In this article we will discuss about:- 1. Meaning and Features of Mass Selection 2. Types of Mass Selection 3. Modifications 4. Procedure 5. Merits and Demerits.
Meaning and Features of Mass Selection:
Mass selection is one of the oldest methods of crop improvement. In this method, individual plants are selected on the basis of phenotype from a mixed population, their seeds are bulked and used to grow the next generation. In other words, it is a method of breeding in which individual plants are selected on the basis of phenotype from a mixed population, their seeds are bulked and used to grow the next generation.
The main features of varieties developed by mass selection in self and cross pollinated species are given below:
1. Application:
Mass selection is applicable to both self and cross pollinated species. However, it is more commonly used in the improvement of cross pollinated crops than in self pollinated species. This method is rarely used in vegetatively propagated crops.
2. Genetic Constitution:
In self pollinated crops, a mass selected variety is homozygous but heterogeneous, because it is a mixture of several pure-lines. In cross pollinated crops, such varieties are mixture of homo and heterozygotes and are heterogeneous, because they consist of several homo and heterozygous genotypes.
3. Adaptation:
Mass selected varieties have wide adaptation and are more stable against environmental changes due to heterogeneity which provides better buffering capacity. In other words, mass selected varieties have broader genetic base than pure lines. They exhibit more or less stable performance. However, adaptability is more in cross pollinated crops than in self pollinated species.
4. Variation:
They are composed of several pure lines in self pollinated crops and of several homo and heterozygous genotypes in cross pollinated crops. Hence there is heritable variation in the mass selected varieties, besides environmental variation. The heritable variation provides them good buffering capacity.
5. Selection:
Selection is effective in case of mass selected varieties of self pollinated crops due to presence of heritable varieties. However, further selection in the mass selected varieties of cross pollinated crops may lead to inbreeding depression.
6. Quality:
A variety developed by mass selection is less uniform in the quality of seed than pure-lines due to presence of heritable variation.
7. Resistance:
Mass selected varieties are less prone to the attack of new diseases due to genetic diversity. In other words, they are more resistant or tolerant to new diseases.
8. Roguing:
Periodic removal of off type plants is essential to maintain the yield of mass selected varieties.
Types of Mass Selection:
There are two types of mass selection, viz.:
1. Positive mass selection, and
2. Negative mass selection.
These are defined below:
1. Positive Mass Selection:
When desirable plants are selected from a mixed population and their seeds are mixed together to grow further generation, it is referred to as positive mass selection. This process is continued for several years. Generally, old varieties or land races are used as the base material for mass selection. Selection of desirable plants or positive approach is in common use in mass selection.
2. Negative Mass Selection:
When only undesirable off type of plants are removed from the field and rest are allowed to grow further, it is known as negative mass selection. This is generally used for varietal purification in seed production and certification programs. This helps in maintaining high level of genetic purity in the varieties especially in the self pollinated species.
The success of mass selection mainly depends on three factors, viz.:
(i) Variability in the base population,
(ii) Mode of inheritance of character to be improved, and
(iii) Heritability of the character.
Mass selection is more successful in old heterogenous variety or land races than in improved varieties.
Modifications of Mass Selection:
Mass selection is practiced both in self and cross pollinated crop plants. However, it is more common in cross pollinated species than in self pollinated species.
There are two defects of mass selection as given below:
(a) No Control on Pollination:
In mass selection there is no control on the pollination. The selected plants are pollinated both by superior and inferior pollen parents.
(b) Selection is based on Phenotype:
The phenotypic performance is greatly influenced by environmental factors such as soil heterogeneity.
In order to overcome these defects, three modifications of mass selection have been suggested in cross pollinated species.
These modifications are:
(i) Rejection of inferior pollen plants,
(ii) Use of composite pollen, and
(iii) Stratification of the field.
A brief description of each method is presented below:
(i) Rejection of Inferior Plants:
In this method inferior pollen parents are removed before flowering and rest are allowed for inter-mating. This controls pollination by inferior plants.
