Plant breeding methods ABT320 Dr Rabia Amir Lecture

Plant breeding methods ABT-320 Dr. Rabia Amir Lecture 13 LOGO

MODERN METHODS OF PLANT BREEDING v Modern methods are developed by the plant breeders with the help of modern scientific tools. v Modern methods of plant breeding include: • Mutation Breeding • Polyploidy Breeding • Distant Hybridization • Biotechnological Methods

MUTATION BREEDING v Mutations are heritable changes in the phenotypes of organisms. v These changes are the results of chemical changes at the level of genes. v Such changes are capable of bringing about new and heritable character variations in crop plants and such variations can be selected and used for the establishment of crop varieties with new characters.

MUTATION BREEDING v Mutations occur in nature in very low frequency. Such mutations are called spontaneous mutations. v Mutations induced with the help of certain chemical or physical agents (mutagens or mutagenic agents) are called induced mutations. v This approach of plant breeding in which new variations of crops with desirable characters are developed with the help of induced mutations is called mutation breeding.

MUTAGENS AND THEIR MODE OF ACTION v Mutagens are the physical or chemical agents used to enhance the frequency of mutations: A. Physical Mutagens B. Chemical Mutagens

PHYSICAL MUTAGENS v These are physical agents that are capable of inducing mutations. v Radiations can be generally classified into two classes based on their energy levels. v Radiations with lower energy levels are capable of causing excitations at the level of nitrogen bases of the genetic material and they are called non-ionizing radiations. v UV light is a good example of non-ionizing radiations.

PHYSICAL MUTAGENS v Radiations with high energy level are capable of causing both excitation and ionization at the level of nitrogen bases. They are called ionizing radiations. v X-rays, gamma rays, alpha particles, beta particles etc are the examples of ionizing radiations.

CHEMICAL MUTAGENS v There are chemicals that are capable of enhancing the frequency of mutations. v The chemical mutagens are mainly classified into three categories on the basis of their nature of action. § Alkylating Agents § Base Analogues § Acridine Dyes

ALKYLATING AGENTS v They induce mutations by adding an alkyl group (ethyl or methyl group) to the nitrogen bases. The major alkylating agents are: § Ethyl Methane Sulphonate (EMS) § Methyl Methane Sulphonate (MMS) § Ethylene Imines (EI) etc. v Since the actions of alkylating agents resemble the actions of radiations they are known as radiomimetic chemicals. AT-GC transition and GC-AT transition

BASE ANALOGS v These are chemicals analogous to nitrogen bases. v They can get incorporated into DNA at the time of replication and can cause wrong base pairing resulting in mutations. v 5 -bromo uracil and 2 -amino purine are the common base analogues used as mutagens. v AT-GC transition

ACRIDINE DYES v These are chemicals capable of getting inserted between two bases of DNA, thus leading to addition or deletion of single or a few base pairs at the time of replication. v Proflavin and acriflavin are two such examples. v Frame shift

TYPES OF INDUCED MUTATIONS v At the molecular level, induced mutations are changes, additions or deletions of nitrogen bases. v Accordingly, they can be classified into: § transitions § transversions § frame shifts

TYPES OF INDUCED MUTATIONS v Transition mutation is the replacement of one purine with another purine or one pyrimidine with another pyrimidine. v Transversion mutation is the substitution of a purine by a pyrimidine or vice versa. v Frame shift mutation is the change in the reading frame of the gene by the addition or deletion of nitrogen bases.

PROCEDURE OF MUTATION BREEDING v It includes: 1. Selection of the material 2. Choice of the mutagen 3. Mutagen Treatment 4. Handling of the mutated populations in the case of seed propagated species 5. Handling of mutated populations in the case of clonally propagated species

SELECTION OF THE MATERIAL v The first step in mutation breeding is the decision on the nature of variations to be induced and the selection of the appropriate material for mutagen treatment. v Depending upon the method of propagation, seeds or other propagules can be selected for treatment. v For in vitro mutagenesis, callus or similar in vitro material is selected.

CHOICE OF THE MUTAGEN v Based on the nature of mutation to be induced and the knowledge on the nature of action of the mutagen, the appropriate mutagen is selected. v Generally, chemicals are preferred for the seed treatment and radiations are preferred for vegetative propagules, pollen etc.

MUTAGEN TREATMENT v In the case of chemical treatment, presoaking materials in water or solutions of some other chemicals enhance the effect of mutagens. This is called pre-treatment. v Later, the materials are transferred into the solutions of the mutagen. v A concentration close to LD 50 of the mutagen is considered optimum. v LD 50 is the dose of the mutagen that causes 50 percent mortality of the treated material.

MUTAGEN TREATMENT v In the case of physical mutagens, the source of the mutagen is kept at a safe distance and the treatment is remotely controlled. v Gamma ray treatment is carried out in protected experimental areas known as gamma gardens. v The duration of treatment is also decided based on the information available.

