As previously noted, breeding cross-pollinated
species tends to focus on population improvement rather than the improvement of
individual plants, as is the focus in breeding self-pollinated species. In
addition to methods such as mass selection that are applicable to both
self-pollinated and cross-pollinated species, there are specific methods that are
suited to population improvement. Some methods are used less frequently in breeding.
Further, certain methods are more effective and readily applied for breeding
certain species than others. After studying this chapter, the student should be
able to:
1 Discuss the method of mass selection in
cross-pollinated species.
2 Discuss the concept of recurrent selection.
3 Describe the methods of half-sib and full-sib
selection.
4 Discuss the method of S1 and S2 selection.
5 Discuss the development of synthetic cultivars
6 Discuss the application of
backcross technique in cross-pollinated species.
The concept of population improvement
As stated in the introduction, the methods of
selection for improving self-pollinated species tend to focus on improving individual
plants. In contrast, the methods of improving cross-fertilized species tend to focus
on improving a population of plants. A population is a large group of
interbreeding individuals. The application of the principles and concepts of
population genetics are made to effect changes in the genetic structure of a
population of plants. Overall, breeders seek to change the gene frequency such
that desirable genotypes predominate in the population. Also, in the process of
changing gene frequencies, new genotypes will arise. It is important for
breeders to maintain genetic variability in these populations, so that further improvements
of the population may be achieved in the future.
To improve the population,
breeders generally assemble germplasm, evaluate, self-selected plants, cross
the progenies of the selected selfed plants in all
possible combinations, bulk, and develop inbred
lines from the populations. In cross-pollinated species, a cyclical selection
approach, called recurrent selection, is often used for intermating. The
cyclical selection was developed to increase the frequency of favorable genes
for quantitative traits. Various methods of recurrent selection are used for
producing progenies for evaluation, as will be discussed here. The procedures
for population improvement may be classified in several ways, such as according
to the unit of selection – either individual plants or family of plants. Also,
the method may be grouped according to the populations undergoing selection as
either intrapopulation or interpopulation. Intrapopulation improvement Selection
is practiced within a specific population for its improvement for specific
purposes. Intrapopulation improvement is suitable for improving populations
for:
_ Which the end product will be a
population or synthetic cultivar.
_ Developing elite pure lines for
hybrid production.
_ Developing mixed genotype
cultivars. Interpopulation improvement Methods
of interpopulation improvement entail selection on the basis of the performance
of a cross between two populations. This approach is suitable for use when the
final product will be a hybrid cultivar. Interpopulation heterosis is
exploited.
Concept of
recurrent selection
The concept of recurrent
selection was introduced in Chapter 4 as a cyclical and systematic technique in
which desirable individuals are selected from a population and mated to form a
new population; the cycle is then repeated. The purpose of a recurrent
selection in a plant breeding program is to improve the performance of a population
with respect to one or more traits of interest, such that the new population is
superior to the original population in mean performance and in the performance
of the best individuals within it.
The
source material may be random mating populations, synthetic cultivars, single
cross, or double cross. The improved population may be released as new cultivar
or used as a breeding material in other breeding programs. The most desirable
outcome of recurrent selection is that the improved population is produced
without reduction in genetic variability. In this way, the population will
respond to
future improvement.
The
success of a recurrent selection program rests on the genetic nature of the
base population. Several key factors should be considered in the development of
the base population. First, the parents should have high performance regarding
the traits of interest and should not be closely related. This will increase
the chance of maximizing genetic diversity in the population. It is also
recommended to include as many parents as possible in the initial crossing to
increase genetic diversity. Crossing provides opportunity for recombination of
genes to increase genetic diversity of the population. More rounds of mating
will increase the opportunity for recombination but it increases the duration
of the breeding program. The breeder should decide on the number of generations
of intermating that is appropriate for a breeding program.
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