In this article we will discuss about:- 1. Brief Account of Breeding Methods in Crop Plants 2. Breeding Populations in Plant Breeding 3. Classification.
Brief Account of Breeding Methods in Crop Plants:
Plant introduction is applicable to all three groups of crop plants, viz., self-pollinated, cross pollinated and asexually propagated species. It is an oldest and rapid method of crop improvement.
The introduced material may be used in three ways, viz.:
(1) Directly as a variety,
(2) As a variety after selection, and
(3) As a parent in the hybridization for development of variety or hybrid.
Pure line selection is applicable to self-pollinated species. It is also used sometimes in cross pollinated species for development of inbred lines. A single best pure line is released as a variety. Thus a pure line variety is homozygous and homogeneous population.
Mass selection is common in cross pollinated species and rare in self-pollinated and asexually propagated species. In self-pollinated crops, a mass selected variety is a mixture of several pure lines. Thus it is a homozygous but heterogeneous population. In cross pollinated species, a mass selected variety is a mixture of several hetero and homozygotes. Thus, it is a heterozygous and heterogeneous population.
Progeny selection is used in cross pollinated species. A variety developed by this method is heterozygous and heterogeneous population because it consists of several hetero and homozygotes.
Pedigree method is applicable to both self and cross pollinated species. In self-pollinated crops progeny of a single best homozygote is released as a variety. Thus a variety developed by this method has a homozygous and homogeneous population. In cross pollinated species, it is used for development of inbred lines.
Bulk and single seed descent methods are used in self- pollinated species. Progeny of a single best homozygote is released as a variety by these methods. Thus, varieties developed by these methods are homozygous and homogeneous.
Backcross method is applicable in all three groups of crop species. This method is used for transfer of oligogenic characters from a donor source to a well adapted variety. This method is also used for development of multi-lines, isogenic lines and transfer of male sterility. This method is more effective in transferring oligogenic characters than polygenic traits. The end product of backcross method is similar to parent variety except for the character which has to be transferred from the donor source.
Multiline varieties are developed in self-pollinated species. They are mixture of several isogenic lines, closely related lines or unrelated lines. Thus, a multiline variety is a homozygous but heterogeneous population.
Clonal selection is used in asexually propagated species. In this method progeny of a single best clone is released as a variety. Such variety has heterozygous but homogeneous population.
Heterosis breeding is used in all the three groups. However, it is common in cross pollinated and asexually propagated species and rare in self-pollinated species. A hybrid variety has homogeneous but heterozygous population.
Synthetic and composite varieties are developed in cross pollinated species. Such varieties consist of several homozygotes and heterozygotes and thus constitute a heterogeneous population.
Mutation breeding is common in self-pollinated and asexually propagated species and rare in cross pollinated species. A mutant variety differs from parent variety in one or few characters. A Mutant differs from a segregant in two main ways. Firstly, the frequency of segregants is very high and that of mutant is extremely low (0.1%). Secondly, mutant differs from parent variety in one or few characters, where as a segregant differs from parent material in several characters.
Polyploidy breeding is common in asexually propagated species and rare in self and cross pollinated species. A polyploid variety differs from parent variety in chromosome numbers and exhibit gigant morphological characters.
Distant hybridization is used in all the three types of crop species. However, this method is used for transferring some desirable genes from wild species to the cultivated ones. Generally, backcross method is used for transfer of oligogenic characters and pedigree method for transfer of polygenic characters.
Transgenic breeding is applicable to all three types of crop species. This method is used to solve specific problems which cannot be solved by conventional breeding techniques. This method will serve as a tool and cannot be used as a substitute for conventional breeding methods.
Recurrent selection is common in cross pollinated species and rare in other two groups. It is used for accumulating favourable genes in a population i.e. for population improvement. Other approaches which are used for population improvement include disruptive mating, diallel selective mating (DSM) and biparental mating. DSM is used in self-pollinated species and other two techniques can be used both in self and cross pollinated species.
