Department of Botany
Bappa Sri Narayan Vocational Post
Graduation College
(KKV), Charbagh, Lucknow
Plant Hybridization as an Alternative
Technique in
Plant Breeding Improvement
Alisha Maraaz
University Roll No.- 2110084010034
1
, TABLE OF CONTENT
Serial Topic Page
No.
No.
1 Introduction 3-6
2 Review of Literature 7-9
3 Procedure of Hybridization Techniques 10 - 22
4 Methods of Hybridization in Self-Pollinated Groups 23 – 30
5 Conclusion 31
6 Summary 32 - 33
7 References 34 - 42
2
, INTRODUCTION
Genetic variability within the species has been efficiently utilized by breeders in their efforts to improve crops.
However, the existing variability in any given plant breeding population is not sufficient for modern plant
breeding purposes, and hence the need to broaden the existing gene pool of crops. Introduction of new traits
in plants largely relied on sexual crosses between different genotypes within or between closely related
species. However, due to the presence of various reproductive barriers, gene transfer has been restricted to
sexually-compatible species, thus limiting the possibilities of modifying and improving crop plants. Many
desirable and agronomically interesting traits may only be found in distantly related species or even in
unrelated plants. Since they constitute a genetic resource potential, an array of techniques identifies and
isolates these genes and transfer them into crops. Therefore, in cases in which genetic variation is limited, then
the most feasible approaches involve the application of transgenic and hybridization approaches to exploit the
desirable traits genes from different species. Hybridization between distant plant genera is a driver of genome
evolution and new species formation. Distant hybridization generates novel germplasm by causing genetic
recombination. Where interesting genes have been identified and isolated, they have been transferred by
transformation, however in cases where genes coding for certain traits have not been identified, wide
hybridization has been the technique of preference. Hybridization is the natural or artificial process of
producing hybrids through crossing two individuals from different populations that are genetically different.
This process does not change the genetic contents of organisms but rather produces new combinations of genes
which could have certain desirable characteristics or phenotypes. This technique also circumvents problems
such as sexual incompatibility, polyembryony, and male or female sterility encountered in conventional sexual
crossing. In crop improvement, hybridization is done for one of the following reasons. Firstly, to create a
variable plant population for selecting hybrids within these populations with certain desirable combination of
characteristics. Secondly, to combine certain desirable characteristics in certain crops into a single individual
or thirdly, to exploit and utilize hybrid varieties. Whatever the intension of the breeder, the overall aim of
hybridization is always to create genetic variation when two genetically different plants are brought together
in the first filial generation. There are two main categories of hybridization techniques; sexual and somatic.
Sexual hybridization, commonly known as wide or distant hybridization, hybrid combinations are produced
within specific taxonomic distances. Sexual hybridization techniques have been used over time to produce
better as well as new crops such as triticale, which is a crop species produced from the sexual cross between
wheat (Triticum vulgar) and rye (Secale cereale) in 1875. However wide/distant hybridizations of individuals
in different species and even genera have been achieved. When two species in the same genera are crossed,
this is referred to as inter-specific hybridization, while crossing of two individuals in different genera is
referred to as inter-generic hybridization. These kinds of crossing are important because they break species
3
, barriers for transfer of genes and therefore, make it possible to transfer genomes of one species to another
which results in phenotypic or genotypic changes in the progeny. Somatic hybridization on the other hand
results when somatic cells are fused instead of gametes. This technique unlike sexual hybridization is done in
vitro and requires specific handling of the materials to be fused. Precisely, somatic hybridization is done via
protoplast fusion and it has become an important tool for ploidy manipulation in plant improvement schemes,
allowing researchers to combine somatic cells from different cultivars, species, or genera, resulting in novel
allotetraploid and autotetraploid genetic combinations. After the successful establishment of plant protoplast
isolation and fusion techniques, this hybridization strategy was realized, first by fusing the protoplasts of
Nicotiana tabacum and Nicotiana glauca. In the gramineae family, the first ever somatic hybrid plantlet was
a protoplast fusion of rice (Oryza sativa L.) and barnyard grass (Echinochloa oryzicola), which was done in
1987. This technique can facilitate conventional breeding, transfer of genes such as disease resistance genes,
rapid growth rate genes, more product formation rate genes, drought resistance genes and heat or cold
resistance genes, from one species to another, and cultivar development by bypassing some problems
associated with conventional sexual hybridization including sexual incompatibility, nucellar embryogenesis,
and male or female sterility. This write-up provides an overview regarding the utilization of sexual and somatic
hybridization as a method of transferring alien genes to crop species. The potential of somatic hybridization
for restoring ploidy level in polyploid species after breeding at reduced ploidy level, as well as the challenge
of resynthesizing allopolyploid species, will also be discussed. Focus on documented work in crops belonging
to Gramineae family, methodologies used and the fate of the transferred alien DNA in the specific hybrids and
their progeny will be highlighted.
