Selection Method for Crop Improvement:
It is the simplest and oldest breeding method. It is also called as German method or German method of broad breeding because once it was used nicely in Germany for improving the sugar beets and small grains such as rye and wheat.
It can be defined as preservation of certain individual plants of desirable characters. In simplest form selection means choosing plants of one’s choice. It is the basis of all crop improvement. Even today it is most common method of crop improvement among the cultivators.
Types of Selection Method:
1. Natural Selection:
This is a natural process. It operates in the nature without human interference. According to the Darwin’s principle “Survival of the fittest” plants which survive through the adversities of nature are preferred and the weaker ones are wiped out. Thus, nature itself selects the fittest organisms.
So, natural selection favours these characters which are essential for survival of a species. The selection pressure ultimately resulted in the appearance of many differences between species and subspecies. Natural selection has given the cultivated crops and ‘ecotypes’ in plants.
2. Artificial Selection:
It can be defined as to choose certain individual plants for the purpose of having better crop from a mixed population where the individuals differ in characters. Here the selecting agent is man. Man exploits the variations existing among the species. He picks of a few plants of better qualities from mixed populations and tries to propagate them.
There are 4 methods of artificial selection:
(i) Mass selection
(ii) Progeny selection
(iii) Pure line selection and
(iv) Clonal selection,
(i) Mass Selection:
It can be defined as selection of a number of phenotypically superior plants heads or seeds from the field population, harvesting and bulking their produce together for sowing the next year’s crop and repeating this process till desired characters are achieved.
This is the simplest and oldest method of crop improvement practiced by farmers. Mass selection is based on phenotype (external characters) and the harvested seeds are composited without progeny testing.
It cannot bring any new change in the genotype. It can be practiced in both self-pollinated and cross-pollinated crops. In cross pollinated crops it is usually practiced for population improvement purposes, whereas in self pollinated crops it has only a limited use of maintaining purity of a variety.
Procedure:
Most vigorous plants from the mixed population of a crop are selected. These plants are thrashed together and a mixture of seeds is obtained. This mixture of seeds is a mass, and, therefore, it is known as mass selection.
The mixture so obtained is sown for raising the new crop from which selection is made similarly in the next year. This process of selection is continued till the plants show uniformity in the desired characters and they constitute a new variety.
A general procedure for developing a new variety by mass selection is as follows:
Merits:
(A) It is more of an art than a science because it needs no scientific knowledge.
(B) Simplest, easiest and quickest method of crop improvement. _
(C) Pollination need not to be controlled to provide a new variety.
(D) To meet the need of the farmers, it is the only method for improving the local or wild varieties.
Demerits:
(A) Importance is given to phenotypic characters only.
(B) There is no control over pollination, which causes greater heterozygosity and as a result the desirable qualities gradually diminish.
(C) It is not possible to increase the yield because:
(a) Importance is given to material characters only.
(b) Environmental effects cannot be separated out.
(c) Pollination may be both by superior and inferior pollens.
(D) This method of crop improvement is not applicable to self-pollinated crops (due to less amount of heterozygosity).
(E) In cross pollinated crops variety produced is heterozygous i.e., mixture of different genotypes.
(ii) Progeny Selection:
Progeny selection is commonly used in cross-pollinated and often cross-pollinated crops. Progeny (offspring) can be defined as new individual organisms that results from the process of sexual or asexual reproduction.
Progeny selection is the selection procedure in which superior plants are selected from a heterogeneous population on the basis of performance of their progenies. In other words, primary selection refers to selection of plants from a diverse population on the basis of their progeny test.
The test of genotypic value of an individual based on the basis of performance of its progeny is called progeny test. The test evaluates the breeding value of a singe plant by the performance of its progeny. In 1856, a French man Louis Leveaue de Vilmorin utilized the progeny test to increase the sugar content in the wild sugar beet, hence it is also known as Vilmorin principle.
Procedure:
Progeny selection is done in various ways but the simple method is ear-to-row method. This method was developed by Hopkins (1908) and is extensively used in maize.
Merits:
(i) This method is very simple and convenient.
(ii) In the identification of superior genotypes, this method is more effective than mass selection because it is based on progeny test and not on the phenotypes of the individual plant.
(iii) Inbreeding may be avoided if sufficiently large number and diverse progenies are selected.
Demerits:
(i) There is no control over pollination. Plants are allowed to open pollinate.
(ii) Certain methods of progeny selection are complicated.
(iii) If the progeny of each plant is tested in isolation, it would require lot of area which is not practically possible.
(iii) Pure Line Selection:
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.
The method of pure line selection has been developed from the classical work of W.L. Johanssen (1903), a Danish botanist, and is commonly used to improve the self- pollinated crops.
He worked on the Princess Variety of beans (Phaseolus vulgaris) and put forwarded pure-line theory. He first of all suggested the term pure line and defined it as the descendants of a single self-fertilized organism of homogeneous factorial composition.
Later on many authors defined the term pure line and some of the important definitions given are:
Progeny of a single individual obtained by selfing is called a pure line.
Sinnot, Dunn and Dobzhansky (1950)
A pure line is an inbred homogenic strain. Darlington and Mater (1952)
A strain of an organism that is pure genetically (homozygous) because of continued breeding through other means. Hayes, Immer and Smith (1955)
A strain in which all members have descended by self-fertilization from a single homozygous individual. Poehlman (1959)
Simply pure line can be defined as:
A group of plants all obtained from a single self-fertilized homozygous plant.
