Essay on the Various Plant Breeding Methods

A Generalized Classification of Selection Procedure:

1. Intra-population selection based on additive variance

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(a) Based on sample size and degree of inbreeding

(i) Individual or pure-line or pedigree selection

(ii) Mass pedigree selection

(iii)Mass or bulk selection

(b) Based on family structure and relatives

(i) Individual selection

(ii) Family selection

(iii) Within-family selection

(iv) Combined selection

(c) Based on number of traits selected

(i) Tandem selection

(ii) Independent culling level

(iii)Index selection

2. Inter-population selection based on non­additive variance

1. Reciprocal recurrent selection

2. Recurrent selection for gca

3. Recurrent selection for sca

II. Stabilizing selection.

III. Disruptive selection.

Plant Introduction:

Taking a genotype or a group of genotypes of plants into a new environment where they are not being grown before.

The materials are introduced from other countries or continents.

Movement of crop variety from one environment into another within the same country is also introduction.

e.g. within the country introduction are the wheat in West Bengal and rice in Punjab.

Classification:

(i) Primary introduction:

When the introduced variety is well suited to the new environment it is released for commercial cultivation without any alteration in the original genotype.

e.g. dwarf wheat variety Sonar 64, Lerma Rojo dwarf rice variety – T.N. I, IR8, IR-28.

(ii) Secondary Introduction:

The introduced variety may be subjected to selection to isolate a superior variety. Alternatively it may be hybridized with local variety to transfer one or few characters from this variety to the local ones.

e.g. Kalyan Sona and Sonalika wheat variety selected from materials introduced from CIMMYT, Mexico.

Procedure for Plant Introduction:

1. Procurement of Germplasm

i. Gift, Purchase or Collection

ii. Propagules

2. Quarantine

i. Plant produce imported for consumption

ii. Plant propagules imported for cultivation and Research

iii. Quarantine procedure

3. Cataloguing

4. Evaluation

5. Multiplication

6. Distribution

Purpose or Objectives of Plant Introduction:

i. Provide an entirely new crop species e.g. Maize, Potato, Tobacco.

ii. Directly released as New varieties e.g. Sonara 64, Larma Rojo

iii. Utilization in crop improvement programmes e.g. Pusa Ruby tomato was derived from Meeruty x Sioux

iv. Saving a crop from a disease or pest coffee was introduced in South America from Africa to prevent losses from leaf rust.

v. Utilisation in scientific studies.

vi. Used for Aesthetic value.

Merits:

i. Provides entirely new crop plants.

ii. Provides superior variety.

iii. Introduction are the only feasible means of collecting germplasm and to protect variability from genetic erosion.

iv. Very quick and economical method of crop improvement.

v. Crop species may be introduced in new disease from areas to protect them from damage e.g. coffee and rubber.

Demerits:

i. Weeds entering through plant introduction e.g. phyllaries minor.

ii. Diseases: Late blight of potato from Europe, coffee rust from Srilanka.

iii. Bunchy top of banana from Srilanka.

iv. Insect pest. Potato tuber moth from Italy, wolly aphids of apple.

v. Ornamental turned weeds Water hyacinth, lantana camara.

vi. Threat to ecological balance: Eucalyptus spp introduced from Australia causes a rapid depletion of the sub soil water reserves.

Selection:

Selection operates only genetic variability occuring in plants or crops.

1. Mendelian variation:

All variations occur through gene mutation, macromutation => maize, Brassica micromutation => e.g. Rice, barley, tomato.

2. Interspecific hybridization:

=> Less contribution to evolution => sterile progenies

=> Introgressive hybridization => importance for evolution

e.g. Tripsacum x primitive maize

↓

maize

3. Polyploidy:

Autopolyploidy => more vigour, robust

1. Auto triploids => Banana, water melon (3x)

2. Auto tetraploids potato (4x)

3. Auto hexaploid => sweet potato Allopolyploidy => 50% of plants evolved Allohexaploid => wheat, triticale. Allotetraploid => cotton, tobacco

Germplasm:

(a) Land Races. Naturally selected strains/ species

(b) Obsolete variety Systematic breeding but not cultivated now.

(c) Varieties in cultivation

(d) Wild forms and relatives

(e) Breeding lines.

Pureline selection:

A large number of plants are selected from a self pollinated crop. The selected plants are harvested individually. The selected individual plants are grown in individual rows then evaluated and best progeny is selected.

Characters of pureline:

(1) All the individuals are homozygous.

(2) Variation in pureline is due to environment.

(3) Genetically become variable with time.

Procedure of Pureline Selection:

Pureline selection consists of four major steps, viz. (1) selection of a heterogeneous population from which purelines have to be isolated, (2) Isolation of purelines by individual plant selection, (3) Testing of purelines in field trials, (4) Release of the best pureline as a variety. The year wise procedure is given as follows:

First year:

An old variety or land race is used for pureline selection. Single plants are selected from the heterogeneous population keeping in view the objective of selection. The number of individual plants to be selected may vary form 200 to 1000 in various crops.

