In mass selection plants are selected on the basis of
Answers
Explanation:
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.