Or
A strain made up of the progeny of a single self-fertilized homozygous individual. Johanssen (1903) isolated 19 different lines on the basis of seeds weight (large and small seeds) from the original seeds of Princess variety and planted them separately( Beans are self-fertilized; the seeds in the original lot were homozygous, for genes affecting seed weight. Selection within the original mixed lot of beans was effective in isolating lines that were genetically different).
Seeds obtained from these plants were analysed for size and weight. He found that seeds obtained from each plant varied in size and weight but on average the large seeds parents produced progeny with heavier seeds and the smaller seed parents give rise to plants with smaller and lighter seeds. He observed that a pure line (Pure Line No. 1) produced beans averaging 0.64 g in weight (Fig. 3).
Another pure line (Pure Line No. 19) produced beans averaging 0.35 g in weight. The average seed weights of progenies of beans selected from Pure Line No. 1 were similar to those of the parent line. Likewise, progenies of seeds selected from Pure Line No. 19 were similar to their parent line in average seed weight.
On the basis of his experiments he concluded that:
i. Selection with in the original lot of beans was effective because the plants have genetic variation.
ii. Continuous breeding leads to homozygosity.
iii. Variations in the pure line results from environmental factors only.
iv. Selection within a pure line is not effective because all the plants in a pure line have exactly the same genotype.
Procedure:
Generally a period of 10 to 11 years is required to produce a variety by pure line selection in cross pollinated crop.
It is as follows:
Merits:
1. This is the only method to improve the local varieties of self-pollinated crops. Best genotype for yield, disease resistance, insect resistance, earliness, quality etc. can be isolated from a heterogeneous or mixed population of an old variety.
2. This method is easier than hybridization (emasculation and crossing over)
3. The variety developed by this method is extremely in appearance and performance and, therefore, are more attractive.
4. This method is also used both in self and cross-pollinated crops for production of pure lines and inbreeds.
Demerits:
1. It is very lengthy and laborious process.
2. This method is applicable to self-pollinated crops. It cannot be used for development of varieties in cross-pollinated crops.
3. This method can isolate only superior genotypes from the mixed population. It cannot develop new genotypes.
4. Extremely homozygosity may result in low yield and other undesirable characters.
5. Due to high degree of homozygosity, variations among the varieties are also limited. Therefore, their adaptability to varied environmental condition is also poor.
(iv) Clonal Selection:
A variety that is propagated vegetative from a single plant is called clone or progeny of a single plant obtained by asexual reproduction is known as clone or all the vegetative progenies of a single plant are called a clone.
A clone may be defined as a group of individuals of like genotypic composition traceable through asexual reproduction to a single ancestral zygote. Selection of desirable clones from the mixed population of vegetative propagated crops is known as clonal selection.
Or
Clonal selection is the selection and propagation of the desirable variations between the clones as well as within a clone. Clonal selection generally may be practiced with non-flowering or those species which produce seeds poorly or only under special conditions.
Generally, these species are divided into four categories. These are:
(i) Non flowering species:
Garlic, ginger, betel etc.
(ii) Low seed setting species:
Sugarcane, potato, sweet potato etc.
(iii) Normal flowering and seed setting species:
Citrus, mango, pear, peach apple, litchi, loquat and many ornamental plants (to maintain the heterozygous balance vegetative reproduction is essential).
(iv) Apomictic species:
Agave, Onion, Lilium, Allium, Jasminum, Roses, Bryophyllum etc. (no seeds produced when apomixis occurs).
Characters of Clone:
(i) All the individuals of a clone are genotypically and phenotypically identical.
(ii) Genetically, all the members of a clone are homogeneous and heterozygous.
(iii) Clones are as stable as pure lines.
(iv) A clone is also multiplied vegetatively in future generation.
(v) Mutations is the only means of creating variability.
Procedure:
Selection of clones is done from mixed population of vegetatively propagated crop. However, the unit of selection differs from crop to crop for e.g.,
Stem Cutting:
Sugarcane, betel vine, black pepper, fodder grasses, some ornamental and hedge plants.
Grafts and buds:
Mango, citrus, apple, many fruit trees, rose, and many ornamental plants.
Tuber:
Potato.
Suckers:
Banana, Pine-apple, aloe, agave, Chrysanthemum.
Bulbs:
Onion, Garlic, tulip, lilies.
Runners:
Oxalis, Doob grass.
Stolons:
Mint, Mentha etc.
Rhizomes and Corms:
Ginger, Canna, Turmeric, Colocasia, etc.
Root and root cuttings:
Sweet potato, Dahlia, Asparagus, mango ginger etc. The year-wise scheme of clonal selection is as follows:
The variety produced is stable possessing all the original characters of the parental clone as such.
Merits:
i. It is the only method to improve the clonal crops.
ii. It offers an opportunity to exploit desirable mutations in somatic parts of plants.
iii. It also helps to eliminate unproductive and undesirable types.
iv. This method is helpful in conserving hybrid vigour for several generations.
v. Varieties are as stable as pure lines and easy to maintain because there is no danger of deterioration due to segregation and recombination.
Demerits:
(i) In this method no new genetic variability can be created.
(ii) This method is applicable only to vegetatively propagated crops.
Achievements:
(i) New varieties of potato like Kufri red and Kufri safed; Pedda Nelum in mango- Pidi Monthan; Bombay green, high gate in Banana etc. 