Selection Method of Plant Breeding
Selection Method of Plant Breeding
Selection is the oldest method of plant breeding and is the basis of all crop improvement programmes. The selection may be either (i) natural which takes a longer time for bringing about improvement (this improvement may not be in the direction desired by the plant breeder, because improvement will take place for the plant to survive and compete, not for the plant to produce more for human needs) or it can be (ii) artificial, which can bring about improvement within few years (this improvement wall take place in 1 desired direction).
In selection process, new variability cannot be created, since individual plants or groups of plants having certain desirable attributes are selected from a population. Only these selected plants are allowed to reproduce, so that, selection will be effective only for the heritable variation selected from the A base population. A number of selection methods are available for improvement in-self-pollinated, cross-pollinated and asexually propagated crops. These selection methods include the following- (i) mass selection; (ii) pure-line selection; (iii) progeny selection; (iv) clonal selection.
I. Mass Selection
Mass selection in cross-pollinated crops is usually practised for population improvement purposes, whereas in self-pollinated crops it has only a limited use of maintaining purity of a variety. In this method, a group of superior and similar plants are selected and the seed from such plants is harvested and composited (mixed). Since no progeny testing is done, poor strains are also likely to be carried along with the good ones (selection is based only on phenotype), but these poorer types are gradually eliminated when the mass selection is repeated till desired improvement is achieved.
Genetic Basis of mass selection
Mass selection in self-pollinated crops is generally practised to purify the existing pure-line varieties, since genetic variability in these crops is introduced by spontaneous mutations and/or by occasional natural crossing and recombination between pure lines grown in adjoining areas. Further, in the self-pollinated crops, individual plants of a population are nearly homozygous so that the selected plants will not show any segregation in the progeny and the response to mass selection in later generations will be of a very low magnitude. On the other hand, in cross-pollinated crops, since every plant is heterozygous, due to recombination and segregation of genes, the selected superior and phenotypically similar plants will continue releasing enough variability in the progeny, even in the later generations. This results in the simultaneous improvement of the original population, for several desirable attributes. Therefore, response to selection in cross pollinated crops will continue for several cycles of selection.
Procedure of mass selection
In mass selection, plants are selected from the base population on the basis of their phenotypes, such as growth habit, plant height, duration, resistance to diseases and pests, grain characters, etc. Generally 500 to 1000 plants are selected and bulked. Such selections may be either from the whole field or the field may be divided into several smaller fields. Equal numbers of plants are selected from each smaller field and the seed from all the selected plants is composited. In this manner, one cycle of mass selection is completed and the cycle may be repeated for several generations, till the desired level of purity and improvement is achieved. The improved population is tested for its performance against the check variety and if found superior, it may be either released as a variety or used as an improved base population for any plant breeding programme.
Factors affecting mass selection
In mass selection, the selection of plants, ear-heads (inflorescences) or fruits is practised on the basis of phenotypic appearance alone. Therefore, the success of mass selection will depend upon the following factors-
Heritability is the proportion of phenotypic variability that is heritable-so that if His heritability, and Vg and Vp are genotypic and phenotypic variances, then H= Vs/Vg The selection will be more effective for the characters which have high heritability, because only then the phenotype will reflect the genotype to a large measure, so that the superior phenotype will have a better chance to give a superior progeny.
Selection intensity means the number of plants selected per 100 plants. This intensity of selecting plants from the base population should be optimum; too high or too low will result in low level of success. This optimum selection intensity should be between 2 to 20 per cent based upon the objectives, and the variability in the population.
A close linkage between two desirable characters will be useful and will lead to higher level of success, whereas a close linkage of a desirable attribute with an undesirable one may create problems, so that the level of success will be reduced greatly. Therefore, if knowledge of these linkages is available, this will help in planning the selection strategy.
Extent of variability-
High level of variability in the base population offers better opportunities for selection and higher level of success. On the other hand, absence of variability or low level of variability in the original population adversely affects the success of selection.
Advantages of mass selection
Following are the major advantages of mass selection.
- It is a very easy, quick and simple method, since crossing, selfing and progeny testing are not involved.
- Extensive and large scale yield trials are not necessary. Thus reducing the time and cost.
