Plant Breeding Methods

Additional new reference:George Acquaah. 2006. Principles of Plant Genetics and Breeding. Blackwell Publishing.ISBN: 9781405136464http://www.blackwellpublishing.com

•Plant Breeding Methods

• Mass Selection

• Bulk Breeding Method

• Single-seed Descent

• Pedigree Breeding

• Cross-pollinated Methods

•High lysine rice

Mass Selection

•Selection of individuals

•Sampling seed of selected individuals to plant next generation

•Oldest method of crop improvement

•Improvement of heterogeneous native populations or landraces

Mass Selection Higher percentage of desirable genotypes

Method can only be used in environments where trait is expressed - may not be suitable for off-season winter nurseries

Effectiveness is function of heritability

Manage field to enhance differences: eg. irrigate excessively to increase disease pressure

• Grow population• Allow random mating

• Harvest and bulk seed from desirable plants

• Plant new generation• Repeat

X X X X X X X X X X X X X X X X X XX X X X X X

X X X X X X X X X X X X X X X X X XX X X X X X

Figure 3.12

MASS SELECTION

(Source: Tinker, 2008)

Bulk Breeding Method (Source: Acquaah, 2006)

Bulk Breeding Method

• Procedure for inbreeding a segregating population until the desired level of homozygosity is reached

• Easy way to maintain populations

• Natural selection permitted to occur in target environment

Single Seed Descent MethodAction

Grow F1 plants, harvest allF2 seeds per plant

Grow F2 population, harvestone seed per plantGrow F3 population, harvestone seed per plantGrow F4 population, harvestone seed per plant

Space-plant to grow F5, select best single plantsGrow F5-derived plant rowsIn the F6 generation (F5:6)Yield Test in F7 (F5:7 rows)

Yield Test in F8 (F5:8 rows)Yield Test in F9 (F5:9 rows)Large-scale seed increase for variety release

(Source: Acquaah, 2006)

Single-seed Descent Method

• Easy way to maintain and inbreed (very little record keeping)

• All genotypes are sampled – useful for random genetic studies– Natural selection does not influence

population

• Well suited to off-season winter nurseries

Pedigree Breeding Method (Source: Acquaah, 2006)

Additional Methods:

(Primarily, but not exclusively, suited to Cross Pollinators

Recurrent Selection (Source: Acquaah, 2006)

Recurrent Selection

• Families created–Parents crossed in all possible

combinations• Families and plants/families evaluated• New set of superior parents selected• Inter-mated in all possible combinations,

forming next generation cycle -- improved

Ear-to-row selection, generalized scheme(Source: Acquaah, 2006)

Full-sib breeding method (Source: Acquaah, 2006)

Selfed-progeny Performance Breeding Method(Source: Acquaah, 2006)

Half-sib Selection, with a Progeny Test(Source: Acquaah, 2006)

Half-sib Selection, with a Testcross

Allows most suitable testcross

(Source: Acquaah, 2006)

Backcross Breeding

X

Single Gene Transfer :Linkage Drag with Traditional Backcross Breeding

Donor variety

ResistanceGene

CommercialVariety

New Variety

Single Gene Transfer :Genetic Engineering is a form of Backcrossing

Donor species

ResistanceGene

TransformablePlant

Transformed Plant

geneinsertion

Single Gene Transfer :Genetic Engineering is a form of Backcrossing

TransformablePlant

Transformed Plant – typically poor agronomic type

geneinsertion

Thus, backcross to commercial variety via traditional backcross breeding procedure

(A = recurrent parent, B = non-recurrent, donor parent) step 1: cross (A x B) F1 (50% recurrent parent) [50% of genome from A plus 50% of unrelated genome from B] step 2: backcross (A x F1)BC1F1 (75% recurrent parent)[50% of genome from A plus 50% of genome from F1, which itself is 50% A]therefore [50% + 50%(50%)] = 75% A genome

step 3: backcross (A x BC1F1)BC2F1 (87.5% recurrent parent)[50% of genome from A plus 50% of genome from F1, which itself is 75% A]therefore [50% + 50%(75%)] = 87.5% A genome

Backcross Breeding Method

step 4: backcross (A x BC2F1)BC3F1 (93.75% recurrent parent)[50% of genome from A plus 50% of genome from F1, which itself is 87.5% A]therefore [50% + 50%(87.5%)] = 93.75% A genome  step 5: backcross (A x BC3F1)BC4F1 (96.875% recurrent parent)[50% of genome from A plus 50% of genome from F1, which itself is 93.75% A]therefore [50% + 50%(93.75%)] = 96.875% A genome

General equation for average recovery of the recurrent parent: 

1 - (½) n+1

where, n is the number of backcrosses to the recurrent parent.

for the F1, n= 0;

for BC1, n=1;

for the BC2, n=2;

for the BC3, n=3, etc.

