Q2

M1 Q1

M2 Q2r

Q2 is the target allele we want to backcross

into recurrent parent, which has Q1 to begin

with.


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Gametes produced by an F1 heterozygous at

both QTL and marker locus.

Gamete Frequency

M1 Q1 1/2(1-r)

M1 Q2 1/2( r )

M2 Q1 1/2( r )

M2 Q2 1/2(1-r)


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BC1F1 Genotype frequencies for a marker locus

linked to a target gene.

Genotype Frequency

M1M1Q1Q1 1/2(1-r)

M1M1Q1Q2 1/2( r )

M1M2Q1Q1 1/2( r )

M1M2Q2Q2 1/2(1-r)


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Recombination

P(Q1Q1|M1M2)=r

Assume r=10%

Select one plant based on marker

genotype alone, 10% chance of losingtarget gene

Probability of not losing gene=(1-r)

For t generations, P=1-( 1-r )t

For 5 BC generations, probability of losingthe target gene is P=1-(.9)5=0.41


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Flanking Markers

Best way to avoid losing the target gene

is to have marker loci flanking it

MA1 rA Q1 rB MB1

MA2 Q1 MB2


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BC1F1 genotype frequencies using marker loci

Flanking the target gene

Genotype Frequency

MA1MA1Q1Q1MB1MB1 1/2(1-rA)(1-rB)

MA1MA1Q1Q2MB1MB1 1/2rArB

MA1MA2Q1Q1MB1MB1 1/2rA(1-rB)

MA1MA1Q1Q2MB1MB1 1/2(1-rA)rB

MA1MA1Q1Q1MB1MB2 1/2(1-rA)rB

MA1MA1Q1Q2MB1MB2 1/2rA(1-rB)MA1MA2Q1Q1MB1MB2 1/2rArB

MA1MA2Q1Q2MB1MB2 1/2(1-rA)(1-rB)

Total 1


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Flanking Markers

Probabilityof losing the target gene after selecting

On flanking markers:

P(MA1MA2Q1Q1MB1MB2|MA1MA2MB1MB2)

Example: If the flanking markers have 10% recombination

Frequency with the target gene:, the probability of losing

The gene after 1 generation is P=0.024. The probability

Of losing the gene after 5 generations is P=0.1182


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Other Considerations

Backcross breeding is viewed as aconservative approach

The goal is to improve an existing cultivar

Meanwhile, the competition moves past


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Backcross Populations

May be used as breeding populationsinstead of F2, for example

Studies have shown that the variance in a

backcross population can exceed that ofan F2

Many breeders use 3-way crosses, whichare similar to backcrosses

backcrossing breeding

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