- Varieties developed through mass selection are better adapted.
- This method with some modification is also employed for seed production.
- Through mass selection, synthetic and composite varieties of cross pollinated crops may be improved for few characters like duration, plant type, grain characters, etc.
Limitations of mass selection
Following are some of the limitation of mass selection as a method of plant breeding.
- Mass selection is not very suitable for improvement in characters having low heritability.
- In self-pollinated crops, mass selection is used only for purification and improvement of mixtures of land varieties and pure lines; no real improvement in pure lines can be achieved.
II. Procedure of Pure Line Selection
It is the most commonly used method of selection in self-pollinated crops. Selection of pure lines is made a genetically variable population. There are three important steps for pure line selection.
- A large number of individual plants are selected from the genetically variable original population. The number of such selections may vary between 200 and 1000 depending upon the time, facilities and labour available in the programme. Selection of plants is done on the basis of easily observable characters like duration, height, disease resistnace, etc. Each selected plant is harvested separately.
- During the next year, progeny rows are grown by planting the seeds of cach selected plant in one row, so that a row represents progeny of an individual selected plant. Individual plants are selected between progenies and not within progenies for desired characters since plants within a progeny row are supposed to be genetically similar. In this manner, selection may be repeated for one or few years (3rd and 4th year) and a large scale rejection is made in the progenies grown.
- The finally selected progenies are evaluated by testing their performance in replicated yield trials for 2-3 years against the check varieties. The promising strains are selected, identified, multiplied and released as a new pure line variety.
Factors causing variation in pure lines
Heritable variation however, may arise in the pure lines also after few years due to the following factors-
At the time of harvesting and storage of particular pure line, it may get contaminated with another pure line and this mixture is multiplied over the years. Even though such contamination can eliminated by exercising due care. Mixtures can also be purified by exercising mass selection.
Spontaneous mutations may bring about heritable changes in the pure lines and these mutations are more often deleterious rather than useful. Such heterogeneous mixture resulting from spontaneous mutations can be purified by mass selection. Further, if a useful spontaneous mutation rarely occurs, it can be utilised for plant breeding. For instance, useful spontaneous mutation for dwarfness occured in rice, and was utilized for the production of a number of dwarf rice varieties including ‘Shyama’.
Even in the self-pollinated crops, a small proportion of pollen may be received due to cross-pollination producing natural hybrids, if different pure lines are growing in vicinity of each other. Such natural hybrids would release variability in the following generations through segregation of genes. This natural crossing may be avoided by planting border rows, and its effects may be avoided by regular mass selection.
Due to some meiotic abnormalities in, the crops like wheat, oats, and barley, chromosomal aberrations may be introduced in pure line varieties. This may ultimately give rise to variations, both genotypic and phenotypic.
Achievements of pure line selection
Following are some of the major achievements in plant breeding made through pure line selection-
- In the past, enough genetic variability was available in the local desi varieties. Pure selections were made in such varieties producing pure line varieties like NP 4 and NP 552 in wheat; T1 in cowpea and T1 in moong.
- Pure line selections were also made in the introduced varieties like Kalyan Sona in Wheat and Shining Moong 1 in moong.
- Variations may also arise in the pure line varieties. Some of these variations were selected giving rise to pure line varieties e.g., Pusa Baisakhi moong (from T 44), B 76 urd (from No. 1766) and Chafa gram (from No. 816)
Merits of pure line selection
Following are some of the merits of pure line selection-
- Pure line varieties are uniform, and therefore to the farmers and consumers these are more attractive than the varieties developed through mass selection.
- Varieties developed through pure lines are easily identified during seed certification programmes.
Demerits of pure line selection
Following are some of the demerits of pure line selection-edvaj
- Varieties developed through pure line selection are less adaptable and less stable to the environmental fluctuations.
- Only available genetic variation is utilized in.pure line selection, so that recombinations are not made and utilized.
- The time required for variety development in pure line selection is more than the time required in mass selection method.
III. Progeny selection
Progeny selection is mainly employed in cross-pollinated crops and it overcomes some of the demerits of mass selection. It is based on principle of testing the progenies of selected plants, and the final selection is based on this test. This is done in various ways but one of the most common procedures used in maize is ear-to-row method. Development by Hopkins in 1908.
