Transcript Backcross Breeding

Backcross Breeding
History of Backcrossing
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Harlan and Pope, 1922
Smooth vs. rough awn
Decided to backcross smooth awn
After 1 BC, progeny resembled Manchuria
Terminology
• Recurrent parent (RP) - parent you are
transferring trait to
• Donor or nonrecurrent parent (DP) source of desirable trait
• Progeny test - when trait is recessive
Single dominant gene for disease
resistance- pre flowering
• Cross recurrent parent (rr) with resistant
donor 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
Single dominant gene for disease
resistance- pre flowering
• BC2 F1 plants evaluated, rr plants
discarded, Rr plants crossed to RP
• …. BC4 F1 plants evauated, rr plants
discarded, Rr plants selfed to produce BC4
F2 seeds, which are 1RR: 2 Rr: 1rr
• BC4 F2 plants evaluated before flowering,
rr discarded, R_ selfed and harvested by
plant, then progeny tested. Segregating
rows discarded, homozygous RR rows kept
and tested.
Single dominant gene - post
flowering
• Cross susceptible RP (rr) with resistant DP (RR) all F1s are Rr
• Cross F1 to RP to produce BC1 progeny which are
1 Rr: 1 rr
• BC1F1 plants crossed to RP, trait evaluated before
harvest, susceptible plants discarded
• BC2F1 plants (1 Rr:1rr) are crossed to RP, trait
evaluated before harvest, susceptible plants
discarded
Single dominant gene - post
flowering
• 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
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 RR
progeny. BC1 F1 plants that are Rr will produce
BC1F2 progeny that segregate for resistance.
Single recessive allele progeny test in same season
• BC2 F1 plants from heterozygous (Rr) BC1
plants are crossed to RP; those from
susceptible (RR) BC1 plants are discarded
• BC2 F2 selfed seed is harvested for
progeny testing
• Progeny tests are conducted before BC3F1
plants flower. Only plants from (Rr) BC2
plants are crossed to RP
Single recessive allele progeny test in same season
• Each BC4F1 plant is progeny tested.
Progeny from susceptible BC3 plants are
all susceptible and family is discarded
• If progeny test completed before
flowering, only homozygous resistant (rr)
plants are selfed. Otherwise, all plants
selfed and only seed from (rr) plants
harvested.
• Additional testing of resistant families
required.
Single recessive allele - progeny
test 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 is
harvested. BC1F3 progeny crossed to RP to
produce BC2F1 seeds.
Single recessive allele - progeny
test in different season
• BC2F1 plants crossed to RP to obtain BC3F1
seeds 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 to
produce BC4F1 seeds
Single recessive allele - progeny
test in different season
• BC4 F1 plants selfed and produce
1RR:2Rr:1rr progeny
• BC4F2 plants selfed and resistant ones
harvested by plant
• Resistant families tested further
Importance of cytoplasm
• For certain traits (e.g. male sterility) it is
important that a certain cytoplasm be
retained
• In wheat, to convert a line to a male
sterile version the first cross should be
made as follows: Triticum timopheevi
(male sterile) x male fertile wheat line.
From that point on, the recurrent parent
should always be used as the male.
Probability of transferring
genes
• How many backcross progeny should be
evaluated?
• Consult table in Fehr, p. 367; for example
in backcrossing a recessive gene, to have
a 95% probability of recovering at least 1
Rr plant, you need to grow 5 backcross
progeny.
Probability of transferring
genes
• To increase the probability to 99% and the
number of Rr plants to 3, you must grow
14 progeny
• If germination is only 80%, you must
grow 14/0.8 = 18 progeny
Recovery of genes from RP
• Ave. recovery of RP = 1-(1/2)n+1, where n
is 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 be
homozygous for the 10 alleles of the RP;
remainder will vary.
Recovery of genes from RP
• Selection for the RP phenotype can hasten
the recovery of the RP
• If the number of BC progeny is increased,
selection for RP can be effective
Linkage Drag
• Backcrossing provides opportunity for
recombination between the favorable
gene(s) from the RP and the unfavorable
genes that may be linked
• Recombination fraction has a profound
impact: with c=0.5, P(undesirable gene
will be eliminated) with 5 BC is 0.98
• with c=0.02, P(undesirable gene will be
eliminated) with 5 BC is 0.11
Backcrossing for Quantitative
Characters
• Choose DPs that differ greatly from RP to
increase the likelihood of recovery of
desired trait (earliness example)
• Effect of environment on expression of
trait can be a problem in BC quantitative
traits
Backcrossing for Quantitative
Characters
• Consider selfing after each BC
• Expression of differences among plants
will be greater
• May be possible to practice selection
• Single plant progeny test will not be
worthwhile; must use replicated plots
Other Considerations
• Marker assisted backcrossing
• Assume that you have a saturated genetic
map
• Make cross and backcross
• To hasten the backcrossing process, select
against the donor genotype (except for
the marker(s) linked to the gene of
interest) in backcross progeny
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
Model
Two alleles at marker locus M1 and M2
Two alleles at target gene, Q1 and Q2
M1
M2
Q1
r
Q2
Q2 is the target allele we want to backcross
into recurrent parent, which has Q1 to begin
with.
Gametes produced by an F1 heterozygous at
both QTL and marker locus.
Gamete
M1
M1
M2
M2
Frequency
Q1
1/2(1-r)
Q2 1/2( r )
Q1
Q2
1/2( r )
1/2(1-r)
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)
Recombination
• P(Q1Q1|M1M2)=r
• Assume r=10%
• Select one plant based on marker
genotype alone, 10% chance of losing
target gene
• Probability of not losing gene=(1-r)
• For t generations, P=1-( 1-r )t
• For 5 BC generations, probability of losing
the target gene is P=1-(.9)5=0.41
Flanking Markers
• Best
way to avoid losing the target gene
is to have marker loci flanking it
MA1
MA2
rA
Q1
Q1
rB
MB1
MB2
BC1F1 genotype frequencies using marker loci
Flanking the target gene
Genotype
MA1MA1Q1Q1MB1MB1
MA1MA1Q1Q2MB1MB1
MA1MA2Q1Q1MB1MB1
MA1MA1Q1Q2MB1MB1
MA1MA1Q1Q1MB1MB2
MA1MA1Q1Q2MB1MB2
MA1MA2Q1Q1MB1MB2
MA1MA2Q1Q2MB1MB2
Total
Frequency
1/2(1-rA)(1-rB)
1/2rArB
1/2rA(1-rB)
1/2(1-rA)rB
1/2(1-rA)rB
1/2rA(1-rB)
1/2rArB
1/2(1-rA)(1-rB)
1
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
Other Considerations
• Backcross breeding is viewed as a
conservative approach
• The goal is to improve an existing cultivar
• Meanwhile, the competition moves past
Backcross Populations
• May be used as breeding populations
instead of F2, for example
• Studies have shown that the variance in a
backcross population can exceed that of
an F2
• Many breeders use 3-way crosses, which
are similar to backcrosses