I. SOMATIC HYBRIDIZATION:
Plant protoplasts can be prepared by treatment of plant cells with specific lytic enzymes which remove the
cell wall. Protoplast fusion is a physical process during which two or more protoplasts come into contact with
each other in the presence of fusion-inducing agents like polyethylene glycol (PEG). This is an inexpensive
and rapid mechanism whereby two genetically different protoplasts, isolated from somatic cells, are fused to
obtain parasexual hybrid protoplasts containing heteroplasmic cytoplasm and two fused parent nuclei.
Protoplasts of a variety of plants can be fused using PEG, and the hybrid products will regenerate cell walls
and divide.
CLASSIFICATION OF SOMATIC HYBRIDS:
Somatic hybrids can be classified into three types: symmetric somatic hybrids, asymmetric somatic hybrids,
and cytoplasmic hybrids (cybrids) based on how they are developed. Symmetric somatic hybridization refers
to the combination of nuclear and cytoplasmic genetic information of both parents. Asymmetric somatic
hybridization is incomplete, with the loss of some cytoplasmic or nuclear DNA, and this type of hybridization
has been used to introduce fragments of the nuclear genome from one parent (donor) into the intact genome
4
Bappa Sri Narayan Vocational Post
Graduation College
(KKV), Charbagh, Lucknow
Plant Hybridization as an Alternative
Technique in
Plant Breeding Improvement
Alisha Maraaz
University Roll No.- 2110084010034
1
, TABLE OF CONTENT
Serial Topic Page
No.
No.
1 Introduction 3-6
2 Review of Literature 7-9
3 Procedure of Hybridization Techniques 10 - 22
4 Methods of Hybridization in Self-Pollinated Groups 23 – 30
5 Conclusion 31
6 Summary 32 - 33
7 References 34 - 42
2
, INTRODUCTION
Genetic variability within the species has been efficiently utilized by breeders in their efforts to improve crops.
However, the existing variability in any given plant breeding population is not sufficient for modern plant
breeding purposes, and hence the need to broaden the existing gene pool of crops. Introduction of new traits
in plants largely relied on sexual crosses between different genotypes within or between closely related
species. However, due to the presence of various reproductive barriers, gene transfer has been restricted to
sexually-compatible species, thus limiting the possibilities of modifying and improving crop plants. Many
desirable and agronomically interesting traits may only be found in distantly related species or even in
unrelated plants. Since they constitute a genetic resource potential, an array of techniques identifies and
isolates these genes and transfer them into crops. Therefore, in cases in which genetic variation is limited, then
the most feasible approaches involve the application of transgenic and hybridization approaches to exploit the
desirable traits genes from different species. Hybridization between distant plant genera is a driver of genome
evolution and new species formation. Distant hybridization generates novel germplasm by causing genetic
recombination. Where interesting genes have been identified and isolated, they have been transferred by
transformation, however in cases where genes coding for certain traits have not been identified, wide
hybridization has been the technique of preference. Hybridization is the natural or artificial process of
producing hybrids through crossing two individuals from different populations that are genetically different.
