PRINCIPLES OF CROP PRODUCTIONABT-320

(3 CREDIT HOURS)

LECTURE 04PLANT HYBRIDIZATIONPOLYPLOID BREEDING

PLANT HYBRIDIZATIONGenotype peculiarities of two or more different varieties or species of plants could be brought together only by crossing them. The mating or crossing of two plants or lines of dissimilar genotype is called hybridization. In plant hybridization, one of the plants is taken as the female plant and the other as the male plant. Pollen grains from the male parent are made to pollinate the stigma of the flowers of the female parent. The seeds obtained from such a cross are called F1 seeds and the progeny raised from it is called F1 (First Filial) generation. The F1 is selfed to produce F2 and the subsequent generations like F3, F4 etc are raised in the same way. These generations are called segregating generations and they are handled differently based on the scope and objectives of the breeding program.

OBJECTIVES OF HYBRIDIZATIONThe major objectives of hybridization are:

A. Combination BreedingCombination breeding is the transfer of one or more characters from other varieties to a particular variety. These characters may be oligogenic or polygenic. In this way, genes for disease resistance, quality traits etc can be transferred.

B. Transgressive BreedingTransgressive breeding is based on transgressive variation of characters in segregating generations like F2. A cross is made between two strains of plants and the F2 is screened for transgressive variations.

Transgressive segregation is the segregation of characters beyond the parental limits, in the segregating generations like F2.

C. Production of HybridsHybrid (F1) plants show higher vigor and yield when compared to parents, in some cases. This phenomenon is called hybrid vigor. F1 seeds can be raised in bulk through hybridization and distributed directly for cultivation, especially in cross-pollinating crops.

TYPES OF HYBRIDIZATIONBased on the genetic difference between parents, hybridization can be classified into:

1. Inter-varietal Hybridization2. Distant Hybridization

INTER-VARIETAL HYBRIDIZATIONThe cross between the members of the same species (intra-specific) is called inter-varietal hybridization. In this type of hybridization, different cross patterns can be used.

1. Simple CrossIn this case, two parents are used to produce an F1 hybrid.

2. Complex CrossesIn complex crosses, more than two parents are involved. Such crosses can be called convergent crosses since they bring genes from different sources together.

DISTANT HYBRIDIZATION• Hybridization between the members of different species or hybridization

beyond species level is called distant hybridization. Thus, it may be interspecific (intra-generic) or inter-generic. When conventional methods of hybridization fails, para-sexual methods are used in such cases.

• Para-sexual hybridization is the technique of fusing somatic protoplasts when reproductive cells fail to fuse or fertilize.

THE PROCESS OF HYBRIDIZATIONThe major steps involved in the process of hybridization are:

1. Selection of Parents2. Emasculation3. Bagging4. Tagging5. Pollination6. Harvesting F1 Seeds

7. Further handling of the plants

SELECTION OF PARENTSThe choice of the parents depends on the objective of the cross. In combination breeding, the genetic diversity of the parents is not important. In the case of transgressive breeding, genetically diverse plants are selected as parents. If the characteristics of the parents are not completely known, they are evaluated for the agronomic features.

EMASCULATIONIn the case of crops with bisexual flowers, stamens of the flowers of the female parents are removed or the pollen grains are killed. This process is called emasculation. Mechanical, physiological or genetic methods of emasculation are used, depending upon the nature of the crop and the cross.

MECHANICAL METHODS OF EMASCULATION

Here, the anthers are removed from the flowers of the female parents. Hand emasculation and suction method are generally used. For hand emasculation, the flower buds are opened carefully before anthesis (First opening of the flower) and the anthers are removed with the help of forceps. Care should be taken so that the gynoecium of the flowers in not damaged. In suction method, the petals are removed from the flowers before anthesis, with the help of forceps. Then, a thin rubber or glass tube attached to a suction hose is used to suck the anthers from the flowers.

PHYSIOLOGICAL METHODS OF EMASCULATION

Here, the anthers are killed with the help of heat treatment, cold treatment or alcohol treatment.

GENETIC EMASCULATIONGenetic or cytoplasmic male sterility factors are introduced into the female parents to make them sterile.

BAGGINGThe emasculated inflorescences of female plants are covered using butter paper bags or cloth bags. However, in the case of cross-pollinated crops, male plants may also be bagged if desired, so as to avoid pollen mixture. The bags are removed 2-3 days after pollination.

TAGGINGEmasculated flowers are tagged properly after bagging. Circular or rectangular tags may be used. Details of the cross, date of emasculation, date of pollination and the number of flowers emasculated must be noted on the tag. Carbon pencil or permanent ink may be used for tagging.

POLLINATIONMature, fertile and viable pollen grains are collected from the male parent and dusted on the stigma of the female parent. Care should be taken to see that the pollen grains are dusted at the optimum stage of viability.

HARVESTING F1 SEEDS

Crossed seeds are harvested carefully and stored to raise the F1 generation.

FURTHER HANDLING OF THE PLANTS

Further handling of the hybrids depends on the objective of the cross. In the case of hybrid seed production, the F1 seeds are directly released to farmers. In the case of combination breeding and transgressive breeding, F2 is raised and the most appropriate solution program is used.

POLYPLOIDY BREEDING

In somatic cells, chromosomes are present in homologous pairs whereas in gametes chromosomes are present in single set. Hence, each organism has two types of chromosome numbers, the somatic chromosome number (2n) and the gametic chromosome number (n). However, each genetic set is formed of either a group of different chromosomes or a few groups of such chromosomes. Hence in some cases, the gametic set consists of a few numbers of identical sets. 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). Hence n may be equal to x, 2x, 3x etc. When n=x, the organism is diploid, when n=2x, the organism is a tetraploid and when n=3x, it is a hexaploid (2n = 2x, 4x and 6x respectively). Besides the type of variation, absence or additional presence of individual chromosomes can also be seen in organisms. Such variations can be exploited in plant breeding because they bring about desirable character changes in many cases.

VARIATIONS IN CHROMOSOME NUMBERTYPE CHARACTERS

1. EUPLOIDY Numerical changes in the entire genome(a) Monoploidy Only set of gamete (x)(b) Haploidy Only the haploid (gametic) set of genomes (n)(c) Diploidy Two sets of genomes (2x)(d) Polyploidy More than 2 sets of genomes (3x onwards)

(i) Triploidy 3x(ii) Tetraploidy 4x(iii) Pentaploidy 5x(iv) Hexaploidy 6x

2. 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 (2n - 1)

(ii) Nullisomy Loss of one chromosome pair from the set (2n - 2)(b) Hyperploidy Additional presence of chromosomes along with the diploid set

(i) Trisomy Addition of one chromosome to the set (2n + 1)(ii) Tetrasomy Addition of one pair of chromosomes (2n + 2)

HAPLOIDY BREEDING• Haploids can be used in many ways in plant improvement. They are

useful for the development of pure lines and inbred lines and for the production of aneuploids. Pure lines can be obtained by chromosome doubling of haploids. Such pure lines can be used as cultivars or parents in hybridization.

• PRODUCTION OF HAPLOIDSHaploids originate spontaneously in small numbers. 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.

THE END