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Planning rice breeding programs for impact. Choosing parents and managing a pedigree breeding program. Learning objectives. Strategies for choosing parents Types of crosses F2 population size Describing lines and crosses Pedigree versus bulk selection

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planning rice breeding programs for impact

Planning rice breeding programs for impact

Choosing parents and

managing a pedigree breeding program

learning objectives
Learning objectives
  • Strategies for choosing parents
  • Types of crosses
  • F2 population size
  • Describing lines and crosses
  • Pedigree versus bulk selection
  • Traits for early-generation selection
  • How many generations?
  • Field design for pedigree nurseries

IRRI: Planning breeding Programs for Impact

question what is important when choosing parents
Question:What is important when choosing parents?
  • At least 1 adapted parent should be used
  • Improved donors should be used to avoid linkage drag
  • “Pre-breeding” of improved donors is important
  • MAS allows major gene to be moved with less linkage drag
  • At least 1 high-quality parent should be used
  • Backcross to high-quality parent may be necessary

IRRI: Planning breeding Programs for Impact

types of crosses
Types of crosses
  • 2-way, 3-way (“topcrosses”), and double crosses can be used if lines are of similar performance or quality, but break up adapted linkage blocks and epistatic combinations

IRRI: Planning breeding Programs for Impact

types of crosses5
Types of crosses

Backcross populations

When use them?

if the objective is to improve one adapted parent with genes from a donor

(BC1 and BC2 populations have proven very useful in developing lines with improved drought stress tolerance in IR64 background at IRRI)

IRRI: Planning breeding Programs for Impact

backcrossing with selection to improve ir64 for drought
Backcrossing with selection to improve IR64 for drought

Cross

% recurrent parent

Plants crossed

Selection

IR64 x donor

F1 x IR64

50

1-5 crossed

None

20 crossed

None

BC1F1 x IR64

75

20 crossed

None

BC2F1 x IR64

87.5

X

20 selfed

None

87.5

BC2F2

X

BC2F3

Screened under severe stress

87.5

2000 selfed

IRRI: Planning breeding Programs for Impact

question how many crosses should one make
Question:How many crosses should one make?
  • Most programs make too many crosses, examine populations that are too small
  • Choosing parents carefully is more important than making many crosses.
  • Some programs make very few crosses with diverse parents

E.g. Witcombe and collaborators in India and Nepal:

    • Crossed Kalinga III/IR64
    • Farmers selected both upland- and lowland-adapted cultivars from the cross

IRRI: Planning breeding Programs for Impact

describing crosses
Describing crosses
  • Parents are separate by slashes (“/”):

e.g. IR 64/Swarna

  • In complex pedigrees, additional slashes are used to describe more recent crosses.

e.g. IR64/Swarna//PSBRC 80 (PSBRC 80 wascrossed to plants derived from IR64/Swarna)

  • Backcrosses are denoted by a number indicating number of doses of recurrent parent, followed by an asterisk or ‘x’.

e.g. if Kalinga III is crossed to IR64 and then the F1 is backcrossed to IR 64, the cross is described as:

Kalinga III/2*IR 64

IRRI: Planning breeding Programs for Impact

describing lines
Describing lines
  • Each cross should receive a unique, consecutive number

e.g. The IRRI cross IR55419-04/Way Rarem is:

IR74371

(This number can be thought of as referring to the F1.)

  • Individual plants selected from segregating generations (F2 onward for a single cross) are given a unique plant number. Lines derived from these plants are identified by the plant number:

e.g. The line derived from the 24th F2 plant

selected from IR74371 is: IR74371-24.

IRRI: Planning breeding Programs for Impact

describing lines10
Describing lines
  • If a generation is harvested in bulk, without single plant selection, the resulting population is denoted “B”.

e.g. If the F2 of IR74371 is harvested in bulk, without selection, it is denoted IR74371-B

IR74371-B-7 denotes the 7th F3 plant selected from a bulk F2

Exercise

Describe the selection history of

line IR74371-B-5-6-B-B-43

IRRI: Planning breeding Programs for Impact

homozygosity of a line
Homozygosity of a line

For a line in F generation n, the proportion of loci that are homozygous, relative to the number of heterozygous loci in the F1, is:

Exercise

  • What proportion of loci that were heterozygous in the F1 are homozygous in the F5 ?

1 – (0.5)n-1

6.25%

IRRI: Planning breeding Programs for Impact

homogeneity of a line
Homogeneity of a line
  • Homogeneity means sameness or uniformity
  • Lines derived from homozygous plants are homogeneous
  • Lines derived from heterozygous plants are segregating and non-uniform
  • The degree of genetic uniformity of a line is determined by the level of homozygosity of the plant from which it was derived

e.g. A single plant selected from an F3 bulk has a homozygosity level of 1-(.5)2 = 0.75, relative to the F1. A line derived from this plant by selfing is fixed at 75% of the loci that were heterozygous in the F1.

homogeneity of a line13
Homogeneity of a line

Lines in the same inbreeding generations can have very different levels of homogeneity!

