Hybridization in Plants | Agriculture Optional Notes for UPSC PDF Download

Introduction

Hybridization is the act of mating two organisms with significant genetic differences, and this can occur naturally or be facilitated artificially. It's crucial to understand that hybridization doesn't alter an individual's genetic makeup; instead, it introduces variation by generating a fresh combination of alleles. The primary objective of hybridization is to promote heterozygosity and decrease homozygosity within the population's genotypes.

Hybridization

In simple terms, hybridization refers to the process of mating two organisms from genetically diverse groups or even different species. This age-old technique has been used to enhance genetic diversity within populations. Hybridization is applied to both animals and plants, primarily for commercial advantages. Traditional hybridization methods are aimed at creating genotypes with desirable traits, such as resistance to pests and increased flowering potential in plants, to enhance their market value. In the case of animals, hybridization is employed to introduce genetic diversity and heterozygosity in the genome.

Hybridization relies heavily on the mating of two genetically distant strains of the same species. However, the presence of various reproductive barriers has historically limited breeding to sexually compatible groups, leading to restricted gene flow and consequently, limited opportunities for improving crop genotypes.

Need For Heterozygous Genotypes

Hybridization is carried out to encourage the presence of heterozygous individuals as opposed to a predominantly homozygous generation. The primary motivation behind this is to enhance the genetic makeup of crops and introduce commercially valuable characteristics, such as drought resistance. During hybridization, advantageous traits are carefully chosen, and plants are bred accordingly. Heterozygous hybrids possess the traits inherited from both parent plants, making them candidates for having these favorable attributes. Heterozygous hybrids are selected and cultivated.

Another rationale for promoting heterozygosity is the introduction of genetic diversity. Genetic variability within a population enhances its chances of survival. A positive outcome of achieving genome heterozygosity through hybridization is heterosis, which can be attributed to dominance, over-dominance, or epistasis. Heterosis results in superior performance of hybrid offspring in relation to the selected traits. This phenomenon is also known as hybrid vigor or outbreeding enhancement.

Types of Hybridization

Hybridization can be categorized into two main types: sexual hybridization and somatic hybridization. Sexual hybridization is the more traditional method and is subject to the constraint of sexual compatibility. On the other hand, somatic hybridization is a more contemporary technique carried out in a controlled environment (in vitro), involving the fusion of two protoplasts.

Sexual Hybridization

Sexual hybridization is the process of mating plants either within the same species or between different species to produce offspring with heterozygous genetic compositions. Sexual hybridization can be further categorized into interspecific hybridization and intergeneric hybridization.

• Interspecific Hybridization: This involves the hybridization of plants from two distinct species within the same genus.
• Intergeneric Hybridization: In this type of hybridization, plants from two different genera are crossed.

These forms of hybridization are crucial for transferring the genetic material of a species to distantly related species, thereby enriching the genetic diversity and creating a broader gene pool.

The Procedure of Hybridization

Hybridization typically involves eight key steps:

• Plant Selection: This step involves the careful choice of parental plants for the hybridization process. The selected plants should be healthy and suited to the given environmental conditions.
• Homozygosity: Achieving homozygosity in the parental plants is vital to establish pure genetic lines by eliminating undesired traits. This is usually attained through self-pollination or selfing of the parental plants over multiple generations.
• Emasculation: Emasculation is the removal of male reproductive organs from flowers. This is primarily done in bisexual flowers and avoided in unisexual ones. Anthers or stamens (male reproductive organs) must be removed without harming the ovules. Emasculation is carried out before pollen shedding and can be done using methods like scissors, hot water treatment, alcohol treatment, or suction.
• Bagging: Bagging is a technique where the ovules of a flower are covered to prevent cross-pollination by foreign pollen. These protective bags are typically made from materials like paper, butter paper, or vegetable parchment paper.
• Tagging: This step involves attaching a label to the emasculated plant, which provides information about field records, emasculation date, crossing date, and the name of the plant to which it is crossed.
• Crossing: Crossing is the process of artificially facilitating cross-pollination. In this step, pollen from selected parents is placed on the stigma of the flower to allow fertilization.
• Harvesting: The seeds produced from this cross are collected and stored, along with the original tagging information.
• F1 Generation: The seeds give rise to the first filial generation (F1), which is then subjected to hybrid selection for desired traits.

Selection of Hybrids

This step is of utmost significance in the production of viable hybrids. Various methods are employed for hybrid selection, with the most common and straightforward approach being based on the hybrid's observable traits, known as morphological markers. Other techniques involve the use of molecular markers and cytogenetic analysis.

• Selection via molecular markers entails the amplification of specific genomic regions that contain markers associated with traits like fertility restoration and specific ribosomal DNA sequences. Methods like AFLP (amplified fragment length polymorphism), RAPD (rapid amplification of polymorphic DNA), and SSR (single sequence repeat polymorphism) are utilized for this purpose.
• An efficient method for selection involves screening the secondary metabolites produced by the hybrid. The secondary metabolites synthesized by the offspring are both quantitatively and qualitatively distinct from those of their parent plants. Some commonly studied secondary metabolites include compounds like phenolics, terpenoids, alkaloids, isothiocyanates, and flavonoids.

Results of Hybridization

The outcomes of hybridization encompass both positive and negative effects on plants, and they can be summarized as follows:

• Heterosis: Heterosis, also known as hybrid vigor, is a phenomenon in which hybrid offspring exhibit improved performance in specific traits. This can include phenotypic advantages over their parent plants, such as enhanced resistance to biotic and abiotic factors, increased yield, and accelerated growth.
• Sterility and Inviability: These are significant obstacles to successful hybridization and can result from issues like incompatible mating, chromosomal rearrangements, or the underexpression of specific genes due to epistatic interactions.
• Negative Impacts: Unsuccessful hybridization can lead to detrimental effects such as hybrid breakdown, arrested pollen tube growth, and embryo abortion.

Conclusion

Hybridization is a method for mating two individuals of the same or different species to bring about specific alterations in the organisms. It can be applied to both plants and animals. Sexual hybridization includes interspecific and intraspecific techniques. Hybridization has both advantageous and detrimental consequences, with negative outcomes such as hybrid breakdown and inhibited pollen tube growth.