The following points highlight the four main of plant breeding methods. The methods are: 1. Mass Selection 2. Pure Line Selection 3. Recurrent Selection 4. Clonal Selection.
Method # 1. Mass Selection:
When a large number of plants of similar phenotype are selected and their seeds are mixed together to constitute a new variety is called mass selection. The population obtained from the selected plants would be more uniform than the original population.
Purpose:
In case of self-pollinated crops, mass selection procedure has following objectives:
(a) Improvement of Local Varieties:
In local varieties sometimes there are mixtures of several types which may differ in flowering maturity time, disease resistance, plant height, etc. Elimination of poor quality plants help to get uniform performance of the variety.
(b) Purification of Existing Pure-Line Varieties:
Maintenance of purity of the existing pure line varieties is done through this method. The pure line varieties sometimes tend to become variable with time due to mechanical mixtures, natural hybridisation and mutation. Through regular mass selection the purity of the pure line varieties is maintained.
(c) Production of New Varieties from Heterogeneous Local Land Races:
By increasing the frequency of superior genotype, the population character can be changed. This change is a function of heritability and the number of genes conditioning the trait under selection.
In case of cross pollinated crops, inbreeding must be avoided as it leads to loss in vigour and yield. But in mass selection several plants are selected and their seeds are mixed together to raise the next generation, so inbreeding is avoided or kept minimum.
Procedure:
The process involves selection or retention of a large number of individual plants on the basis of superior phenotype, compositing the harvested seeds without progeny-testing, the mixture of seeds is sown in mass to raise the next generation. This process continues till the desired level of uniformity is achieved.
The procedure for mass selection may be outlined in brief as follows:
First Year:
A large number (200-2000) of plants are selected on their performance and phenotypic characters like vigour, plant type, disease resistance or other desirable characteristics. The seeds are composited together.
Second Year:
The composite seeds are planted in a preliminary yield trial along with standard varieties as checks as well as the variety from which the selection is made to determine whether there is any improvement or not.
Third to Sixth Year:
The composite seeds from previous year collection are evaluated in coordinated yield trial in several locations within the same agro climatic zone. Initial evaluation trial for one year then uniformity regional trial for two or more years is done.
Seventh Year:
If the result is suitable for cultivation and performance is well then certification of seeds is necessary and released by central or state variety release committee.
The procedure of mass selection whenever is coupled with progeny testing then that helps in maintaining the purity of pure line varieties. For this procedure, in the second year the individual plant progenies are planted individually and poor performers are rejected.
The remaining progenies are mixed together to constitute the variety. The process may be repeated every few years to keep the variety pure as often as it is found necessary.
The modified procedure of mass selection is represented below schematically:
Merits and Demerits Merits:
1. Since a large number plant is selected, the variety is more stable in performance over different environments as it is more adapted than a single pure line. So the varieties developed through mass selection are more widely accepted than pure line.
2. Extensive and prolonged field trials are not necessary. This reduces the time and cost needed for developing a new variety. This method is also less labour consuming.
3. Mass selection retains considerable genetic variability, so another mass selection after few years improves the variety.
4. This method can be applied to cross pollinated crops.
Demerits:
1. The varieties developed through mass selection show variation and are not uniform as pure line varieties, and strict selection for uniformity would lead to inbreeding depression.
2. The improvement of variety through mass selection is poorer than through pure line selection.
3. When the mass selection is done without progeny test, the homozygosity of the selected plants is not assured. In the self-pollinating species as there are chances for cross pollination to some extent, some plants must be heterozygous. In this method the genetic superiority either may be due to environment or due to genotype.
4. Pure line selection is more used than mass selection, only improvements of local old varieties are done through this method.
5. In seed certification programme, it is more difficult to identify the mass selected variety than the pure line.
6. This method utilizes only the variability already exists in the population, improvement is done only through selection. So the limitation is that it cannot generate new genetic variability.
Achievements:
Mass selection is effective when the population has the following characters:
(a) High genetic variability for different traits.
(b) The characters should be highly heritable in nature.
(c) The crop is grown under low population density.
(d) If only one particular character is chosen.
At present use of mass selection is limited to purification of pure line varieties of self-pollinated crops, because the superiority of the pure lines would be lost quickly if their purity is not maintained through mass selection.