  • Example: consider 2 lines in the F6. One derived from a single F3 plant, the other from an F5 plant.
    • In the F3 plant, 75% of loci are homozygous
    • In the F5 plant, 93.75% of loci are homozygous
  • An F3 –derived F6 line is therefore more homogeneous than an F5 derived F6 line, although the plants in each are equally homozygous.
    • For the example above:
      • The F3 –derived line in the F6 is denoted as: F3,6
      • The F5 –derived line in the F6 is denoted as: F 5,6
what is your opinion on pedigree versus bulk selection
What is your opinion on: Pedigree versus bulk selection
  • Advancing in bulk to F3 is almost always more efficient than pure pedigree selection
  • Even using bulk method, mild selection for plant type, grain shape can be applied
  • About 2000 plants should be carried per generation

IRRI: Planning breeding Programs for Impact

how many generations should pedigree selection continue
How many generations should pedigree selection continue?

2 factors to consider:

  • Phenotypic (visible) uniformity

If the line is not visibly uniform, it needs reselection

  • Genetic heterogeneity remaining within the line.

Genetic heterogeneity (variability) is the “fuel” of selection

Selecting among genetically homogeneous plants is pointless

IRRI: Planning breeding Programs for Impact

slide16

Exercise

      • Construct a table for lines established from the F3 through the F8 generation, indicating the proportion of segregating loci expected within the line.

25

12.5

6.25

3.13

1.56

0.78

IRRI: Planning breeding Programs for Impact

controlling field variability in pedigree nurseries
Controlling field variability in pedigree nurseries
  • Field variability can have a big impact on even highly heritable traits like height, DTF etc.
  • Use repeated checks, but not too many
  • Select among nearby lines (gridding)
  • Treat groups of advanced sister lines as replicates of a family; select on the basis of family means rather than individual line performance (or select the best lines from the best families)

IRRI: Planning breeding Programs for Impact

selection based on family means
Selection based on family means

Conventional arrangement

Range 1

IR70000-1-1-1

IR70000-1-1-2

IR70000-1-1-3

IR70000-2-14-1

IR70000-2-14-2

IR70000-2-14-3

Range 2

IR70002-3-1-1

IR70002-3-1-2

IR70002-3-1-3

IR70005-5-7-1

IR70005-5-7-2

IR70005-5-7-3

Range 3

IR70007-6-4-1

IR70007-6-4-2

IR70007-6-4-3

IR70010-3-8-1

IR70010-3-8-2

IR70010-3-8-3

IRRI: Planning breeding Programs for Impact

selection based on family means19
Selection based on family means

Blocking by families

Block 1

IR70000-1-1-1

IR70000-2-14-1

IR70002-3-2-1

IR70005-5-7-1

IR70007-6-4-1

IR70010-3-8-1

Block 2

IR70000-1-1-2

IR70000-2-14-2

IR70002-3-2-2

IR70005-5-7-2

IR70007-6-4-2

IR70010-3-8-2

(randomize)

Block 3

IR70000-1-1-3

IR70000-2-14-3

IR70002-3-2-3

IR70005-5-7-3

IR70007-6-4-3

IR70010-3-8-3

(randomize)

IRRI: Planning breeding Programs for Impact

any questions or comments
Any questions or comments?

IRRI: Planning breeding Programs for Impact

summary 1
Summary 1
  • Most successful crosses have at least 1 high-quality, adapted parent that is preferred by farmers in the TPE
  • BC1- or BC2-derived populations may be efficient for generating high-quality, high-yield lines because they leave adapted gene blocks in the elite recurrent parents intact
  • At least 2000 plants should be screened in the F2
  • A standard pedigree description system should be used
  • Bulk inbreeding in the F2 and F3 generations is used by some breeders to inexpensively produce uniform lines

IRRI: Planning breeding Programs for Impact

summary 2
Summary 2
  • Pedigree selection should focus only on highly heritable, easily-scored traits (unless family selection is used.)
  • There is little genetic variability among F6 sister lines derived from the same F5 plant
  • Regularly occurring checks should be included in pedigree nurseries, but their frequency should not exceed 10% of the total number of plots
  • Selection among families of closely-related lines can be used in advanced pedigree generations. Individual lines in a family can be treated as replicates, and planted in different blocks to overcome the effect of field variability

IRRI: Planning breeding Programs for Impact