Procedure of progeny selection
The procedure of progeny selection in a modified car-to-row method involves the following steps-
- First year- A number of plants (50-100) are selected from the base population, on the basis of their superior phenotype. The selected plants are allowed to be open-pollinated and are harvested individually. Some of the seed is utilized for second year and the remaining seed of each selected plant is kept in reserve.
- Second year- The small amount of seed of individual selected plants is grown in separate progeny rows, each having 10-50 plants. Progeny rows are evaluated and superior ones are identified.
- Third year- The reserved seed (kept after first year) of superior progenies (identified on the basis of second year testing) is composited (mixed) to produce the next generation. This completes one selection cycle with progeny testing. In the same manner, one or more additional cycles of selection are practised from the composited population grown after first selection cycle. Later, some more modifications were suggested in the above modified ear-to-row method, so that various selection schemes including half-sib family selection, full-sib family selection and S-family selection were suggested.
Advantages of progeny selection
Following are some of the major advantages of progeny selection-
- Progeny testing helps in separating the environmental effect from the genetic effect on the phenotype, since initially plants are selected only on the basis of phenotypes.
- Inbreeding may be avoided if sufficiently large number and diverse progenies are selected.
Disadvantages of progeny selection
Following are some of the disadvantages of progeny selection-
- As the selected plants are allowed to open-pollinate there is no control over pollen parent.
- Certain modifications of progeny selection make the task of selection complicated.
IV. Clonal Selection
In asexually propagated crops, progenies derived from a plant resemble in genetic constitution with the parent plant. This is possible because the reproduction is based purely on mitosis, producing cells similar to the parent cell. The meiosis, which leads to variation due to segregation and recombination is completely absent. Such groups of plants which are derived from a single plant through asexual means are known as clones. Therefore, plants of a clone should be genotypically as well as phenotypically alike making selection within a clone futile. If some phenotypic differences are there, these may be due to the effect of environment.
Characteristics of Clones
Following are some of the important characteristics of clones in addition to a few discussed above-
- Generally, clones are stable but there may still be degeneration of clones after few years due to loss in vigour and productivity through mutation and susceptibility to diseases and pests.
- Plants of a clone are highly heterozygous and homogeneous, since individual plants of an asexually propagated crop are heterozygous by nature. Therefore, such crops are prone to inbreeding depression.
- Genetic variation within a clone may be brought about primarily through mutation, but mechanical mixture and occasional sexual reproduction may also cause such variability.
- Clones are always propagated through vegetative means, whereas pure lines and inbreds propagate through sexual means. However, pure lines. Inbreds and clones are all homogeneous although clones, unlike pure lines and inbreds are heterozygous.
Procedure of clonal selection
Selection of clones is done from mixed population of vegetatively propagated plants or from the old unimproved variety having genetic variability resulting due to various reasons discussed above. Therefore, in clonal selection, we make use of the genetic variability already available in a population. Following is the year-wise scheme of clonal selection.
- First year- Superior individual plants from the base population are selected for characters having high heritability like plant height, disease resistance, days to flowering and maturity etc.
- Second year- Individual plant progenies or clones are grown and superior clones are identified based upon their morphological appearance.
- Third year- Selection clones are grown alongwith a standard check variety and preliminary yield trial is done. Observations during trial are made on disease characteristics, quality, etc. If necessary, separate disease nursery may be planted and artificial disease epiphytotics may be created. Superior clones are selected on the basis of these trials.
- Fourth to ninth year- Superior clones selected from previous years are put to multi-location trials for a few years and superior clones are identified. Such identified clones are multiplied and released as a new variety.
Merits of clonal selection
Following are some of the merits of clonal selection-
- It is an easy and quick method of breeding for asexually propagated crops. In fact, it is the only major method of breeding for vegetatively propagated crops.
- Additional variability generated through hybridization may also be handled through clonal selection.
Demerits of clonal selection
The only demerit of clonal selection is that only the naturally available variability is utilized for clonal selection. No new genetic variability is created.
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