Bulk Selection Methods

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Bulk Selection Methods
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The Basic Scheme

• Grow out the F2 generation. Allow natural
  selection to occur. Harvest seed from remaining
  plants.
• Grow out the F3 generation. Allow natural
  selection to occur. Harvest seed from remaining
  plants.
• Continue for several generations. Select single
  plants, grow progeny rows, harvest seed for yield
  testing.

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Florell (1929)

• 19 wheat crosses grown as bulks
• First selections in F5 or F6
• Selected 9-49 heads per cross
• Planted head rows, kept best (looking) 45
• Increased seed for yield test
• 33 of 45 beat the check. Top yielding line was
  55 bu/a vs. 37 bu/a for check

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A procedure for inbreeding a segregating
population until the desired level of
homozygosity is achieved. The seed used to grow
each inbreeding generation is a sample of that
harvested from plants the previous generation.
The bulk method is used predominately with inbred
species. It is totally unsuited for fruit crops
and most vegetables. Unlike the mass selection
method, no human imposed selection is made during
successive generations of inbreeding in bulk.
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Bulk selection was developed in Sweden in the
early 20th century. It was trying to handle
segregating generations of a winter wheat hybrid
that attempted to combine winterhardiness and
high yield. The bulk, grown over several years
in Swedish winters, was influenced by natural
selection during the approach to homozygosity.
Natural selection increased the frequencies of
winter-hardy types in the population.
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Because of the importance of natural selection in
this method, the breeder should carefully choose
environments in which natural selection is likely
to favor the desired genotypes in the population,
i.e., a population segregating for disease
resistance should be grown in the presence of the
pathogen in order to reduce the productivity of
susceptible plants. Because of this concern for
the environment, the population undergoing the
bulk method would rarely be grown in off-season
nurseries.
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The central issue in bulk population breeding is
the nature of this correlation Is the ability
to survive in competition related to agricultural
worth?
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Genetic ConsiderationsGenotypic frequencies in a
population inbred by the bulk method are
determined by four variables associated with
natural selection in a heterogeneous population.
1) genetic potential of a genotype for seed
productivity 2) competitive ability of a
genotype 3) influence of the environment on the
genotype expression 4) sampling of genotypes to
propagate the next generation
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There is no way to know if certain F2 plants have
progeny in F3 generation, or later generations.
There is also no way to predict the genetic
variability for any character in any generation.
If variables favor the desired genotype, the
frequency of desired genotypes will increase if
not, youre out of luck!
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Subdivision/types of Bulk MethodPractical
short-term breeding Up to F2 to F6 generation.
Survival among pure line genotypes Survival of
different plant types in hybrid - derived
populationsLong-term Evolutionary breeding
Composite cross method
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Long-term Evolutionary breeding Composite cross
methodThe two methods are separate entities.
If one selects out of F4, F5, or F6, having
bulked in F2, the probably impacts of natural
selection will be small in most cases. This does
not mean insignificant, however. On the other
hand, if one selects out of a bulk propagated for
15 to 20 generations, the impact of natural
selection may be much greater.
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Survival among pure line genotypes Harlan, J.,
and Martini, 1938. J. Agric. Res. 57189-199.
They reported on the effects of natural
selection in cultivated plants. Mixed 11 pure
lines of barley and grew them for 4-12 years at
Ag. Experiment Stations from Davis, CA to Ithaca,
NY. The change in composition was monitored at
each location.
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Results1 or 2 lines became dominant very
quickly at each location. Natural selection in
barley is a significant force at all locations
and a force of great magnitude at some locations.
14
Similar experiments were conducted by Suneson and
Wiebe (1942) with four varieties of California
barley. They mixed equal amounts of the
varieties and a census was taken annually over a
9 year period of propagation. They also measured
the mean agronomic performance in adjacent plots.
Atlas quickly dominated the bulk population.
Hero and Vaughn were virtually eliminated.
But differences in productivity of the cultivars
was small.
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The experiment was not conclusive in establishing
a relationship between yielding ability and
survival. Vaughn, the highest yielder, was the
poorest competitor so there was not a very
strong positive association between these
characteristics. The marked differences in
competitive ability must have depended on other
characteristics of competitive ability besides
yield. Later Josh Lee (1960) provided
information that might explain superiority of
Atlas over Vaughn. Atlas accounted for 55
of spikes in mixed stands, but in a pure stand it
has 40 fewer spikes than Vaughn. He found
that Atlas also produced a larger root system
then Vaughn.
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An important feature of these experiments, which
may not necessarily relate to the bulk population
method, is that these experiments all involved
inbred homozygous lines. But the predominant use
of the bulk method is to self-in-bulk until
homozygosity is reached. Segregation will occur
in bulk hybrids with the result that the
competing genotypes are not expected to be
constant from one generation to another. In
previous studies, the populations investigated
were very simple in terms of components pure
lines. It was not possible to assay for the
survival of an allele or allelic combination per
se because that allele or allelic combination was
always associated with all other alleles in the
genotype.Only at the time of homozygosity (F6 to
F8) will the situation begin to represent that of
variety mixtures.
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Plant height segregated in this cross in a
monofactorial basis. Samples of the segregating
generations were from the F2 to the F6
generation. One sample was grown with zero
Nitrogen and a second with high N. Obviously,
the short plants were at a disadvantage in
competition with tall ones in this bulks in both
cultures. However, the disadvantage was
intensified under high N conditions. In a pure
stand the short plants outyielded the tall by a
mean of 35. In this study, natural selection
was counterproductive to the goal of breeding
high yielding rice cultivars. Tall genotypes had
vigorous vegetation which gave them a decided
competitive advantage for light interception.
Such a high differential in reproductive rates
caused the demise of the short, high yielding
genotypes very rapidly.
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Jennings recommends roguing the population to
remove the good competitors in order to eliminate
the bad effects of competition in segregating
populations. But Allard concludes that
agronomically poor types are generally poor
competitors. So differences in opinion exist.
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Composite Cross Method Suneson, C. 1956.
Agron. J., 48188-190. The composite cross
method can be called evolutionary plant
breeding because it uses composite crosses and
natural selection. The rationale is survival
ability. Survival ability is sometimes
detrimental to productivity in a pure stand but
has special significance for determining
agricultural fitness. Develop Composite Cross
II crossed together 28 diverse cultivars of
barley (378 crosses). Grown in bulk from F2 to
F29 without selection.
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Periodically compare yielding ability of CCII
with Atlas using remnant seeds from various
generations. AtlasF3 to F4
67.6F7to F8 85.0F11 to F14 88.0F15
to F20 106.0F21 to F23
113.8F24 135.7
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The composite bulk was conspicuously inferior to
Atlas in yielding ability and general agronomic
appearance in early generations. There was a
gradual improvement, however, in both yielding
ability and agronomic type until, by F15 the bulk
equaled Atlas and then continued to improve.
So natural selection did cause significant
increased in yield. After 15 generations 1)
Continued natural selections for significant
gain in yield.2) Cyclic hybrid recombination
alternated with natural selection.3)
Conventional selection and testing to identify
best lines.
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Evolutionary plant breeding in barley had
produced numerous cultivars. It can be thought
of as a stretched bulk method, resting on the
dynamics of natural selection. It is very unique
in the length of its development period. It is
theoretically and practically sound, and its
greatest value would be as an adjunct breeding
program. Because of the long wait before
selection can begin, it is not feasible as a
single method breeding program. The department
and your farmers might will not be very happy if
it takes you over 20 years to develop a
cultivar!! Perhaps a compromise would be to
treat all crosses by the short-term bulk method,
forming derived lines in the F4 F6 generations
for the breeding project. If the population is
judged useful for the future, it could be
continued as an evolutionary breeding entity.
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Pros

