Agriculture Reference
In-Depth Information
eries which magnify useful differences and hence
make selection easier, are extremely valuable in
the selection phase of wheat breeding. For
example in Nebraska, early-generation winter
wheat populations are grown at Mead, Nebraska,
an area of relative unimportance to wheat produc-
tion, because any plant that can survive there will
survive anywhere in Nebraska. This location pro-
vides an effective environment to differentiate
and select for an adequate level of winter survival
throughout the northern Great Plains (Color
Plate 26).
The value of a selection site is predicated upon
the selected phenotype being representative of
the desired genotype. In the previous example,
winter survival is a good example, because spring
growth habit genes at most loci are dominant to
genes for winter growth habit; hence the desired
phenotype is usually the genetically homozygous
recessive plant. Similarly, optical kernel sorting
for recessive traits can be extremely useful for
enriching the segregating population for the
desired phenotype or genotype. Examples include
selection for white kernels in a population segre-
gating for red and white kernels, or waxy wheat
in a population segregating for waxy and nonwaxy
kernels, or hard kernels in a population segregat-
ing for kernel hardness (Dowell et al., 2006).
Similarly, perfect markers can be highly success-
ful for allele enrichment in populations, as noted
previously. The advantage of phenotypic selec-
tion, either in selection nurseries or by optical
kernel sorting, is that it represents direct selection
and can handle a large number of seeds or plants
at little cost. The main disadvantage is that the
traits must be highly heritable. The main advan-
tage of MAS is that it can involve DNA from seed
or leaf tissue for low-heritability traits, traits that
are diffi cult to measure, or traits that can only be
evaluated in the adult-plant stage. As an indirect
selection procedure, the main disadvantage of
MAS is the marker may not be diagnostic for the
trait of interest depending on the population (Ellis
et al., 2007), and though it is becoming less
expensive, MAS remains expensive if used
extensively.
Previously we mentioned allele enrichment
strategies. By far the most common method of
allele enrichment is using molecular markers
where the markers identify early-generation
progeny that have the QTL allele of interest.
Those without it can be discarded. DePauw et al.
(2007) reported how the
Xucw71_5utr
CAPS
marker is used to enrich the frequency of
Gpc-B1
for elevated protein content in topcross F
1
popu-
lations. In crosses in which
Gpc-B1
occurs at 25%
gene frequency (1
Gpc-B1gpc-B1
:1
gpc-B1gpc-B1
),
the heterozygotes are retained and the homozy-
gotes which lack the gene are discarded, pro-
ducing a shift in gene frequency from 25% to
50%. In other topcross F
1
populations in which
the frequency of
Gpc-B1
is 75% (1
Gpc-B1gpc-
B1
:1
Gpc-B1Gpc-B1
), only the homozygotes with
Gpc-B1
are retained, producing a shift in gene
frequency from 75% to 100%.
However, due to cost and population size
requirements for using markers, reduction in
population size is advised fi rst by removing unde-
sirable types for other traits before the application
of marker selection. For example, consider the
goal to develop a hard wheat cultivar that is resis-
tant to Fusarium head blight. In the F
2
progeny
of a single cross between a soft wheat cultivar with
two QTLs for resistance to Fusarium head blight
and a hard wheat cultivar susceptible to Fusarium
head blight, it might be best to optically sort and
remove the soft kernels fi rst, so only the remain-
ing hard kernels would be tested using markers
for Fusarium head blight resistance genes. To
compare the unsorted with the sorted popula-
tions, the frequency of hard (1/4) times the fre-
quency of homozygous Fusarium head blight
resistant lines (1/16) equals 0.0156, or 1.6% of
the population. Using 96-well plates for MAS
and 4 plates (i.e., 384 individuals), 6 individuals
could be selected. However, if the soft kernels
were removed fi rst, then 1/16 of the remaining
population would be homozyogous resistant, or
potentially 24 of 384 individuals. Remember that
while 24 individuals is a small population size to
represent other segregating alleles in the popula-
tion, it is fourfold greater than the 6 individuals
available in the absence of selection for hardness;
initial population size could be scaled up to
generate a more desirable size of the enriched
population.