This process does not change the genetic contents of organisms but rather produces new combinations of genes
which could have certain desirable characteristics or phenotypes. This technique also circumvents problems
such as sexual incompatibility, polyembryony, and male or female sterility encountered in conventional sexual
crossing. In crop improvement, hybridization is done for one of the following reasons. Firstly, to create a
variable plant population for selecting hybrids within these populations with certain desirable combination of
characteristics. Secondly, to combine certain desirable characteristics in certain crops into a single individual
or thirdly, to exploit and utilize hybrid varieties. Whatever the intension of the breeder, the overall aim of
hybridization is always to create genetic variation when two genetically different plants are brought together
in the first filial generation. There are two main categories of hybridization techniques; sexual and somatic.
Sexual hybridization, commonly known as wide or distant hybridization, hybrid combinations are produced
within specific taxonomic distances. Sexual hybridization techniques have been used over time to produce
better as well as new crops such as triticale, which is a crop species produced from the sexual cross between
wheat (Triticum vulgar) and rye (Secale cereale) in 1875. However wide/distant hybridizations of individuals
in different species and even genera have been achieved. When two species in the same genera are crossed,
this is referred to as inter-specific hybridization, while crossing of two individuals in different genera is
referred to as inter-generic hybridization. These kinds of crossing are important because they break species
3
, barriers for transfer of genes and therefore, make it possible to transfer genomes of one species to another
which results in phenotypic or genotypic changes in the progeny. Somatic hybridization on the other hand
results when somatic cells are fused instead of gametes. This technique unlike sexual hybridization is done in
vitro and requires specific handling of the materials to be fused. Precisely, somatic hybridization is done via
protoplast fusion and it has become an important tool for ploidy manipulation in plant improvement schemes,
allowing researchers to combine somatic cells from different cultivars, species, or genera, resulting in novel
allotetraploid and autotetraploid genetic combinations. After the successful establishment of plant protoplast
isolation and fusion techniques, this hybridization strategy was realized, first by fusing the protoplasts of
Nicotiana tabacum and Nicotiana glauca. In the gramineae family, the first ever somatic hybrid plantlet was
a protoplast fusion of rice (Oryza sativa L.) and barnyard grass (Echinochloa oryzicola), which was done in
1987. This technique can facilitate conventional breeding, transfer of genes such as disease resistance genes,
rapid growth rate genes, more product formation rate genes, drought resistance genes and heat or cold
resistance genes, from one species to another, and cultivar development by bypassing some problems
associated with conventional sexual hybridization including sexual incompatibility, nucellar embryogenesis,
and male or female sterility. This write-up provides an overview regarding the utilization of sexual and somatic
hybridization as a method of transferring alien genes to crop species. The potential of somatic hybridization
for restoring ploidy level in polyploid species after breeding at reduced ploidy level, as well as the challenge
of resynthesizing allopolyploid species, will also be discussed. Focus on documented work in crops belonging
to Gramineae family, methodologies used and the fate of the transferred alien DNA in the specific hybrids and
their progeny will be highlighted.
I. SOMATIC HYBRIDIZATION:
Plant protoplasts can be prepared by treatment of plant cells with specific lytic enzymes which remove the
cell wall. Protoplast fusion is a physical process during which two or more protoplasts come into contact with
each other in the presence of fusion-inducing agents like polyethylene glycol (PEG). This is an inexpensive
and rapid mechanism whereby two genetically different protoplasts, isolated from somatic cells, are fused to
obtain parasexual hybrid protoplasts containing heteroplasmic cytoplasm and two fused parent nuclei.
Protoplasts of a variety of plants can be fused using PEG, and the hybrid products will regenerate cell walls
and divide.
CLASSIFICATION OF SOMATIC HYBRIDS:
Somatic hybrids can be classified into three types: symmetric somatic hybrids, asymmetric somatic hybrids,
and cytoplasmic hybrids (cybrids) based on how they are developed. Symmetric somatic hybridization refers
to the combination of nuclear and cytoplasmic genetic information of both parents. Asymmetric somatic
hybridization is incomplete, with the loss of some cytoplasmic or nuclear DNA, and this type of hybridization
has been used to introduce fragments of the nuclear genome from one parent (donor) into the intact genome
4
