Breeding a New Genetic Plant Variety: 5 Steps | Plant Breeding

The main steps in breeding a new genetic variety of a crop are: 1. Collection of Variability  2. Evaluation and Selection of Parents  3. Cross Hybridization among the Selected Parents  4. Selection and Testing of Superior Recombinants  5. Testing, Release and Commercialization of New Cultivars.

Step # 1. Collection of Variability:

Genetic variation is the source of raw material for selection. Variation means differences among individuals of a population or species for a specific character. Genetic variations are heritable. Heritable variation is due to genotype and is transmitted from one generation to other. Such variations are useful in selection.

Genetic variations can be created by:

In many plants pre-existing genetic variability is available from wild relatives of the crop. Such natural existing genes, available in wild varieties, species and relatives of cultivated species. Only on availability of such genes, effective exploitation can be done for breeding programmes. Germplasm collection is the entire collection of (plants/seeds) having all the diverse alleles for all genes in a given crop.

Step # 2. Evaluation and Selection of Parents:

Selection must have started with the primitive man changing his mode of life from a nomad to an agriculturist. Unconsciously with the desire to ensure himself of the seed to sow a further crop, the primitive agriculturist must have practiced selection.

It is an important and essential operation in all breeding programmes. It is also the oldest breeding method and is the basis of all crop improvements. The selection was entirely based on phenotype in the pre-Mendelian era as nothing was then known about genotype.

Most of the best phenotypes selected by the breeder being mostly heterozygous, did not give the same results or even deteriorated due to segregation in the next generation. This compelled the breeder to make a new selection of the superior at every sowing. And moreover sometimes the superior phenotype was due to the favourable environmental conditions and not hereditary and thus the desired results could not be obtained.

The germplasm is evaluated so as to identify the plants with desirable combination of characters. The selected plants are multiplied and used in the process of hybridization. Purelines are created wherever desirable and possible.

Selection in Self-Pollinated Crops:

Self pollinated crops gradually become homozygous. Best homozygous individuals can be easily isolated in such crops. By selection, pure lines are produced (e.g., HUW – 468). Pure-line represents the progeny of homozygous plant. Due to self-pollination, seeds of such crops can be used year after year. Any variation, if present is due to environmental change.

Selection in Cross-Pollinated Crops:

Cross-pollinated crops are heterozygous. Such crop contains mixture of many genotypes. Some plants are with superior genotype; still others are with inferior genotype. Selection in such plants, targets at enhancing the frequency of superior genotypes with reducing the heterozygosity. Selection is a continuous process here due to presence of several types of genotypes.

Types of selection:

Selection can be divided into two types:

(a) Natural Selection, and

(b) Artificial Selection.

(a) Natural Selection:

It has resulted in evolution and occurs in nature. Here the fittest survives and remaining ones disappear. Due to this natural selection, our cultivated crops and ecotypes in plants have originated. It is a continuous process and is operating in nature as one of the natural factors. Artificial selections and hybridization are based on various ecotypes i.e., climatic or regional races.

(b) Artificial Selection:

Here the various types of plants are selected from a mixed bulk by plant breeders. This type of selection is also called artificial selection and can be defined as Artificial selection is to choose certain individual plants for the purpose of having better crop from a mixed population where the individuals differ in characters.

Step # 3. Cross Hybridization among the Selected Parents:

Hybridisation in animals is known for over 2000 years. Cotton Mather (1716) noticed first artificial plant hybrid in corn. Joseph Kolreuter (1760), a German botanist carried out first hybridisation experiments in tobacco. It was after the work of Mendel in 1900; principles for mechanism of inheritance in plants were understood. Now hybridisation has become one of the most commonly used and best methods of crop improvement.

Hybridisation is the crossing of two plants differing from each other genotypically in one or more traits.

Selection cannot bring out anything which is not already in genes. It only helps to isolate the good genes but fails to make the genes better. Since useful characters are usually present scattered in different races, varieties etc., only by hybridisation it is possible to combine all the characters in a single variety.

Its main objectives are:

(i) To produce a single variety having combination of good characters.

(ii) To exploit and utilize the hybrid vigour.

(iii) To increase and generate the genetic variations through recombinations.