• "backcrossing" refers to repeated crossing of hybrid progeny back to one parent.

•  Usually associated with improving cultivar of self-pollinated species or an inbred of a cross-pollinating species for trait governed by single gene.

•  provides precise way of improving a cultivar that excels in a large number of attributes, but is deficient in one characteristic.

•  provides gains of predictable value from improved trait.      

1. Single dominant gene, character evaluated on single plant basis preflowering, :

Example: transfer of dominant gene (RR) conferring resistance to Roundup herbicide and wish to recover the entire genome of a superior agronomic recurrent parent, adding the RR gene. Season 1: rr x RR F1 seeds (Rr)

(50% recurrent parent) Season 2: Grow the F1 plants (Rr) and spray with Roundup, pre-flowering. The plants that survive confirm that true hybrids, rather than selfs were formed and that the transfer of the donor gene was successful. Backcross those F1 hybrids (Rr) to the elite recurrent parent (rr) to recover superior agronomic recurrent parent genome:

rr x Rr BC1F1 seeds (50% will be rr, 50% will be Rr)

(___% recurrent parent) 75

Season 3: Grow the BC1F1 plants (rr and Rr) and spray with Roundup, pre-flowering. The 50% of the plants that survive confirm that the transfer of donor gene was successful. Backcross those to the elite recurrent parent (rr) to recover superior agronomic genome:

rr x Rr BC2F1 seeds (50% will be rr, 50% will be Rr)

(___% recurrent parent)

Season 4: Grow the BC2F1 plants (rr and Rr) and spray with Roundup, pre-flowering. The 50% of the plants that survive confirm that the transfer of donor gene was successful. Backcross those to the elite recurrent parent (rr) to recover superior agronomic genome:

rr x Rr BC3F1 seeds (50% will be rr, 50% will beRr)

(___% recurrent parent)

87.5

93.75

Season 5: Grow the BC3F1 plants (rr and Rr) and spray with Roundup, pre-flowering. The 50% of the plants that survive confirm that the transfer of donor gene was successful. Backcross those to the elite recurrent parent (rr) to recover superior agronomic genome:

rr x Rr BC4F1 seeds (50% will be rr, 50% will beRr)

(___% recurrent parent)

Season 6: Grow the BC4F1 plants (rr and Rr) and spray with Roundup, pre-flowering. The 50% of the plants that survive confirm that the transfer of donor gene was successful. Backcross those to the elite recurrent parent (rr) to recover superior agronomic genome:

rr x Rr BC5F1 seeds (Rr)

(___% recurrent parent)

96.875

98.4375

Season 7: Grow the BC5F1 plants (rr and Rr) and spray with Roundup.

The 50% of the plants that survive confirm that the transfer of donor gene was successful. Note that these BC5F1 plants would be expected to comprise, on average, 98.4375 elite recurrent parent superior agronomic genome, but they exist as heterozygous (Rr) plants. Allow the BC5F1 plants to self-pollinate, producing BC5F2 seeds.

Season 8: Grow the BC5F2 population as single-plants (RR, Rr, and rr) and spray with Roundup.Only the rr plants will be killed, the remaining plants (RR or Rr) that survive can be harvested as separate single-plants, saving the BC5F2:3 seeds.  Season 9: Grow the BC5F2:3 plant rows and spray with Roundup.One-third of the rows will be homozygous dominant (RR) and 2/3 of the rows will be segregating (¼ of the plants within the segregating rows will be observed to have died after spraying with Roundup).

Harvest only the homozygous dominant rows, and bulk all seed from the rows which appear uniform for height, maturity, and other observable traits together in one large single bag, as breeders seed stock for release or further yield evaluations.

ImplementationSatisfactory recurrent parent must exist. Backcross procedure produces new cultivar phenotypically similar to one favored by both farmer and processor.   Must still be satisfactory for all traits, (other than the one to be improved), in 6-10 generations down the road.   Commonly used to transfer disease resistance genes, eg powdery mildew and leaf rust in wheat, phytophthora resistance in soybean.   Growing demand for food and processor quality traits, breeders see resurgence in backcrossing compositional traits such as fatty acid and amino acid composition traits to specific lines of interest in soybean, corn, canola, and other crop species.

How many backcrosses should a breeder make? Factors such as:  

1) importance of recovering all characteristics of RP,  

2) relatedness of recurrent and donor parents  

3) selection among backcross progeny for the RP phenotype selection, especially in first 2 backcross generations, will speed recovery of the RP genome.

5 or more backcrosses considered effective for recovery of recurrent parent genome.