HANDLING OF THE MUTATED POPULATIONS IN THE CASE OF SEED PROPAGATED SPECIES v All the germinated seeds are grown to produce the M 1 population. v Generally the mutations will be recessive and most of them can be selected only in later generations. v However, dominant mutations and pseudo-dominant mutations can be selected in the M 1 itself. v The M 1 plants are selfed and the seeds are harvested separately. v The M 2 generation is raised from the seeds collected from the M 1 generation.

HANDLING OF THE MUTATED POPULATIONS IN THE CASE OF SEED PROPAGATED SPECIES v Oligogenic mutations can be selected at this level. Their seeds are grown separately and desirable mutants isolated after necessary trials. v Superior and desirable M 2 plants are selected and M 3 seeds are collected. v M 3 progenies are raised from the seeds and they are evaluated for breeding behavior. v The seeds of true breeding progenies are bulked together to conduct yield trials.

HANDLING OF THE MUTATED POPULATIONS IN THE CASE OF SEED PROPAGATED SPECIES v Preliminary yield trials are conducted in the M 4. v Co-ordinated yield trials are carried out from M 5 onwards. By M 8 or M 9, the most promising lines are selected and released. v In the case of polygenic traits, inferior plants are rejected at M 3 and M 4 levels and based on screening tests, the remaining seeds are bulked and used for yield trials and finally released as new varieties.

HANDLING OF MUTATED POPULATIONS IN THE CASE OF CLONALLY PROPAGATED SPECIES v In vegetatively propagated species, mutations are expressed as chimeras. Chimeras are combinations of genetically different tissues. v In the case of vegetatively propagated crops, the generation raised from the treated propagules is called the VM 1 generation. v Plants showing chimeras can be selected and propagated to produce the VM 2 generation.

HANDLING OF MUTATED POPULATIONS IN THE CASE OF CLONALLY PROPAGATED SPECIES v Solid mutants are identified and selected in VM 2. v In VM 3, the mutations identified in VM 2 are confirmed. v Preliminary yield trials are carried out in VM 4 and co-ordinated trials from VM 5 onwards. v By VM 9 the best line is released as a new variety.

APPLICATIONS OF MUTATION BREEDING v Mutation breeding can be used to develop improved crop varieties, to induce male sterility, for the production of haploids, to create additional genetic variability, and to improve the adaptability of crops.

POLYPLOIDY BREEDING v In somatic cells, chromosomes are present in homologous pairs whereas in gametes chromosomes are present in single set. v Hence, each organism has two types of chromosome numbers, • the somatic chromosome number (2 n) • the gametic chromosome number (n) v However, each genetic set is formed of either a group of different chromosomes or a few groups of such chromosomes. v Hence in some cases, the gametic set consists of a few numbers of identical sets.

POLYPLOIDY BREEDING v Here, each of such sets represents a basic set of chromosomes and the number of chromosomes in such a set can be called the basic chromosome number (x). v Hence n may be equal to x, 2 x, 3 x etc. v Besides the type of variation, absence or additional presence of individual chromosomes can also be seen in organisms. v Such variations can be exploited in plant breeding because they bring about desirable character changes in many cases.

VARIATIONS IN CHROMOSOME NUMBER 1. TYPE EUPLOIDY (a) Monoploidy (b) Haploidy CHARACTERS Numerical changes in the entire genome Only set of gamete (x) Only the haploid (gametic) set of genomes (n) (c) Diploidy Two sets of genomes (2 x) (d) Polyploidy More than 2 sets of genomes (3 x onwards) (i) Triploidy 3 x (ii) Tetraploidy 4 x (iii) Pentaploidy 5 x (iv) Hexaploidy 6 x

2. TYPE CHARACTERS ANEUPLOIDY Change in the number of a one or a few chromosomes (a) Hypoploidy Loss of chromosomes from the diploid set (i) Monosomy Loss of one chromosome from the diploid set (2 n - 1) (ii) Nullisomy Loss of one chromosome pair from the set (2 n - 2) (b) Hyperploidy Additional presence of chromosomes along with the diploid set (i) Trisomy Addition of one chromosome to the set (2 n + 1) (ii) Tetrasomy Addition of one pair of chromosomes (2 n + 2)

HAPLOIDY BREEDING v Haploids can be used in many ways in plant improvement. v They are useful for the development of pure lines and inbred lines and for the production of aneuploids. v Pure lines can be obtained by chromosome doubling of haploids. v Such pure lines can be used as cultivars or parents in hybridization.

PRODUCTION OF HAPLOIDS v Haploids originate spontaneously in small numbers. v Haploid production can be induced by inter-specific cross, use of alien cytoplasm, anther culture, pollination with foreign pollen, use of irradiated pollen, chemical treatment etc.

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