Hybrid Vigour or Heterosis:

The term heterosis was coined by Shull (1914) and in ordinary use hybrid vigour and heterosis are practically synonymous. Hybrid vigour is the superiority of the hybrid over either parent in one or more traits. Vigour in growth, yield, resistance to diseases, pests is agriculturally important.

Heterosis has been exploited at commercial scale both in plants and animals. Heterosis is being exploited in crop plants, ornamentals and fruit crops. In vegetatively propagated plants, heterosis once achieved can be maintained easily. Heterosis is lost by inbreeding. To maintain optimum heterosis, seeds must be produced every year by crossing the pure parental lines, which are constantly maintained.

Technique of Hybridisation:

Hybridisation can be done only by skilled hands. Artificial crossing is necessary for hybridisation. It can be studied under the following headings:

Selection and Isolation of Parents:

Here two healthy plants with desirable characters are selected. Then parent plants are grown separately on isolated plots to avoid cross pollination. Self pollination is carried out in both the parents to produce inbreds. Self pollination is done for 6-8 generations till the majority of plants become homozygous and true breeding. The last generations of both the parents are used for further steps of hybridisation.

Emasculation (Fig. 9.13):

Emasculation is the removal of stamens from one of the parents (if bisexual) before they release their pollen grains. It is done by following methods:

(i) Hand Method:

In this method young buds are opened and stamens are plucked off by using forceps or scissors. This method is generally applicable in plants with large flowers. For small flowered plants this method is tedious and trouble making.

(a) Hot Water or Alcohol Method:

If the flowers are too small and difficult to hand pick, emasculation is done by this method. In this method the stamens or male panicles (inflorescence) are immersed in hot water having temperature at 42°-48°C or in ethyl alcohol for some time mostly 3 to 10 minutes.

Importance of Emasculation:

It prevents the process of self pollination.

(b) Suction Pressure Method:

In this method suction pressure is applied to emasculate the flowers.

(c) Bagging (Fig. 9.14):

Soon after emasculation, the flowers are covered by polythene bags to prevent cross-pollination by undesired pollen grains. In some cases even non-emasculated flowers of the other parent plants are also covered by polythene bags to prevent contamination by unwanted pollen grains. The bags are tied at the base of flower or inflorescence, as the case may be, by threads, copper wires or pins.

(d) Collection of Pollen Grains:

At maturity pollens from other non-emasculated selected parent plants are collected in dry bags.

(e) Crossing:

Fresh pollen should be used for crossing. In wheat and oats the pollen viability is only of few minutes. When the stigma of emasculated flower matures, the polythene bag is removed and stigma is dusted with stored pollen grains. It is advisable to perform crossing early in the morning as in most of the crops stigma becomes receptive at different times in the morning.

(f) Tagging:

A tag with relevant information is attached with plants.

A lag bears the information like:

(i) Date of emasculation,

(ii) Field record number,

(iii) Date of pollination and crossing, and

(iv) Details of male and female parents.

Step # 4. Selection and Testing of Superior Recombinants:

Newly developed variety is checked for its yield, quality, disease and insect resistance before it is released to farmers. The selection process is crucial to the success of the breeding objective and requires careful scientific evaluation of the progeny usually more than one superior progeny plant may become available. Such plants are self pollinated for many generations to get homozygosity. Due to this plant attains a state of uniformity and characters do not segregate in forthcoming generations.

Such tests are carried by ICAR (Indian Council for Agricultural Research) in India. Such evaluation is done at different stations in different regions of India. In every agro-climatic zone, every crop is evaluated for minimum three years. A new pure line, population or hybrid is released after such vigorous testing. Variety release committee recommends the release of improved variety under new name with permission of Government.

Step # 5. Testing, Release and Commercialization of New Cultivars:

The newly selected lines are evaluated for their yield and other agronomic characters of quality, disease resistance etc.

Seed, here means any plant part used for multiplication in plant like grains in wheat, potato tuber, sugarcane stem etc. Grains used for raising the crops are called seeds. Seed with superior characters is called improved seed.

Characters present in improved seed should be:

(i) High purity.

(ii) High germination percentage.

(iii) Free from weed seeds.

(iv) Free from diseases.

Quality of seed is certified by an agency. Such seed is called certified seed. NSC (National Seed Corporation) was established by Government of India to carry on such works.