What is Hybridization?
Hybridization can be defined as the process of crossing two organisms that 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.
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 are focused to produce a genotype with favourable traits like pest resistance, and 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 is 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.
Need For Heterozygous Genotypes
Hybridization is performed to produce and promote heterozygous strains over a homozygous generation. The main reason behind this is to improve the crop genotype and establish commercially important traits in crops, for example, drought resistance. When hybridization is performed, favourable traits are selected and plants are bred. These heterozygotes contain the trait from both parents, thus they are assumed to have favourable 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 of 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 vigour 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.
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:
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.
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.
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.
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.
Tagging: It is the process of attaching a tag to the emasculated plant, which contains information about, the number of field records, date of emasculation, date of crossing, and name of the plant to which it is crossed.
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.
Harvestation: The seeds from this progeny are collected, and are stored with the original tag.
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 producing 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 and 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).
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 negative impacts on the plant, they are as follows:
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.
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.
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.
Conclusion
Hybridization is the technique of breeding two different individuals of the same or other species in order to achieve the desired changes in the organisms. This technique can be used for both plants and animals. Interspecific and intraspecific are the two types of sexual hybridization techniques. There are both positive and negative effects of hybridization. Hybrid breakdown and arrested pollen tube growth are some of the negative impacts of hybridization.
FAQs on Hybridization in Plants
1. What is the primary goal of hybridization in plants?
The primary goal is to combine desirable characteristics from two or more different parent plants into a single, improved offspring, known as a hybrid. These desirable traits can include higher yield, better disease resistance, improved nutritional quality, or greater tolerance to environmental stresses like drought or salinity.
2. What are the main steps involved in the process of artificial hybridization?
Artificial hybridization is a controlled process involving several key steps:
- Selection of Parents: Choosing parent plants with the desired complementary traits.
- Emasculation: The removal of anthers from the female parent's flower before they mature to prevent self-pollination.
- Bagging: Covering the emasculated flower with a bag to prevent contamination from unwanted pollen.
- Collection of Pollen: Gathering mature pollen grains from the selected male parent plant.
- Pollination: Dusting the collected pollen onto the stigma of the bagged female flower.
- Re-bagging and Tagging: The flower is re-bagged, and a tag with details of the cross is attached.
- Harvesting and Testing: The seeds from the hybrid are harvested, grown, and evaluated for the desired combination of traits.
3. How does hybridization differ from the broader field of plant breeding?
Plant breeding is a broad scientific field that encompasses all methods used to improve the genetic makeup of plants for human benefit. Hybridization, on the other hand, is a specific technique or method within plant breeding. While plant breeding also includes techniques like selection, mutation breeding, and polyploidy breeding, hybridization specifically refers to the process of cross-pollinating two genetically different individuals to create a hybrid.
4. What is meant by 'hybrid vigour' or 'heterosis' and why is it significant in agriculture?
Hybrid vigour, or heterosis, is the phenomenon where the hybrid offspring exhibits superior qualities—such as increased growth rate, size, yield, or fertility—compared to both of its parents. This superiority arises from the masking of deleterious recessive alleles and the combination of favourable dominant alleles from both parents. It is highly significant in agriculture as it is the primary reason for the dramatic increases in crop productivity seen in many species like maize, rice, and tomato.
5. Can you provide some successful examples of hybrid plants used in agriculture?
Yes, many modern crops are successful hybrids. Some common examples include:
- Hybrid Maize (Corn): Known for its high yield and uniformity, a classic example of exploiting hybrid vigour.
- Hybrid Rice: Varieties like 'Pusa RH-10' have significantly boosted rice production in many countries.
- Hybrid Tomato: Bred for traits like better shelf life, disease resistance, and uniform ripening.
- Pusa Komal: A hybrid variety of cowpea resistant to bacterial blight.
6. Why is ensuring the homozygosity of parent plants a critical first step in hybridization?
Ensuring the homozygosity (or creating a pure line) of parent plants is critical for achieving predictable and consistent results. A homozygous parent produces only one type of gamete for a particular trait. By crossing two different pure lines, the breeder can be certain that the resulting F1 generation will be genetically uniform (heterozygous) and will reliably express the desired combination of dominant traits. This removes genetic randomness and ensures that the favourable characteristics are dependably passed on to the hybrid progeny.
7. What are the different types of hybridization based on the genetic relationship of the parents?
Hybridization is classified based on the taxonomic relationship between the parent plants:
- Intervarietal (Intraspecific) Hybridization: The cross is made between two different varieties of the same species (e.g., crossing two different varieties of wheat). This is the most common type.
- Interspecific Hybridization: The cross is made between two different species of the same genus (e.g., crossing wheat and rye to produce Triticale). This is often used to transfer traits like disease resistance.
- Intergeneric Hybridization: The cross is made between plants belonging to two different genera. This is the most difficult and rare type of cross due to genetic barriers.
8. What are the major limitations or challenges faced in plant hybridization programs?
Despite its benefits, plant hybridization faces several challenges. A major limitation is hybrid breakdown, where the F2 generation and subsequent generations lose their vigour and show reduced fertility or viability. Additionally, there can be significant incompatibility barriers between distant species or genera, preventing fertilization or the development of a viable embryo. The process is also very time-consuming and labour-intensive, as it can take many years to select parents, perform crosses, and evaluate the resulting hybrids.
