Breeding for Yield

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Breeding for Yield

• PLS 664
• Spring 2007

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Issues

• How do we build yield potential into a cross?
• How do we select for yield in the generations
  prior to yield testing?
• How do we select for yield in the first
  generations of yield testing?

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Yield Potential - where does it come from?

• Good x good
• Dandans data
• tanksley

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Yield Potential - how to breed for it in very
early generations

• Evauating vigor

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Microplots

• Hill plots
• Honeycomb design
• Single row plots

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The first yield test - 1 location, 1 replication

• How meaningful can it be?

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Preliminary Wheat Test Logan Co. 2006
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Next Step

• Multiple Environments
• Why?
• G x E

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Genotype x Environment Interaction

• Two sources
• Differences in scale of genetic variance
• Genotype rank changes
• Genotype rank changes are the real problem

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Genotype Rank Change - The Bane Of The Plant
Breeders Existence
V 1
Yield
V 2
Env. 1
Env. 2
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Yield Potential vs. Yield Stability
Yield
1 2 3 4 5 6 7 8
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Locations
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Yield Potential vs. Yield Stability
Yield
1 2 3 4 5 6 7 8
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Locations
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Yield Potential vs. Yield Stability
We want it all!
Yield
1 2 3 4 5 6 7 8
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Locations
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Heritability Proportion of phenotypic
variance That can be explained by genetic
effects
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The most important function of heritability is
its predictive role. Traits with high
heritability values can be improved with rapidity
and with less intensive evaluation than traits
with low heritability (Nyquist, 1991). For
example, we evaluate tens of thousands of F34,
F45, and F56 lines in short 4-foot rows
annually. We are successful in identifying lines
with appropriate plant height and maturity in
these unreplicated plots because of the high
heritability of these two traits. Selection for
grain yield, on the other hand, a low
heritability trait, begins with F57 lines, grown
in 55 square-foot plots in replicated tests at
several locations. This intensive evaluation
continues over several years for each new set of
F57 lines.
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Definitions of Heritability
Nyquist (1991) pointed out two contexts within
which a trait is hereditary 1) the context of
the trait being determined by the genotype of the
individuals being observed, This context gives
rise to what we call Broad-Sense Heritability
(H), which is that portion of the phenotypic
variance that is genetic in origin,


It reflects the correspondence between phenotypic
values and genotypic values. If the proportion
is high, the genotype plays a large role in
determining the observed phenotype if the
proportion is low, then the alternative is true
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The second context gives rise to what we call
Narrow-Sense Heritability (h2), which is that
portion of the phenotypic variance that is due to
variance in breeding values among the individuals
in a population, or the ratio
.

In this equation refers to additive
genetic variance Or the variance of breeding
values.
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Using our own breeding program as an example,
narrow-sense heritability for plant height and
maturity must be high in our F34 wheat lines,
because we find that the short, early maturing
lines we select will generally produce short,
early maturing F45 progeny. Narrow-sense
heritability gets to the core of the predictive
role of heritability. It expresses the degree to
which the phenotype is determined by the alleles
transmitted from parent to offspring--i.e., is
what we see in this generation what we will get
in the following generation? It gives a practical
interpretation to heritability. It is an
estimate of the phenotypic difference between
lines which one expects to recover in the progeny.
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We can estimate two forms of broad-sense
heritability based on the data that this
experiment has provided  1. Per-plot
basis 2. Entry mean basis
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Heritability on a Per-Plot Basis  This experiment
involved evaluation of genotypes in a two-rep
experiment grown at two locations in two years-
common scenario. In order to have some common
base-line of comparison, the heritability can be
reported on a per-plot basis--the plot being the
lowest unit of observation, or selection.
H (per-plot basis)

(19.8)
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Heritability on an Entry Mean Basis
While the per-plot estimate of heritability
described previously has value as a common
baseline comparison, the phenotypic means that
are available to us from the experiment analyzed
in Table 10.2 were based on data averaged over
eight plots (2 replications at 2 locations in
each of 2 years). Such data would be far more
precise than single plot data not only because we
have two replications at each location, but
because ?2GY, ?2GL and ?2GLY were all significant
sources of variation in the analysis of
variance.of variation also.
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Having done the work, we can estimate the
heritability of the trait based on the Entry
Means over the eight values for each line. The
formula is  H (entry mean basis)


0.495 (or 49.5).
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Thus, having exposed our material to the vagaries
of ?2GL, ?2GY, ?2GLY, and ?2, the phenotypic
means we have calculated are closer
approximations to the true genotypic value than
the single plot estimates. As one scans over the
yields of the lines based on entry mean data, it
can be inferred that for every one unit
difference in observed phenotypic means, 49.5 of
the difference is due to differences at the DNA
level. The remaining 50.5 is due to the
blurring of the true genotypic value by
environmental and experimental error effects.
As we would expect, the performance of the
progeny of a line selected on the basis of its
entry mean would likely be a closer approximation
to the eventual performance of the progeny than
the performance of the progeny of a line selected
on a per-plot basis.
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Heritability on a Single Plant Basis Single
Plants Within a Plot or Grid Pattern
Selection on a single plant basis can be
conducted on plants within plots of, say, S01
lines or half-sib families. Alternatively, one
could grow a heterogeneous population on a large
area of the nursery and subdivide the area into
grids or plots. The superior plant(s) within
each grid would be selected without consideration
of plants in other grids.
Variance Within plots
Variance among plots
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Heritability on a single plant basis would be
estimated as
For convenience we utilize the variances from the
previous example. Heritability on a per plant
basis is estimated as   h2
0.12 (or 12).
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Under reasonable management conditions, we would
assume that plot-to-plot variation measured over
a large number of plants per plot would be less
than plant-to-plant variation within plots
(?2w). Heritability on a per-plant basis is
estimated as
Or 11 percent.