BREEDING SELF-POLLINATED SPECIES 325
16.8.5 Advantages and disadvantages Rouging may be conducted to remove undesirable
genotypes prior to bulking.
There are key advantages and disadvantages of the The breeder may select the appropriate environment
bulk breeding method. to favor desired genotypes in the population. For
example, selecting under disease pressure would elim-
Advantages inate susceptible individuals from the population.
Preliminary yield trials may be started even while the
It is simple and convenient to conduct.
lines are segregating in the F3 or F4.
It is less labor intensive and less expensive in early The single seed descent method may be used at
generations.
each generation to reduce the chance of genetic
Natural selection may increase frequency of desir-
drift. Each generation, a single seed is harvested
able genotypes by the end of the bulking period.
from each plant to grow the next bulk population.
It is compatible with mass selection in self-polli-
The dense planting makes this approach problem-
nated species.
atic in locating individual plants.
Bulk breeding allows large amounts of segregating The composite cross bulk population breeding,
materials to be handled. Consequently, the breeder
also called the evolutionary method of breeding,
can make and evaluate more crosses.
was developed by C.A. Suneson and entails system-
The cultivar developed would be adapted to the
atically crossing a large number of cultivars. First,
environment, having been derived from material
the pars of parents are crossed, then pairs of F1s are
that had gone through years of natural selection.
crossed. This continues until a single hybrid stock
Single plant selections are made when plants are
containing all parents is produced. The method has
more homozygous, making it more effective to eval-
potential for crop improvement but it takes a very
uate and compare plant performance.
long time to complete.
Disadvantages
16.9 Single seed descent
Superior genotypes may be lost to natural selection,
while undesirable ones are promoted during the
The method of single seed descent was born out of a
early generations.
need to speed up the breeding program by rapidly
It is not suited to species that are widely spaced in
inbreeding a population prior to beginning individual
normal production.
plant selection and evaluation, while reducing a loss of
Genetic characteristics of the populations are diffi-
cult to ascertain from one generation to the next. genotypes during the segregating generations.
Genotypes are not equally represented in each gen- The concept was first proposed by C.H. Goulden in
eration because all plants in one generation are not 1941 when he attained the F6 generation in two years
advanced to the next generation. Improper sam- by reducing the number of generations grown from a
pling may lead to genetic drift. plant to one or two, while conducting multiple plant-
Selecting in off-season nurseries and the greenhouse ings per year, using the greenhouse and the off sea-
may favor genotypes that are undesirable in the pro- son. H.W. Johnson and R.L. Bernard described the
duction region where the breeding is conducted, procedure of harvesting a single seed per plant for
and hence is not a recommended practice. soybean in 1962. However, it was C.A. Brim who, in
The procedure is lengthy, but cannot take advantage 1966, provided a formal description of the procedure
of off-season planting. of single seed descent, calling it a modified pedigree
method.
16.8.6 Modifications
16.9.1 Key features
Modifications of the classic bulk breeding method
include the following: The method allows the breeder to advance the maxi-
mum number of F2 plants through the F5 generation.
The breeder may impose artificial selection sooner This is achieved by advancing one randomly selected
(F3 or F4) to shift the population toward an agricul- seed per plant through the early segregating stages.
turally more desirable type. The focus on the early stages of the procedure is on
326 CHAPTER 16
attaining homozygosity as rapidly as possible, without Year 8 and later. Yield trials are conducted in the F8
selection. Discriminating among plants starts after to F10 generations. The most superior line is
attainment of homozygosity. increased in the F11 and F12 as a new cultivar.
16.9.2 Applications Comments
Growing plants in the greenhouse under artificial If the sample is too small, superior genetic combina-
conditions tends to reduce flower size and increase tions may be lost because only one seed from each
cleistogamy. Consequently, single seed descent is best plant is used.
for self-pollinated species. It is effective for breeding It may be advantageous to use progeny rows prior
small grains as well as legumes, especially those that to yield testing to produce sufficient seed as well as
can tolerate close planting and still produce at least to help in selecting superior families.
one seed per plant. Species that can be forced to The breeder may choose to impose some artificial
mature rapidly are suitable for breeding by this selection pressure by excluding undesirable plants
method. It is widely used in soybean breeding to from contributing to the subsequent generations
advance the early generation. (in the early generations). This is effective for quali-
tative traits.
Record keeping is minimal and so are other activities
16.9.3 Procedures such as harvesting, especially in early generations.
Overview
16.9.4 Genetic issues
A large F1 population is generated to ensure adequate
recombination among parental chromosomes. A sin- Each individual in the final population is descendent
gle seed per plant is advanced in each subsequent gen- from a different F2 plant. Each of these plants under-
eration until the desired level of inbreeding is goes a decrease in heterozygosity at a rapid rate, each
attained. Selection is usually not practiced until F5 or generation. Barring the inability of a seed to germi-
F6. Then, each plant is used to establish a family to nate or a plant to set seed, the effect of natural selec-
help breeders in selection and to increase seed for sub- tion is practically non-existent in the single seed
sequent yield trails. descent procedure. Only one seed per plant is
advanced, regardless of the number produced. That
is, a plant producing one seed is equally represented
Steps
in the next generation as one producing 1000 seeds.
