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Term Paper on Gene Flow in Crop Plants
Term Paper Contents:
- Term Paper on the Definition of Gene Flow
- Term Paper on the Types of Gene Flow
- Term Paper on the Modes of Gene Flow
- Term Paper on the Factors affecting Gene Flow
- Term Paper on the Methods of Gene Flow
- Term Paper on the Advantages of Gene Flow
- Term Paper on the Practical Achievements of Gene Flow
- Term Paper on the Undesirable Effects of Gene Flow
Term Paper # 1. Definition of Gene Flow:
Gene flow refers to movement of various alleles from exotic or un-adopted or wild germplasm to the cultivated genotypes. In other words, the movement of alleles from one genotype to another due to open pollination/cross pollination is referred to as gene flow. The term gene flow was coined by Birds sell in 1950.
Main points related to gene flow are given below:
(i) It involves transfer of alleles from one genotype to another.
(ii) Generally gene transfer is from exotic or un-adopted germplasm to cultivated genotypes.
(iii) It also includes movement of alleles from wild relatives or wild species to the cultivated genotypes.
(iv) Open pollination or cross pollination is essential for gene flow.
(v) The rate of gene flow is higher in cross-pollinated species than in often cross pollinated species and self pollinated species.
(vi) The gene flow is influenced by several factors.
(vii) Gene flow has several useful consequences.
(viii) The rate of gene flow can be enhanced through artificial hybridization.
(ix) Some natural devices such as open pollination, male sterility, self incompatibility promote gene flow among the population.
(x) The gene flow may take place among the genotypes of the same species or between distantly related genotypes belonging to two different species of the same genus.
Term Paper # 2. Types of Gene Flow:
The gene flow occurs in nature through open pollination or cross pollination. Hence gene flow is higher in cross pollinated species than often cross pollinated species and self pollinated species. The gene flow can also be achieved through artificial hybridization. The gene flow takes place between various types of plant populations.
Based on mode, and species involved, gene flow can be of different types as follows:
i. Mode of Allele Transfer:
a. Natural Gene Flow:
Movement of alleles from one genotype to another through open pollination or cross pollination
b. Artificial Gene Flow:
Movement of alleles from un-adapted genotype to cultivated genotype using artificial hybridization.
ii. Genotypes Involved:
a. Intraspecific Gene Flow:
Movement of Alleles (natural or artificial) among the genotypes of the same species i.e. from germplasm to cultivated genotypes.
b. Interspecific Gene Flow:
Movement of Alleles (natural or artificial) between two species of the same genus or from wild species to the cultivated species.
c. Intergeneric Gene Flow:
Movement of alleles by artificial means between two genera of the same family.
The gene flow is very common among the genotypes of the same species especially in open pollinated crops. The inter-specific and inter-generic gene flow is achieved by artificial methods.
Term Paper # 3. Modes of Gene Flow:
In plant breeding populations, the gene flow takes place in two ways, viz.:
(i) Natural, and
(ii) Artificial.
These are briefly discussed below:
(i) Natural Gene Flow:
The movement of alleles from one genotype to another in nature by open pollination is called natural gene flow. It occurs in cross pollinated and often cross pollinated species. All those mechanisms that promote cross pollination also promote natural gene flow.
(ii) Artificial Gene Flow:
The movement of alleles from one genotype to another through man made hybridization is referred to as artificial or man-made gene flow. This type of gene flow is possible in all type of crops viz. self pollinated, cross pollinated and often cross pollinated species.
Term Paper # 4. Factors affecting Gene Flow:
The gene flow is a natural process. However, gene flow is promoted by human through artificial crossing between different species. In nature, the gene flow is influenced by various factors.
In the plant populations, the important factors affecting gene flow include:
(i) Mode of pollination,
(ii) Cross compatibility,
(iii) Pollen mobility and
(iv) Hybrid viability.
These are briefly discussed below:
(i) Mode of Pollination:
The rate of gene flow varies depending upon the mode of pollination of a plant species. The gene flow is high in cross pollinated species such as corn, pearlmillet, sunflower, rapeseed etc. and low in self pollinated species such as wheat, barley, rice, urdbean, mungbean, soybean etc.