Division of Breeding Method:
Thus, crop plants are divided into three groups, viz.:
(2) Cross pollinated, and
(3) Asexually pollinated species (Vegetatively propagated).
These are briefly discussed below:
i. Self Pollinated Species:
These are self fertilizing species. In these species, development of seed takes place by self- pollination (autogamy). Hence, self-pollinated species are also known as autogamous species or in breeders. Various plant characters such as homogamy, cleistogamy, chasmogamy, bisexuality etc. favour self fertilization.
Some important features of autogamous species are given below:
i. They have regular self-pollination.
ii. They are homozygous and have advantage of homozygosity, means they are true breeding.
iii. In-breeders do not have recessive deleterious genes, because deleterious genes are eliminated due to inbreeding by way of gene fixation.
iv. In-breeders have homozygous balance and, therefore, are tolerant to inbreeding. In other words, inbreeding does not have any adverse effects on in-breeders.
v. In autogamous species, new gene combinations are not possible due to regular self-pollination.
vi. In-breeders are composed of several component (homozygous) lines. Hence, variability is mostly among component lines.
vii. In-breeders have generally narrow adaptation and are less flexible.
ii. Cross Pollinated Species:
This group refers to cross fertilizing species. These species produce seed by cross pollination (allogamy). Hence, these species are also referred to as allogamous species or out-breeders. Various plant characters which promote cross pollination include dichogamy, moecy, dioecy, heterostyly, herkogamy, self incompatibility and male sterility.
Some important features of outbreeders are given as follow:
i. They have random mating. In such population, each genotype has equal chance of mating with all other genotypes.
ii. Individuals are heterozygous and have advantage of heterozygosity.
iii. Individuals have deleterious recessive genes which are concealed by masking effect of dominant genes.
iv. Out-breeders are intolerant to inbreeding. They exhibit high degree of inbreeding depression on selfing.
v. Cross pollination permits new gene combinations from different sources.
vi. In these species, variability is distributed over entire population.
vii. They have wide adaptability and more flexibility to environmental changes due to heterozygosity and heterogeneity.
iii. Asexually Propagated Species:
Some crop plants propagate by asexual means i.e. by stem or root cuttings or by other means. Such species are known as asexually propagated species or vegetatively propagated species. Such species are found in both self and cross pollinated groups. Generally, asexually propagated species are highly heterozygous and have broad genetic base, wide adaptability and more flexibility.
Breeding Populations in Plant Breeding:
The genetic constitution of plants is determined by mode of pollination. Self-pollination leads to homozygosity and cross pollination results in heterozygosity. Thus, we have to exploit homozygosity in self-pollinated crops and heterozygosity in cross pollinated species, because in-breeders have advantage of homozygosity and out-breeders have advantage of heterozygosity.
Based on genetic constitution, plant breeding populations are of four types, viz.:
(3) Homozygous, and
These are briefly discussed below:
i. Homogeneous Populations:
Genetically similar plants constitute homogeneous populations. Examples of homogeneous populations are pure lines, inbred lines, F1 hybrid between two pure lines or inbred lines and progeny of a clone. Pure lines and inbred lines generally have narrow adaptation.
ii. Heterogeneous Populations:
Genetically dissimilar plants constitute heterogeneous populations. Examples of heterogeneous populations are land races, mass selected populations, composites, synthetics and multi lines. Heterogeneous populations have wide adaptability and stable performance under different environments.
iii. Homozygous Populations:
Individuals with like-alleles at the corresponding loci are known as homozygous. Such individuals do not segregate on selfing. Thus non-segregating genotypes constitute homozygous populations. Example of, homozygous populations are pure lines, inbred lines and mass selected populations in self-pollinated plants. Thus pure lines and inbred lines are homozygous and homogeneous and mass selected varieties of self-pollinated crops and multi lines are homozygous but heterogeneous, because they are mixture of several pure lines.
iv. Heterozygous Populations:
Individuals with unlike-alleles at the corresponding loci are referred to as heterozygous. Such individuals segregate into various types on selfing. This includes F, hybrids, composites and- synthetics. Thus F hybrids are heterozygous but homogeneous and composites and synthetics are heterozygous and heterogeneous. Such populations have greater buffering capacity to environmental fluctuations.