Year 1. Crossing is used to create the base popula- Selection is conducted on homozygous plants rather
tion. Cross selected parents to generate adequate than segregating material. An efficient early genera-
number of F1 for the production of a large F2 tion testing is needed to avoid genetic drift of desir-
population. able alleles. Single seed descent is similar to bulk
Year 2. About 50–100 F1 plants are grown in a selection in that the F6/F7 comprises a large number
greenhouse ground bench or in pots. They may also of homozygous lines, prior to selection among proge-
be grown in the field. Harvest identical F1 crosses nies. A wide genetic diversity is carried on to relatively
and bulk. advanced generations (F6/F7).
Year 3. About, 2000–3000 F2 plants are grown. At
maturity, a single seed per plant is harvested and
bulked for planting F3. Subsequently, F2 is spaced 16.9.5 Advantages and disadvantages
enough to allow each plant to produce only a few
Single seed descent has certain advantages and
seeds.
disadvantages.
Year 4–6. Single pods per plant are harvested to
plant the F4. The F5 is space-planted in the field,
harvesting seed from only superior plants to grow Advantages
progeny rows in the F6 generation.
Year 7. Superior rows are harvested to grow prelim- It is an easy and rapid way to attain homozygosity
inary yield trails in the F7. (2–3 generations per year).
BREEDING SELF-POLLINATED SPECIES 327
Small spaces are required in early generations (e.g., 16.10 Backcross breeding
can be conducted in a greenhouse) to grow the
selections. This application in plants was first proposed by H.V.
Natural selection has no effect (hence it can’t Harlan and M.N. Pope in 1922. In principle, back-
impose adverse impact). cross breeding does not improve the genotype of the
The duration of the breeding program can be product, except for the substituted gene(s).
reduced by several years by using single seed
descent.
Every plant originates from a different F2 plant, 16.10.1 Key features
resulting in greater genetic diversity in each The rationale of backcross breeding is to replace a
generation.
specific undesirable gene with a desirable alternative,
It is suited to environments that do not represent
while preserving all other qualities (adaptation, pro-
those in which the ultimate cultivar will be commer-
ductivity, etc.) of an adapted cultivar (or breeding
cially produced (no natural selection imposed).
line). Instead of inbreeding the F1 as normally done,
it is repeatedly crossed with the desirable parent to
Disadvantages retrieve (by “modified inbreeding”) the desirable
genotype. The adapted and highly desirable parent is
Natural selection has no effect (hence no benefit called the recurrent parent in the crossing program,
from its possible positive impact). while the source of the desirable gene missing in the
Plants are selected based on individual phenotype adapted parent is called the donor parent. Even
not progeny performance. though the chief role of the donor parent is to supply
Inability of seed to germinate or plant to set seed
the missing gene, it should not be significantly defi-
may prohibit every F2 plant from being represented cient in other desirable traits. An inferior recurrent
in the subsequent population. parent will still be inferior after the gene transfer.
The number of plants in the F2 is equal to the
number of plants in the F4. Selecting a single
seed per plant runs the risks of losing desirable 16.10.2 Application
genes. The assumption is that the single seed rep-
The backcross method of breeding is best suited to
resents the genetic base of each F2. This may not
improving established cultivars that are later found to
be true.
be deficient in one or two specific traits. It is most
effective and easy to conduct when the missing trait is
qualitatively (simply) inherited, dominant, and pro-
16.9.6 Modifications
duces a phenotype that is readily observed in a hybrid
The procedure described so far is the classic single plant. Quantitative traits are more difficult to breed
seed descent breeding method. There are two main by this method. The procedure for transferring a
modifications of this basic procedure. The multiple recessive trait is similar to that for dominant traits,
seed procedure (or modified single seed descent) but entails an additional step.
entails selecting 2–4 seeds per plant, bulking and Backcrossing is used to transfer entire sets of chro-
splitting the bulk into two, one for planting the next mosomes in the foreign cytoplasm to create a cyto-
generation, and the other half held as reserve. Because plasmic male sterile (CMS) genotype that is used to
some soybean breeders simply harvest one multi- facilitate hybrid production in species including corn,
seeded pod per plant, the procedure is also referred to onion, and wheat. This is accomplished by crossing
by some as bulk-pod method. the donor (of the chromosomes) as male until all
Another modification is the single hill method in donor chromosomes are recovered in the cytoplasm
which progeny from individual plants are maintained of the recurrent parent.
as separate lines during the early generations by plant- Backcrossing is also used for the introgression of
ing a few seeds in a hill. Seeds are harvested from the genes via wide crosses. However, such programs are
hill and planted in another hill the next generation. A often lengthy because wild plant species possess sig-
plant is harvested from each line when homozygosity nificant amounts of undesirable traits. Backcross
is attained. breeding can also be used to develop isogenic lines
