Science > Biology > Improvement in Crop variety > Selection and Testing of Superior Recombinants
The selection and testing of superior recombinants involve picking up the better ones from the entire crop plants. The collected germ-plasm is now evaluated to identify the plants showing the desirable combination of characters. Selfing the selected plant for several generations to obtain the homozygous inbred pure line. Such selected homozygous inbred pure line plants are used in the process of the next step called hybridization. There are two types of selections: Natural selection and Artificial selection. With respect to crop improvement, we use artificial selection.
Natural Selection:
It is a natural process and operates in nature without human interference. According to Darwin’s principle “Survival of the fittest”, nature itself selects the fittest organisms. Thus, natural selection favours those characters which are essential for the survival of a species.
Artificial Selection:
Artificial selection can be defined as to choose certain individual plants for the purpose of having a better crop from a mixed population where the individuals differ in characters.
Types of Artificial Selection:
Mass Selection:
This is the most common and old method of crop selection. This method of selection depends mainly on the selection of plants according to their phenotype and performance. In this selection, a large number of similarly appearing vigorous plants are selected for the desired trait and their seeds are mixed together. This mixture of seeds is a mass, and, therefore, it is known as mass selection. The seeds are used for cultivation of the next generation. It is useful in the cultivation of cross-pollinated plants such as maize, watermelon, radish, grapes, apples, onion, pear, etc. This method is used to improve the overall population by positive or negative mass selection. Mass selection is only applied to a limited degree
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Year |
Details |
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First year |
Vigorous plants (50 -1000) with similar but desirable traits like height, maturity, disease resistance, productivity etc. are selected from source population and their seeds are mixed and next generation is grown. This process is repeated till desirable results are achieved. |
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Second year |
Seeds carried from first year planted in a preliminary yield trial in isolated plot compartments along with standard varieties as checks. Selected plant types are critically evaluated for phenotype. |
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Third to sixth year |
Co-ordinated or main yield trials of selected ones using standard check for comparison are carried out at several locations just to confirm that selection could perform in the similar manner at different locations within an agro-climatic zone. |
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Seventh to eighth year |
Final trials for check of expected trait in developed variety at regional research laboratories. Mass selected variety is released if it is up to the expectation. Seeds are multiplied. They are named and packed for distribution. |
Advantages of Mass Selection:
- Varieties developed by this method have wide adaptability, wide genetic base and stable.
- It is rapid, simple and least expensive of all plant-breeding procedures. It doesn’t require any scientific knowledge.
- It retains considerable genetic variability.
- It is useful in meeting the need of the farmers
- It is the only method for improving local or wild varieties.
Disadvantages of Mass Selection:
- Variation exists in variety developed by mass selection. The phenotype of these varieties is not as uniform as pure line varieties.
- The selected character of crop exhibit segregation due to natural cross-pollination. Without progeny testing selected types will show segregation in the next generation, hence a test of homozygosity of plants progeny is always required.
- There is less improvement in the crop as compared to pure line selection. The yield depends on the environment.
- Limited use in the improvement of self-pollinated crops.
- Varieties are difficult to identify in the seed certification program than pure lines.
- It is only effective for characters with high heritability.
- Only those varieties which show genetic variation can be improved by this method, as it uses variability present in the population.
- It cannot bring any new change in the genotype.
- There is no control over pollination, which causes greater heterozygosity and as a result the desirable qualities gradually diminish. Also, the pollination may be both by superior and inferior pollens.
Pure Line or Single Plant Selection:
The pure line is the progeny of a single self- fertilized self-pollinated homozygous plant. It can be defined as the process of isolating a desirable homozygous individual from the mixed population and multiplying the same without contamination to release as a new variety. In pure line selection, large numbers of plants are selected from a self-pollinated crop and are harvested individually. The plants with the desired trait or traits are selected out of the variable population in the field. Seeds from selected plants and sown separately to produce progeny by self-pollination. This process is continued for many generations and plants with inferior characters are eliminated in each generation.
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Year |
Details |
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First year |
An old variety or land race is used for pure-line selection. The number of individual plants to be selected may vary from 200 to 1000 in various crops. |
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Second year |
The progeny of each selected plant is grown separately in few rows and evaluated for the character under consideration. The top 15-20 progenies are selected and seed of all plants in each progeny is bulked which constitutes strains. |
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Third Year |
The strains constituted in second years are evaluated in replicated field trials and top performing few strains are selected for further evaluation. |
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Fourth to seventh year |
The selected strains are evaluated in field trials for 2-3 years for yield performance. The best genotype is identified on the basis of yield performance. |
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Eighth to tenth year |
The best performing strain is released and notified as a variety. Then the breeder, foundation and certified seeds are produced. The production of certified seed takes two years after release of a variety. Thus, the seed of new variety reaches the farmers in tenth year. |
Wheat varieties like Kalyan – 27 and PV – 18 are developed by this method.
Advantages of Pure Line Selection Method:
- It is easier than hybridization and requires less skill.
- Due to extreme uniformly, it is easily identified in seed certification and is more supported by farmers and consumers.
- It achieves the maximum possible improvement over the original variety.
- The selected plant shows the desired character for several generations.
Disadvantages of Pure Line Selection Method:
- It is not useful in cross-pollinated crops because it is expensive, laborious.
- The breeder has to devote more time to pure line selection than mass selection. Thus it is time-consuming and laborious. It may take 10-12 years for the development of the new variety.
- There is an upper limit on the improvement in the new variety by the genetic variation present in the original population.
- The new variety doesn’t have wide adaptability and stability in production.
Clonal Selection:
A variety that is propagated vegetatively (asexual reproduction) from a single plant is called clone. Besides clonal selection, interspecific hybridization and mutation breeding are also used for the improvement of asexually propagated crops. These methods have been successfully used in sugarcane and potato.
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Year |
Details |
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First year |
A superior plants are selected from a mixed population of vegetatively propagated crop. Superior plants are selected on the basis of yield, maturity, disease resistance, etc. |
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Second year |
The progeny of each selected plant is asexually propagated and grown separately for seed increase and evaluation. |
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Third Year |
Clones superior for target variety are selected and evaluated in replicated preliminary trials. |
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Fourth to sixth year |
Several clones are evaluated in multi-location trial for 3-4 years using standard variety as a check. Clones which are superior to check variety in yield, disease resistance and quality are identified for release. |
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Seventh to tenth year |
The best clone is released as a variety and its seed is multiplied for distribution. Thus release of new variety by this method takes 9-10 years. |
Advantages of Clonal Selection:
- It is the only method to improve clonal crops.
- The desired clone can be obtained in less than 1 year.
- As there is no danger of deterioration due to segregation and recombination., varieties obtained are as stable as pure lines and easy to maintain.
- It helps in eliminating unproductive and undesirable types.
- This method is useful in conserving hybrid vigour for several generations.
Disadvantages of Clonal Selection:
- This method is applicable only to vegetatively propagated crops.
- In this method, no new genetic variability can be created.
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