Although no proper records are available, but the following varieties in different crops have been developed by mass selection in India:
Maize — Jaunpur local, Tinpakhia, Basri
Pearl Millet — Bajra-207, Bajri-28-15, Bichpuri local, Pusa moti
Mustard — Gurgaon brown sarson
Sorghum — RSI, T22
Cotton — Dodahatti local, Dharwar American, Combodian cotton
Peanut — TMV-1, TMV-2
Except these there are large numbers of examples of mass selection in maize, sugarbeet, alfalfa, red clover available in other countries.
Method # 2. Pure Line Selection:
In this case, a large number of plants are selected from a population of self-pollinated crop, harvested individually and their individual progenies are evaluated and the best progeny is released as a pure line variety. So the pure line is a progeny of a single homozygous self-pollinated plant.
Purpose:
In self-pollinated crops pure-line selection method has several applications:
(a) Improvement of Local Varieties:
It is more favourite method of improvement of local varieties which have considerable genetic variability.
(b) Pure Line Selection for Introduced Varieties:
Introduced varieties are often subjected to pure line selection to develop suitable varieties for that place.
(c) Improvement of Old Pure Line Varieties:
Pure lines become variable with time, i.e., off type plants appear in pure line. From such genetically variable pure lines often again pure line selection method is applied to make it more stable.
(d) Selections for New Characteristics in a Pure Line:
Sometimes new favourable characters which were not considered previously can be selected from the existing variety through pure line selection method.
(e) Pure Line Selection in Hybridization Programme:
Pure line selection method is most applicable in hybridization programme, i.e., selection from segregating generations of crosses. The pure lines in such cases may be selected using pedigree, bulk or backcross methods.
Procedure:
The general procedure of pure line selection involves three steps:
1. Selection of individual plants is done from a local variety. The superior and similar observations are selected assuming those are homozygous.
2. Visual evaluation of individual plant progenies is performed and the superior progenies are harvested separately.
3. Replicated yield trials are carried out for critical evaluation of the progenies. The most promising strain is identified and released as variety.
The procedure for pure line selection may be outlined in brief as follows:
First Year:
A large number of plants (200-3000) are selected from ‘desi’ or local variety or any other mixed population. The seeds are harvested from individual plant separately. The selection is made on easily observable characters like flowering, maturity times, disease resistance, presence of awns, plant height, etc.
Second Year:
Progenies from individual selected plants are grown separately with proper spacing. The progenies are observed visually and poor, weak or defective progenies are rejected, selection is made for very much desirable characters. Disease epiphytotics may be created to test the progenies for disease resistance. If necessary this process may be repeated for one or more years.
Third Year:
This step consists of replicated yield trials for critical evaluation of the progenies. The selected progenies are planted in replicated trial using the best variety as check. If enough seeds are available then preliminary yield trial may be conducted.
Fourth Year:
Replicated yield trials are conducted by the breeder using the best variety as check. The promising strains are observed and harvested for coordinated yield trial for further evaluation.
Fifth to Eighth Year:
The promising strains are evaluated at several locations along with other strains. The most promising strains are identified and released as new varieties.
Ninth Year:
The best progeny seeds are multiplied and released as a new variety through seed certification.
Schematic representation of pure-line selection in self-pollinated crops:
The total time required for the whole programme may vary depending upon several factors. If enough seeds are available in the second year then preliminary yield trial is done in the third year. Depending on the performance in the coordinated multi-location yield trial the time period can be lessened.
Merits and Demerits:
Merits:
1. Pure line selection can achieve maximum possible improvement over the original variety. This is because the variety is the best pure line present in the population.
2. The pure line variety is extremely uniform since all the plants in the variety have the same genotype. Such uniformity is liked both by the farmers and the consumers.
3. Due to extreme uniformity, the variety is easily identified in seed certification programme.
Demerits:
1. As the pure line variety is genetically more homogeneous, the adaptability to various environments is less.
2. This procedure requires more time, space, labour and more expensive yield trials.
3. The upper limit of improvement is set by the genetic variation present in the original population.
4. Once a pure line is developed, the variability is frozen and further selection becomes less effective.
Achievements:
Pure line selection is the most extensively used breeding method in early days of crop improvement in India because at that time many genetically variable ‘desi’ varieties were available.
