Transcript Backcross Breeding

Backcross Breeding
History of Backcrossing
• Harlan and Pope, 1922
• Wanted the smooth awns from European
barleys in the domestic barleys
• Crosses with European types were not
fruitful
• Decided to backcross smooth awn
• After 1 BC, progeny resembled Manchuria
and they were able to recover high
yielding smooth awn types
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
rr x RR
Rr
Single dominant gene for disease
resistance- pre flowering
• Cross F1 to Recurrent Parent to produce
BC1 progeny which are 1 Rr: 1 rr
Rr x rr
R
r
Rr
rr
R allele only present
in heterozygous form
Single dominant gene for disease
resistance- pre flowering
• Evaluate BC1s before flowering and
discard rr plants; cross Rr plants to
Recurrent Parent
Rr – keep
rr - discard
Single dominant gene for disease
resistance- pre flowering
• BC2 F1 plants evaluated, rr plants
discarded, Rr plants crossed to Recurrent
Parent
• BC2 F1 plants evaluated, rr plants
discarded, Rr plants crossed to Recurrent
Parent
• BC4 F1 plants evauated, rr plants
discarded, Rr plants selfed to produce BC4
F2 seeds, which are 1RR: 2 Rr: 1rr
Single dominant gene for disease
resistance- pre flowering
• 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 Recurrent Parent (rr) with
resistant Donor Parent (RR) - all F1s are Rr
rr x RR
Rr X rr
rr; Rr
BC1
Single dominant gene - post
flowering
• Cross F1 to Recurrent Parent to produce
BC1 progeny which are 1 Rr: 1 rr
• Because we can’t evaluate the trait before
flowering, a number of BC1F1 plants must
be crossed to Recurrent Parent, then the
trait is evaluated and susceptible plants
discarded
• This procedure is therefore less efficient
than the pre-flowering trait because we
have made crosses that we cannot use
Single dominant gene - post
flowering
• 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) Recurrent Parent to
resistant (rr) Donor Parent
• F1 plants crossed to Recurrent Parent, BC 1 seeds
are 1 RR:1Rr
• Note now that all BC1 plants are susceptible; we
are interested only in those plants which carry
the resistant “r” allele
• All BC1 plants crossed to Recurrent Parent and
selfed to provide seeds for progeny test
Single recessive allele progeny test in same season
• 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 Recurrent Parent
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) Recurrent Parent to
resistant (rr) Donor Parent
• F1 plants crossed to RP, seeds are 1
RR:1Rr
• Again, we are interested in plants carrying
the resistant “r” allele – we can’t
distinguish them yet from RR types
Single recessive allele - progeny
test in different season
• The difference is now that we cannot do
the progeny test in the same season
because the resistance is expressed late in
plant’s life.
• 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
Recurrent Parent to produce BC2F1 seeds.
Single recessive allele - progeny
test in different season
• BC2F1 plants crossed to Recurrent Parent
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.
Cytoplasmic male sterility in
Wheat
Triticum timopheevi x Elite breeding line
(Male sterile)
(Male fertile)
F1 (female) x RP (male)
Carry out for 4 BC; use male-sterile version of
elite breeding line as female parent in hybrid
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 Donor
Parent and Recurrent Parent differ by 10
loci, 26% of the plants will be homozygous
for the 10 alleles of the Recurrent Parent;
remainder will vary.
Recovery of genes from
Recurrent Parent
• Selection for the Recurrent Parent
phenotype can hasten the recovery of the
Recurrent Parent
• If the number of BC progeny is increased,
selection for Recurrent Parent can be
effective
Linkage Drag
•
When backcrossing, we often get more
than one gene from the donor parent
•
The additional genes may be
undesirable, hence the term linkage drag
•
Backcrossing provides opportunity for
recombination between the favorable gene
and the linked unfavorable genes
Linkage Drag
• Recombination fraction has a profound
impact: with c=0.5, probability that
undesirable gene will be eliminated with 5
BC is 0.98
• with c=0.02, probability that undesirable
gene will be eliminated with 5 BC is 0.11
Backcrossing for Quantitative
Characters
• Choose Donor Parent that differs greatly
from Recurrent Parent to increase the
likelihood of recovery of desired trait
(earliness for 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=0.10
Q2
Q2 is the target allele we want to backcross
into recurrent parent, which has Q1 to begin
with.
Recombination
• Assume recombination between marker
and QTL=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
Flanking Markers
Probabilityof losing the target gene after selecting
On flanking markers:
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
Marker Assisted BC