EGT and Mass Selection

PLS 664April 10, 2007

Early Generation Testing

• Objective: identify those populations that are likely to contain superior lines

• Strategy: eliminate those populations of low potential from the inbreeding process

• Goal: maintain and develop lines from populations with high genetic potential

Jenkins, 1935

• Usual method of estimating combining ability in maize was to inbreed lines, then mate them to a common tester

• Jenkins saved seed from S0:1 lines through many selfing generations, then crossed them to common tester

• Found that combining ability was already determined in S0:1 lines

Self-Pollinated Crops

• Determine the generation for testing

• If it is to be the F2, you will have to grow the F1 in an environment which favors seed production

• A more common choice would be F2:3 lines

Self-Pollinated Crops

• Harvest seed from individual F2 plants

• Plant seeds in F2:3 progeny rows

• Identify the superior rows• Harvest all seed in each selected row in bulk

• Grow replicated tests of F2:4 lines

• Grow replicated tests of F2:5 lines

Self-Pollinated Crops

• Harvest selected F5 plants individually

• Grow F5:6 lines in headrows

• Test F5 - derived lines extensively

Breeder’s Decisions

• Generation to test• Number of reps, locations and years - tradeoff

between early and late generation testing• Separate program for inbreeding or not• Selected lines can be advanced by pedigree, bulk,

or SSD• Number of plants chosen from each hetergeneous

line may vary

Recall that there is all of the additive variance among F2:3 lines and one-half of the additive

variance within F2:3 lines

In later generations of F2 derived lines, there is

still all of the additive variance among lines, and considerable variance within lines, as inbreeding progresses

Genetic Considerations

Genetic Considerations

• Therefore, one may need to take a large number of heads to adequately sample the variation within the F2 - derived line

• Now one must decide how to allocate resources

• Should you sample more lines or more selections within lines?

Pros

• Inferior individuals and crosses are discarded early in the process

• One hetergeneous line may yield more than one cultivar

Cons

• When you commit a lot of resources to early generation testing, you cannot devote as much to thorough evaluation of more inbred material

• If you spend a lot of time testing the early generations, cultivar release may be delayed

Mass Selection : The Prologue

•The formation of a composite propagation stock by the selective

• harvest of individuals from a heterogenous population.

•Oldest method of plant improvement. Humans have been selecting

•desirable seeds for years, even since Neolithic times.

• Quickest way to make progress with undeveloped populations

• and therefore get tenure!!

• Today’s mass selection is performed mostly in concert with an

•established breeding program in a specific population.

• Mass selection can be based upon any method a breeder can

• design to “accept or reject” plants in the population. It is

• only limited by the creativity of the researcher.

Examples of selection criteria

height weight color

texture chemical content shippability

storability timing of maturity taste!!

# tillers shape disease resistance

“kick test”

•Of course, selection success is related to the single plant

• heritability estimate of each trait.

Modes of Implementation

Method #1 = Selection of individuals within a heterogeneous

population. Choosing favorable/desirable plants and harvesting

the seed.

Method #2 = Sampling seed from selected individuals to plant

the next generation.

10 cm

1958: Plant out heterogeneous

population of F3oat seed. Using

a lawn clipper, cut oat plants to

uniform height (determined by check cultivar).

Harvest only top 10 cm in order to select against

short plants. Collected seed will be composed

of complete panicles, partial

panicles, or no panicle.

Mass Selection in Oat : Romero & Frey, 1966

1959 (F4), 1960 (F5), and 1961 (F6): Repeated the procedure.

Evaluated 75 plants from the unselected group of each generation,

75 plants from the selected group of each generation, and 75 plants

of a pure line check.

Results

Significant reduction plant height = 0.47” generation-1

Associated shift towards earliness = 0.23 days cycle-1

Shift towards higher yields = 0.41 grams plot-1. Non-significant

h2D h2I

rg DI

h2D = heritability of directly selected trait

h2I = heritability of indirectly selected traitrg DI = genetic correlation between D&I traits

Mass selection in soybean for maturity and

calcareous soil tolerance : Fehr, unpublished

Yellowing of genotypes indicates lack of ability to utilize available

iron.

Cultivar with desirable levels of tolerance and maturity were planted

as standards.

Plants with more yellowing than standards were removed before

flowering.

One pod/plant was harvest and bulked from selected individuals

** These two examples have been of selections based on individual

plants within the population.

Mass selection of seed size in soybeans : Fehr & Weber,

1968

1963: 4500 F6 plants were planted and 400 plants of early maturity

were selected.

• Top 1/4 of main stem and all branches were removed; remainder of

• plant was threshed together.

• Seed was passed over different sized sieves and 25% of the largest

• and smallest seeds were retained.

• Specific gravity tests in glycerol-water solutions were performed for each size group. 25% of the seeds with a high density and 25% of the seed with a low density were selected.

1964 : 2300 F7 seeds from each group were planted: 400 selected for harvest in same fashion.

• Repeat sizing and specific gravity tests.

•Specific gravity tests in glycerol-water solutions were performed

• for each size group. 25% of the seeds with a high density and

•25% of the seed with a low density were selected.

1964 : 2300 F7 seeds from each group were planted: 400 selected

for harvest in same fashion.

• Repeat sizing and specific gravity tests.

1965 : Process repeated with 1000 F8 seeds from each group.

Results

Linear change in seed size

Progress for high protein- low oil was best in large seed/high specific

gravity set.

Progress for high oil - low protein was best in small seed/low specific

gravity set.

** This is an example of selections and sampling being performed

simultaneously on the harvested seed.

Mass Selection : The Epilogue

Selection can be applied to 1) individual plants, 2) seeds.

Selection involved is 1) artificial selection, 2) naturalselection,

3) both.

Selection for 1) one trait, 2) multiple traits

Sampling methods :

1) random sample of selected seeds bulked

2) equal quantities of seeds harvested & bulked from selected

individuals

3) selection performed on seeds which are used to plant out the

next generation. (Selection & sampling done simultaneously).

Cultivar Purification : mass selection is used routinely

in the maintenance of purity for self-pollinated cultivars

or inbreds of cross-pollinating species. It involves roguing of

off-types (removal of individuals that do not conform to the

normal types).

Genetic Considerations:

• Leads to higher percentages of desired genotypes.

• Effectiveness is a function of h2 of trait on a single plant basis.

• Improving h2 will improve gain.

Double Edged Sword Appears Again

Advantages

• Rapid, inexpensive procedure for increasing frequency of desired

• genotypes. Allard likes it because of the safety and rapidity with

•which mass selection can affect improvement in landraces.

• Can repeat over years until no more progress seems apparent.

Disadvantages

• Can only be used in environment where character is expressed.

•Prevents use of off-season nurseries.

• Limited value for low h2 traits.

Modified Bulk System

• Also known as mass selection• Grow the segregating bulk• Harvest seed from superior individuals

• Combine or “bulk” the harvested seed• This bulk constitutes the population for the next year, or next cycle of selection

Modified Bulk System

• Very simple to implement• Has an excellent track record (dating back to Neolithic times!)

• Combines selection and inbreeding• May be difficult to perceive superior individuals in a heterogeneous population

Modified Bulk System

• Does not work well with low heritabililty traits

• Does allow the breeder to shape the population while inbreeding

UK Modified Bulk System

• F2: select bright clean heads from early, short disease free plants; thresh in bulk

• F3: same procedure• F4: select bright clean heads from early, short disease free plants; thresh into headrow trays

• F5: plant F4:5 headrows; harvest superior headrows separately

• F6: test F4:6 lines in single rep trials at 2-3 locations