Second year:

The progeny of each selected plant is grown separately in few rows and evaluated for the character under consideration. The top 15-20 progenies are selected and seed of all plants in each progeny is bulked which constitutes strains.

Third year:

The strains constituted in second years are evaluated in field trials for 2-3 years for yield performance. In India, the selected entries (strains) are evaluated in All India Coordinated Crop Improvement Project. The best genotype is identified on the basis of yield performance.

Fourth to Tenth year:

The best performing strain is released and notified as a variety. Then the breeder, foundation and certified seeds are produced. The production of certified seed takes two years after release of a variety. Thus the seed of new variety reaches the farmers in tenth year. Merits:

(1) Uniform.

(2) Maximum improvement possible.

(3) Variety is easily identified in seed certification programme.

Demerits:

(1) Less adaptable.

(2) More time, labour and money.

(3) Upper limit and improvement is set by the genetic variation present in original population.

Pedigree method:

Individual plants are selected from F₂ and subsequent generations and their progenies are tested. During this process details about the plants selected in each generation is recorded in pedigree record.

Procedure steps:

Breeding Procedure:

First parents are selected keeping in view the breeding objective. The cross is made between selected parents. The F₁ material is grown using wide spacings. The dominance behaviour for various characters is recorded. In F₂ also the material is grown using wide spacings. Individual plant selection is practised in F₂. The progeny of each selected plant is grown separately which constitutes F₃ generation. In F₃ and F₄ generations, selection is practised within and between families. From F₅ to F₈ between progeny selection is practised and superior progeny are identified and isolated in F₈. These progeny constitute strains.

These strains are evaluated in replicated multilocation trials for a period of 3-5 years. Based on superior performance, the strain is released as a variety. Thus release of new variety by this method takes 14-15 years. The number of plants to be grown and selected in each generation is presented. This is a general procedure of pedigree method. The number of plants to be grown and selected in each generation is not fixed. It may slightly vary from crop to crop.

Merits:

i. Gives maximum opportunity to the breeder to use his skill.

ii. Well suited for characters which are simply inherited.

iii. Transgressive segregants can be easily identified through records.

iv. Information about inheritance is precisely obtained.

Demerits:

i. Maintenance of pedigree record is time consuming.

ii. The success in this method is largely dependent on skill of the breeder.

iii. Selection for yield in F₂ and F₃ is ineffective.

Achievements:

Rice: Krishna, Ratna, Sabarmati, Padma, jaya, Bala, Kaveri, etc.

Wheat: HD 2281, HD 2285, HD 2380, ND 2402, Janak.

Cotton: LH 900, LH 1556, F 846, Vikas, Khandwa 3, Sharda, Abhadita, Sahana, Anjali, Surabhi, Suvin etc.

Pigeonpea: T21, Prabhat

Greengram: T2, T44, T51, Sheela, etc.

Backcross method:

Backcross refers to crossing of F₁ with either of its parents when the F₁ is crossed with homozygous recessive parent, it is known as test cross. A system of breeding in which repeated backcrosses are made to transfer a specific character to a well adapted variety for which the variety is deficient is referred to as backcross breeding.

Main features:

Application:

It is generally used to improve specific character of a well adapted variety for which it is deficient such as resistance to a specific disease.

Parent material:

In this method two types of parents like recipient parent and donor parent involves. The parent which receives a desirable character is known as recipient parent. The parent which donates desirable character is known as donor parent.

Genetic constitution:

The new variety resembles the parent variety in all the characters except for the character under transfer.

Number of backcross:

5 to 6 backcross are sufficient to retain genotype of original variety with new character.

Procedure:

Transfer of dominant gene:

Suppose wilt resistance in cotton is controlled by a dominant gene (RR). The donor parent is a strain (В) from the germplasm. The resistance has to be transferred to an adapted variety (A) which is susceptible to wilt. The adapted variety (A) will be used as recurrent parent and strain (B) as donor parent. The F₁ will be wilt resistant but heterozygous (Rr). Backcrossing of F₁ (Rr) with susceptible variety (rr) will produce resistant and susceptible plants in equal number in BC₁ F₁ (1 Rr: 1 rr).

The resistant cotton plant (Rr) can be identified by growing the material in wilt sick plot. The resistant plants (Rr) are then backcrossed to the adapted variety. Generally, 6-8 backcrosses are sufficient to obtain identical plants to adapted except for the added genes for wilt resistance.

The wilt resistant plants are heterozygous (Rr). They are selfed for one genereation to obtain homozygous (RR) resistant plants. All the resistant true breeding plants are bulked and new variety is released. The variety developed in this way is identical to the adapted variety (A) expect for wilt resistance.