• Automatic increase in proportion of homozygous
  plants with each generation
• May increase mean yield via natural seln.
• Final purification of lines simplified
• Selection among crosses can occur before
  selection within crosses begins, thus the elite
  crosses are the ones that remain in the program

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Cons

• Plants of one generation not all represented by
  progeny the next generation
• Genotypic frequencies and genetic variability
  cannot be defined readily
• Bulk method not suited to greenhouses, off season
  nurseries
• Environment must be suitable to reinforce
  breeding objectives

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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

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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 S01 lines through many
  selfing generations, then crossed them to common
  tester
• Found that combining ability was already
  determined in S01 lines

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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 F23 lines

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Self-Pollinated Crops

• Harvest seed from individual F2 plants
• Plant seeds in F23 progeny rows
• Identify the superior rows
• Harvest all seed in each selected row in bulk
• Grow replicated tests of F24 lines
• Grow replicated tests of F25 lines

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Self-Pollinated Crops

• Harvest selected F5 plants individually
• Grow F56 lines in headrows
• Test F5 - derived lines extensively

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Breeders 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

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Genetic Considerations

• Recall that there is all of the additive variance
  among F23 lines and one-half of the additive
  variance within F23 lines
• In later generations of F2 derived lines, there
  is still all of the additive variance among
  lines, and an increasing amount within lines, as
  inbreeding progresses

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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?

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Pros

• Inferior individuals and crosses are discarded
  early in the process
• One hetergeneous line may yield more than one
  cultivar

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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