(ii) Use of Composite Pollen:
In this method, pollen is collected from the selected plants and bulked. This composite pollen is used to pollinate the selected plants. This also controls pollination by inferior plants.
(iii) Stratification of Field:
This is known as grid method of mass selection and was suggested by Gardner (1962). In this method, the field in which mass selection is to be practiced is divided into small plots in such a way that each plot should have 40-50 plants. Now superior plants are selected in each small plot. This eliminates the effect of environment [soil heterogeneity] because small plots are more homogeneous than larger plots.
The success of mass selection is more with oligogenic recessive characters than with polygenie characters. In case of polygenic traits, large number of individual plants should be selected in each generation to tap maximum desirable alleles. Moreover, mass selection is more effective for characters having high heritability. Generally, oligogenic characters have higher heritability than polygenic characters.
In cross pollinated species, the progeny of each cycle of mass selection should be grown in isolation to prevent contamination with other varieties (mating types) of a crop. The isolation distance differs from crop to crop depending upon the agency of pollination.
Generally, more isolation distance is required for wind pollinated species than insect pollinated ones. In general, isolation distance of 300 metres in case of insect pollinated species and 500-1000 metres for wind pollinated species is sufficient. In self pollinated species distance of 5-10 metres is adequate.
Procedure of Mass Selection:
Mass selection consists of various steps, viz., selection of base population, selection of desirable plants from base population and mixing their seeds to rise next generation, evaluation in field trials, and releasing as a new variety.
The general procedure of mass selection is given below:
First Year:
An unimproved old variety or land race is used as a base population which is grown in a large plot. Then individual plants (55-1000) are selected on the basis of phenotypic performance for characters like height, maturity, disease resistance, productivity etc. The selected plants are harvested at maturity and their seeds are mixed together to grow next generation. This process is repeated till desirable results are achieved.
Second Year:
The crop is grown from the bulk seed of selected plants in a separate field using standard variety as a check for comparison of performance. In other words, the material is evaluated in preliminary yield trial. If mass selection is used for purification of old mixed variety, the same old variety can be used as check for comparison.
Third to Sixth Year:
The performance of bulk is evaluated for yield and adaptation in main yield trials for 3 to 4 years using standard check for comparison.
Seventh and Eighth Year:
The variety is released and named in seventh year and seed is multiplied. In the eighth year the seed is ready for distribution.
Merits and Demerits of Mass Selection:
Merits:
(i) This is a good method for improvement of old varieties and land races. This is also used for the purification of improved varieties.
(ii) Mass selected varieties are more stable in their performance than pure-lines. In other words, they have more buffering capacity than pure-lines due to heterogeneity.
(iii) Mass selected varieties provide good protection against diseases.
(iv) Mass selection is a simple and quick method of crop improvement. It takes about 8 years for the release of a new variety, whereas pure-line selection takes about 10 years in the development of new variety.
(v) This method is applicable to both self and cross pollinated species.
Mass selection has played more significant role in developing new varieties in cross pollinated species than in self pollinated species.
In India, mass selection has been useful in the development of improved varieties in cross pollinated crops like maize, pearl millet and mustard, and in often cross pollinated species like cotton and sorghum. In self pollinated species it has been rarely used because pure line selection is more effective in such crops than mass selection.
Demerits:
(i) The selection is based on the phenotypic performance. The superior phenotype is not always an indication of superior genotype. The real breeding value of single plants can be judged from the performance of their progeny. Progeny test is not carried out in mass selection.
(ii) In cross pollinated species, there is no control on the pollination. The selected plants are pollinated by both superior and inferior pollen parents. This results in rapid deterioration of variety developed by mass selection.
(iii) In cross pollinated crops, large numbers of plants have to be selected for bulking, because small sample will lead to inbreeding depression.
(iv) The produce of varieties developed by mass selection is less uniform man pure lines. This is because mass selected varieties are mixture of several pure lines in self pollinated crops and consists of several genotypes in cross pollinated species.
(v) In self pollinated species, pure line selection is more effective than mass selection, because pure line selection leads to isolation of best line from a mixed or heterogeneous population.
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