Transfer of recessive gene:

Suppose w’ilt resistance in cotton is governed by a recessive gene (rr). In such case, a progeny of each backcross will segregate into two genotypes (RR and Rr) which cannot be identified. Therefore, it is necessary to self the population after each backcross obtain resistant homozygous recessive plants (rr). The resistant plants are identified by owing the F₂ material in wilt sick plot. The resistant plants are backcrossed with adapted variety. Here each backcross is followed by one selfing, whereas with dominant one continuous backcrosses are made.

Merits:

i. It retains all desirable characters of a popular adapted variety.

ii. It is a useful method for transfer of oligogenic character like disease resistance.

iii. The male sterility and fertility restorer genes are transferred to various agronomic bases.

iv. It is the only breeding method which is used for interspecific gene transfer.

Demerits:

i. The backcrosses have to be made for 6-8 generations.

ii. Sometimes undesirable character also transferred to the new variety.

Achievements:

Backcross method has been widely used for the development of disease resistant varieties in both self and cross pollinated species. It has also been used for interspecific gene transfer and development of multiline varieties in self pollinated species. Several varieties resistant to various diseases have been developed by this method in wheat, cotton and several other crops. In cotton varieties V797, Digvijay, Vijalpa and Kalyan which belong to Gossypium herboceus have been developed by backcross method.

Mass selection:

Definition: A number of plants are selected on the basis of their phenotype and the open- pollinated seed from them is bulked together to raise the next generation.

Procedure:

First year: (i) Several plants selected on the basis of phenotype (ii) open pollinated seed from the selected plants harvested and bulked.

Second year: (i) Bulk seed from the selected plants grown (ii) Mass Selection may be repeated.

Third to fifth year: Main yield trials are carried out to determine the performance of superior mass selected strains in comparison with standard varieties as checks.

Sixth to eight year: Trials on the cultivator’s holding.

Eighth year: The variety is produced and named.

Application:

i. It cannot bring any new change in the phenotypes of the population but can only sort out the different types already existing it.

ii. It can be applied to any crop which posesses some amount of heterozygosity i. e. variability, which is the basis of mass selection.

In cross pollinated crops mass selection is more effective with following types of crop materials in which heterozygosity is found invariably:

i. Wild varieties

ii. Local varieties growing on farmer’s fields

iii. Introduced varieties

iv. Mixed varieties of a crop

Uses:

i. Production of new varieties

ii. Purification of unimproved varieties

iii. Maintaining the purity of a variety

Achievements:

i. First commercial types of hirsutum cotton in India e.g. Cambodias

ii. Variety of Groundnut such as TMV-1, TMV-2

iii. Maize – T13, Jaunpur, Darjeeling white round

iv. Bajra – Pusa moti

v. Potato – K122

Demerits:

Varieties developed show variation and are not uniform as pureline varieties.

In the absence of progency test, it is not possible to determine whether the selected plants are homozygous.

Improvement is less than that of pureline selection.

Single seed descent method:

A breeding procedure used with segregating populations of self pollinated species in which plants are advanced by single seeds from one generation to the next is referred as single seed descent method.

Procedure:

Modified from Bulk method (one seed is selected):

Merits:

i. This is a simple, convenient, less expensive and time saving method.

ii. Large number of crosses can be evaluated by this method, because less space and labour is required in each generation.

iii. This method is able to retain variability in a breeding population.

Demerits:

i. It does not provide opportunity to select for superior plants till F₅ generation.

ii. The frequency of getting desirable genotypes in the advanced generation is reduced.

iii. The identity of superior Ft plants cannot be maintained in this method.

Bulk method:

In this method F₂ and subsequent generations are harvested as bulk to grow the next generation. The duration of bulking may be 6-7 generations. Selection can be made in each generation but harvesting is done as bulk. At the end of bulking period single plant selection is made and tested for yielding ability.

Procedure:

The bulk breeding method consists of four important steps, viz. (1) bulking period, (2) progeny selection and isolation of superior progeny, (3) multilocation trials of superior progeny, and (4) release of the best progeny as a variety. These steps are briefly discussed below:

Bulk period. The F₁ plants are grown and their F₂ seeds are harvested in bulk. The F₂ plants are raised from a sample of F₂ seeds and F₃ seeds are harvested in bulk. This process is repeated until the desired level of homozygosity is achieved. In general, bulk period is allowed upto F₅ generation. The material is subjected to biotic and abiotic stresses during bulking period to eliminate undersirable genotypes.

Progeny selection:

In F₆ the material is space planted and individual plant selection is practised. The progeny of each selected plant is grown separately in F₇ and superior progeny are selected and isolated in F₇ and F₈.

