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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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Single recessive allele -progeny test in same season
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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Single recessive allele -progeny test in same season
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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Single recessive allele - progenytest in different season
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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Single recessive allele - progenytest in different season
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