Hybridization in Plants

What is Hybridization?

Hybridization in simple terms is defined as the breeding of two different organisms from genetically diverse groups or species. Hybridization is a very old technique that has been used to increase the genetic variability among the population. Hybridization is performed on animals as well as plants, this is done to ensure the maximum benefit from the commercial point of view. Classical hybridization techniques were focused to produce a genotype with favorable traits like pest resistance, high flowering potential among plants, to increase their commercial values. Hybridization is also performed in animals to induce genetic variability or heterozygosity of the genome.

Hybridization in Plants

In the classical breeding approach, the genetic variability among the species has been greatly utilized to promote plant improvement, but as the need for commercialization increased, classical breeding techniques were not sufficient to provide the genotype heterozygosity within a population. Hybridization was largely dependent on the sexual cross between two genetically distant strains of the same species, but due to the presence of various reproductive barriers, breeding was limited to sexually compatible groups, thus limiting the gene flow, which resulted in limited opportunities to improve the crop genotype.  Hence new approaches for creating a heterozygous gene pool were developed.

Hybridization can be defined as the process of crossing two organisms which are genetically distant from each other. This can be an artificial or natural process. It is important to note that hybridization does not change the genetic composition of an individual, it creates variability by producing a new combination of the allele.  The main goal of this process is to induce heterozygosity and reduce homozygosity in the genotypes of the population.

Need For Heterozygous Genotypes

Hybridization is performed to produce and promote heterozygous strains over homozygous among the filial generation. The main reason behind this is to improve the crop genotype and establish commercially important traits in the crop, for example, drought resistance. When hybridization is performed favorable traits are selected then and plants are bred. Now some of the progeny are heterozygous according to Mendelian genetics. These heterozygotes contain the trait from both parents, thus they are assumed to have favorable traits. Heterozygous hybrids are selected and grown. Another reason for supporting heterozygosity is the induction of variability. It is the genetic variability among the population that ensures a better chance at survival of the population. Another positive impact of heterozygosity of genome achieved by hybridization includes heterosis, which can be attributed to either dominance, over-dominance, or epistasis. Heterosis is the enhanced performance of the hybrid offspring for the selected traits. This is also known as hybrid vigor or outbreeding enhancement. 

Types of Hybridization

Hybridization can be classified into two groups namely, sexual hybridization and somatic hybridization. Sexual hybridization is the comparatively classical approach, it is subjected to the sexual compatibility barrier. Somatic hybridization is a rather modern approach, it is performed in vitro. It can be defined as the fusion of two protoplasts, protoplast is the cellular component that remains after removal of the cell wall. 

Cell component + plasma membrane = protoplast.

Sexual hybridization

Sexual hybridization can be defined as a process where plants of different species or the same species are bred to produce offspring with heterozygous genotypes. Sexual hybridization can be further classified as interspecific hybridization and intergenic hybridization.

Interspecific Hybridization- It can be defined as hybridization between two different species of the same genus.

Intergeneric Hybridization- It can be defined as hybridization between organisms of two different genera. 

These types of hybridization are important to ensure the transfer of the genome of an organism of a particular species to distantly related species. This creates a diversified gene pool.

The Procedure of Hybridization

There are generally eight steps to hybridization, they are as follows,

1. Selection of plant- It is referred to as choosing both the parental plants for the process, the plant must be healthy and can grow in the given condition are the two main prerequisites of the process.

2. Homozygosity-  Inducing homozygosity in the parental plants is important to establish the purity of lines, that is eliminating the unwanted traits. It is achieved by self pollination or selfing of the parental plants over generation to achieve the result.

3. Emasculation- It can be defined as the process of removal of male reproductive organs from the flower. It is mainly performed in bisexual flowers and is avoided in unisexual flowers. The removal of anthers or stamens (male reproductive organ) must be carried out without harming the ovum. It is done prior to pollen shading. There are the following methods that are used for emasculation,  scissors Method, hot water treatment, alcohol treatment and suction.

4. Bagging- It can be defined as a method to cover the ovum of the flower. It is done to prevent cross pollination of the flower by other pollen. The bags are made up of paper, butter paper, and vegetable parchment paper.

5. Tagging- It is the process of attaching a tag to the emasculated plant, which contains information about, number of field records, date of emasculation, date of crossing, name of plant to which it is crossed. 

6. Crossing- It is the process of artificial cross-pollination. In this process pollen from selected parents is placed on the stigma of the flower, to allow fertilization.

7. Harvestation- The seeds from this progeny are collected, and are stored with the original tag.

8. F1 generation- The seeds give rise to the filial one generation which is then subjected to a selection of hybrids among it. 

Selection of Hybrids

It is the most important step to produce viable hybrids. There are various methods for selecting hybrids, the most simple and widely used is selection based on phenotypic traits of the hybrid, these phenotypic traits are called morphological markers. Other techniques include the use of a molecular marker, cytogenetic analysis.

Selection by molecular markers includes amplification of certain part of the genome that has markers related to fertility restoration and specific ribosomal DNA sequences by AFLP (amplified fragment length polymorphism), RAPD (rapid amplification of polymorphic.

DNA), SSR (single sequence repeat polymorphism). They are most widely used and reliable because of the presence of unlimited number of molecular markers in the genome.  

Selection through screening of secondary metabolites produced by the hybrid is also a very efficient method. The secondary metabolites produced by offspring are quantitatively and qualitatively different from their parents, examples of some commonly studied secondary metabolites include phenolic, terpenoid, alkaloid, isothiocyanates, and flavonoid compounds.

Results of Hybridization

Results of hybridization include both the positive and the negative possible impact of it on the plant, they are as follows,

  1. Heterosis- it is the hybrid phenomenon through which hybrid progeny shows enhanced performance in certain traits which may include, phenotypic superiority as compared to parents in terms of biotic and abiotic resistance and, increased yield and growth rate.

  2. Sterility and Inviability- These are the main barriers to hybridization, this can be because of incompatible mating, chromosomal rearrangements, or down expression of certain genes due to epistasis.

  3. Hybrid Breakdown, arrested pollen tube growth, and embryo abortion is also among some of the harmful impacts that can generate in case of unsuccessful hybridization.

FAQs (Frequently Asked Questions)

1. Why is the Homozygosity of Genotypes Important in the Parental Plants?

Ans- It is important to gain purity of gametes to ensure that only favorable traits are selected and transferred into the hybrid progeny. It is achieved by the self-cross of the parental plant.

2. What is Somatic Hybridization?

Ans- it can be referred to as a fusion of two different protoplasts, this is done in vitro. There are the following types of somatic hybrids, symmetric somatic hybrids, asymmetric somatic hybrids, and cytoplasmic hybrids known as cybrids.