Multilocation testing:

The selected progeny constitute strains. In eighth year, preliminary yield trial is conducted. From 10th to 14th year multilocation testing is carried out and the best performing strain is identified on the basis of 3-4 years performance in the multilocation trials. The best strain is released and multiplied for seed distribution in the 15th year. Thus bulk method takes 15-16 years for release of new variety.

The bulk material should be tested under such environment which is expected to favour desirable genotypes. For example, screening for disease resistance should be carried out in the disease prone area. Moreover, the screening should be done under natural field condition rather than in green houses.

Merits:

i. No pedigree record is maintained.

ii. If bulking period is longer natural selection operates and desirable genotypes selected.

iii. Chances of obtaining transgressive segregants more.

Dements:

i. Takes much longer time to develop a new variety.

ii. A large number of progenies are to be selected in each generation which requires much labour, time and space.

iii. We cannot get information on inheritance.

Recurrent selection:

Recurrent selection is reselection generation after generation with inter-breeding of selects to provide for genetic recombination. It is used to improve the frequency of desirable alleles for a character in a breeding population.

Main features:

Application:

It is more commonly used in cross pollinated species than in self pollinated species.

Base population:

A heterozygous base population is required to start recurrent selection.

Important steps:

(i) Selection of superior plants from base population (ii) selfing of selected plants (iii) growing progeny of selected plants in the next season (iv) intermitting among progeny (v) bulking of crossed seed.

Use of end product:

The population developed by recurrent selection can be used in producing homozygous inbreeds by selling, in the production of hybrid varieties, in the production of synthetic varieties.

Impact:

It is used to improve the frequency of desirable alleles for a character in a population.

Types of Recurrent Selection

1. Simple, Recurrent Selection:

A type of recurrent selection that does not include tester is referred to as simple recurrent selection. It is also known as phenotypic recurrent selection. This method is an extension of mass selection. Main features of simple recurrent selection are given below:

1. The tester is not used in this scheme.

2. It does not measure the combining ability.

3. The selection is based on phenotype or simple test.

4. This method is useful only for those characters which have high heritability.

5. This method requires only two seasons for the completion of one selection cycle.

2. Recurrent selection for GCA:

First year:

Superior plants for the character under improvement are selected from the base population. The selected plants are selfed and also crossed to a heterozygous tester having broad genetic base. The selfed seed is kept in cold storage.

Second year:

The crossed seed is sown and the combining ability of the selected plants is evaluated and plants with good gca are identified.

Third year:

The progeny of selected plants with good gca are grown from their selfed seed kept in cold store. These progeny are intermated in all possible combinations and their crossed seed is composited to form a new source population for further selection. This completes original selection cycle. In the same way another cycle can be completed in three years (4th to 6th year).

This is called first recurrent selection cycle. Many such cycles may be made to obtain desired results.

3. Reciprocal Recurrent selection of SCA:

A form of recurrent selection that is used to improve the sea of a population for a character by using homozygous tester is referred to as recurrent selection for specific combining ability (RSSCA). It is also known as half sib recurrent selection with homozygous tester. This method was originally proposed by Hull in 1945.

Table 1: Comparison of recurrent selection for gca and recurrent selection for sea:

4. Reciprocal Recurrent Selection:

A form of recurrent selection that is used to improve both gca and sea of a population for a character using two heterozygous testers is known as reciprocal recurrent selection (RRS). It is also termed as recurrent reciprocal half sib selection.

First year:

Several phenotypically superior plants are selected from population A and B. The pollen of some selected plants of A population is used to cross large number of randomly selected plants of population B. Similarly, pollen of some selected plants of В population is used to cross large number of plants of population A. All the plants of population^ and В used as pollen parents in the crosses are selfed.

Second year:

The progeny of test crosses made with pollen parents of A and В populations are evaluated in separate replicated trials. The superior progeny are identified.

Third year:

The selfed seed of those A and В plants whose progeny were found among the progeny of A plants and also among the progeny of В plants. The crossed seeds of A block are composited in equal quantity to raise A generation. Similarly, crossed seeds of В block are bulked to raise B₁ generation. This completes original cycle of selection.

Fourth year:

The A, and B, populations are grown from the composite crossed seeds of respective population obtained in third year. Then operations of first year are repeated.

Fifth year:

The operations of second year are repeated.

Sixth year:

The operations of third year are repeated.

The last three years constitute first cycle of reciprocal recurrent selection. Such selection cycles may be continued till the desired improvement is achieved.

Merits:

i. It is an efficient breeding method for increasing the frequency of superior genes in a population.

ii. It helps in maintaining high genetic variability.

iii. Selection is made on the basis of test cross performance

Demerits:

i. It is not used directly for the development of new varieties.

ii. It involves lot of selection, crossing and selfing work.

iii. It permits selfing which leads to loss of genetic variability.