Classification of Breeding Methods:
Various approaches (viz. selection, hybridization, mutation, etc.) that are used for genetic improvement of crop plants are referred to as plant breeding methods or plant breeding procedures or plant breeding techniques. The choice of breeding methods mainly depends on the mode of pollination, mode of reproduction, gene action and breeding objective of crop species.
Plant breeding methods are generally classified on the basis of application in crop improvement (general methods, special methods and population improvement approaches) and hybridization (methods involving hybridization and methods not involving hybridization). Classification of breeding methods is presented in Table 13.2.
Various breeding procedures that are more commonly used for the genetic improvement of various crop plants are known as general breeding methods. Such breeding methods include introduction, selection (pure line selection, mass selection, progeny selection), hybridization (pedigree, bulk and backcross methods), heterosis breeding, synthetic and composite breeding.
On the other hand, those breeding procedures that are rarely used for improvement of crop plants are referred to as special breeding methods. Such methods include: mutation breeding, polyploidy breeding, wide crossing or distant hybridization and biotechnology. Four breeding approaches, viz. recurrent selection, disruptive mating and selection, diallel selective mating system and biparental mating are used mainly for population improvement (Table 13.2).
There are some differences in the breeding methods used for self-pollinated and cross pollinated species. Self-pollinated species are homozygous, hence we can start hybrization directly. Cross pollinated species, on the other hand, are highly heterozygous.
Hence we cannot start hybridization directly. First we have to develop inbred lines by selfing or inbreeding and then only hybridization can be taken up. We have to exploit homozygosity in self-pollinated crops and heterozygosity in cross pollinated species.
Asexually propagated species such as sugarcane, potato, sweet potato, etc., are highly heterozygous. Hence, F1 hybrids in such crops exhibit segregation and selection can be practised in F1 generation. The superior clones are identified and further multiplied. The maintenance or conservation of hybrid vigour is easy in such crops because of asexual propagation.
i. Methods of Breeding Autogamous Species:
Plant breeding methods that are used for genetic improvement of self-pollinated or autogamous species include:
(i) Plant Introduction,
(ii) Pure line selection,
(iii) Mass selection,
(iv) Pedigree method,
(v) Bulk method,
(vi) Single seed descent method,
(vii) Backcross method,
(viii) Heterosis breeding,
(ix) Mutation breeding,
(x) Polyploidy breeding,
(xi) Distant hybridization and
(xii) Transgenic breeding.
Four breeding approaches, viz. recurrent selection, disruptive selection, diallel selective mating, and biparental mating are used for population improvement.
ii. Methods of Breeding Allogamous Species:
Breeding methods that are used for genetic improvement of cross pollinated or allogamous species include:
(i) Plant Introduction,
(ii) Mass and progeny selection,
(iii) Backcross method,
(iv) Heterosis breeding,
(v) Synthetic breeding,
(vi) Composite breeding,
(vii) Polyploidy breeding,
(viii) Distant hybridization, and
(ix) Transgenic breeding.
Mutation breeding is rarely used in allogamous species. Three breeding approaches, viz., recurrent selection, disruptive mating and biparental mating are used for population improvement.
iii. Methods of Breeding Asexually Propagated Species:
Important breeding methods applicable to asexually propagated species are:
(i) Plant Introduction,
(ii) Clonal selection,
(iii) Mass selection,
(iv) Heteross breeding,
(v) Mutation breeding,
(vi) Polyploidy breeding,
(vii) Distant hybridization, and
(viii) Transgenic breeding.
Mass selection is rarely used in asexually propagated species.
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