A large number of improved varieties were developed by this method in many self-pollinated crops:
Wheat — NP4, NP6, NP12, Pb-8, CI3, K46
Rice — M-351, Vidisa 60-1, Patni 6, Aispuri, BP 53
Cotton — Coimbatore 2, Gadag 1, MCU1
Barley — C-251, C-50, K-12
Tobacco — NP28, NP63, NP70
There are many more examples of achievements through pure-line selection in India as well as in abroad. Apart from the above examples there are many more pure lines available in case of pulses like Cicer arietinum, Vigna radiata, Vigna mungo, Arachis hypogea, oil yielding Brassica (both ‘rai’ and ‘toria’), jute and in many vegetables also. For long time many pure line varieties are dominating in Indian agriculture.
Method # 3. Recurrent Selection:
This selection method is a variation of progeny selection, the main difference is that the selected progenies are allowed for all possible intercrosses through open pollination to provide all kind of recombination. So the method can be defined as reselection generation after generation with interbreeding of selects to provide for genetic recombination.
There are four main types of recurrent selection:
(a) Simple Recurrent Selection:
In this method a number of plants with desirable phenotypes are selected and self-pollinated. In next generation the progenies from the selected plants are grown separately, and are allowed to intercross in all possible combinations. Equal amount of seeds from each plant are composited to produce the next generation. This completes the original selection cycle.
For recurrent selection several desirable plants are selected from the composite population and are self-pollinated. Progeny rows are grown and all possible intercrosses are made by hand. Equal amount of seeds from all the intercrosses are composited to produce next generation. This is the first recurrent selection cycle. This population may be subjected to further more recurrent selection cycle in the same way.
Recurrent selection is effective in increasing the frequency of desirable genes in the population. It is most suited for characters with high heritability. Simple recurrent selection is considerably more efficient than selection with self-pollination.
(b) Recurrent Selection for General Combining Ability:
This method is based on the combining ability of the lines or plants. In this system a number of superior plants are selected from the source population, the selected plants are called S0 plants, which are selfed and crossed to heterozygous tester of broad genetic base. This selfed seed is kept in cold storage.
The crossed seeds of S0 plants with tester is used to evaluate the combining ability of various S0 plants. Now the S0 plants with good performance are grown in third year from the selfed seeds kept in storage. Those are intercrossed in all combinations and composite of intercrossed seed is then used to establish a new population for further selection. This cycle may be repeated.
(c) Recurrent Selection for Specific Combining Ability:
This method is same as recurrent selection for general combining ability except that the tester is an inbred line, i.e., the tester here is with narrow base. So here the differences in the performance of S0 plants in crosses are due to specific combining ability alone. Great care should be taken in inbred line selection which will be used as tester, if it is of inferior quality then the whole scheme will fail.
(d) Reciprocal Recurrent Selection:
This is the method to improve simultaneously two populations from two diverse sources, A and B, one population serves as a tester for the other. Both the populations are selfed as well as out-crossed with the other, the superior progenies are selected.
Method # 4. Clonal Selection:
A clone may be defined as a group of plants derived from a single plant by vegetative propagation.
The clone can be characterised by the following characters:
(a) Clone is homogeneous,
(b) Individuals of a clone are either homozygous or heterozygous,
(c) Clones are stable in nature,
(d) Variability can be induced through mutation,
(e) Clone is propagated vegetatively.
Normal stem, runner, sucker, stolon, tuber, rhizome, bulb and root or root cuttings can be used as clones.
The procedure of clonal selection is the selection of desirable clones from the mixed population of vegatatively propagated plants. The technique lies in selecting and propagating the best clone based on its performance.
The steps may be summarised as follows:
1. The collection of all possible clonal variability.
2. Critical evaluation of each clone and each member of a clone for yield and quality following test for disease and pest infestation.
3. The disease free and high yielding clones are selected and evaluated and multiplied as variety.
Advantages of clonal selection are:
1. Easy Maintenance:
In this method there is no chance for out-crossing or loss of seed viability.
2. Very Quick:
As there is no time requirement for seed development, single clone can be identified very easily and can be multiplied straight way to give new variety.
3. Permanent Hybrid:
Heterotic clones can be exploited for any length of time without the need to produce hybrid seed as in seed crops.