Thus those factors which promote cross pollination such as dicliny dichogamy, heterostyly, herkogamy, self incompatibility and male sterility also promote gene flow. Similarly those factors which prevent cross pollination such as bisexuality, homogamy cleistogamy, chasmogamy and cross incompatibility etc. also prevent gene flow.
(ii) Cross Compatibility:
The another important factor that affects the rate of gene flow is cross compatibility. The gene flow can occur between those cultivated and wild species which are sexually compatible. In other words, gene flow will take place when the cross between two individuals or genotypes or species is compatible. The gene flow will not occur between those species having cross incompatibility.
The gene flow can occur between genetically engineered and non-genetically engineered genotypes if there is cross compatibility. The cross incompatibility is observed in distantly related species or between species having ploidy differences both in cross and self pollinated species.
(iii) Pollen Mobility:
The gene flow is governed by pollen mobility. The pollen mobility depends on pollen weight, pollen longevity and pollen production. The pollen grains with light weight, abundance production and more longevity are expected to travel long distance and effect cross pollination. This promotes gene flow, because such pollen grains can blow with wind to long distance. Heavy pollen grains with low longevity will restrict or prevent gene flow.
(iv) Hybrid Viability:
The hybrid viability also governs the rate of gene flow. The cross between two species say cultivated and wild may be compatibly but the hybrid may not be viable. This prevents gene flow between species. If the hybrid between two species is viable and fertile it will enhance the rate of gene flow.
The interspecific hybrids are fertile when there is chromosomal homology between two species. When there is lack of chromosomal homology between two species, the hybrid would be sterile.
Term Paper # 5. Methods of Gene Flow:
The gene flow occurs in nature through open pollination. It can also be promoted by various breeding approach.
There are two broad methods of man-made gene flow, viz.:
(i) Conventional methods, and
(ii) Modern methods.
These are briefly discussed below:
(i) Conventional Methods:
The conventional methods of gene flow among the plant breeding populations include backcross method, bulk method, pedigree method, single seed descent method. There are four populations improvement approaches, viz. recurrent selection, distruptive selection, biparental mating and diallel selective mating which are also used for man made gene flow. All these methods involve crossing and hence are referred to as hybridization methods.
These methods permit combining of characters or genes from different sources into one genotype and hence are also known as combination breeding approaches. Backcross methods is used for transfer of one or two characters from donor parent to the good agronomic base.
Pedigree, bulk an single seed descent methods are used for combining several characters of two genotypes together. Population improvement procedures are used for creating genetic variability, breaking undesirable linkages and broadening genetic base of the population.
(ii) Modern Methods:
Modern methods include:
(i) Direct gene transfer using biotechnological tools and
(ii) Somatic hybridization.
There are four methods of direct gene transfer viz.:
i. Agrobacterium mediated gene transfer,
ii. Particle gun method,
iii. Electroporation method and
iv. Micro-injection.
These techniques are used for direct gene transfer bypassing sexual process.
The gene can be transferred from one species to another in single step. In somatic hybridization, the gene transfer is achieved through protoplast fusion. These techniques are used for gene flow across the species and genera. It permits gene transfer even from unrelated species or organisms.
Term Paper # 6. Advantages of Gene Flow:
Gene flow has several desirable consequences such as:
(i) Creation of genetic diversity,
(ii) Combining desirable genes,
(iii) Formation of new species,
(iv) Development of hybrids,
(iv) Improvement in adaptation,
(v) Broadening genetic base etc.
These are briefly discussed below:
(i) Genetic Diversity:
Genetic diversity refers to variety of genes and genotypes found in a particular crop species. It provides protection from biotic and abiotic stresses resulting in insurance against crop losses. The movement of alleles from one genotype to on other enhances genetic diversity. The new variability is also created through combination and recombination of genes between two genetically different genotypes.
Genetic diversity in a population provides opportunities for selection of superior genotypes. In other words, selection is effective when there is genetic diversity in a population. Genetic diversity also provides broad genetic base and wide adaptation to a population. In other words, it provides buffering capacity to a population.
(ii) Combining Desirable Genes:
The gene flow permits transfer of desirable alleles from different sources into one genotype. For example, alleles for yield, quality, resistance to biotic and abiotic stresses, earliness, and suitable plant type can be combined together through man made gene flow.
(iii) Formation of New Species:
Gene flow permits formation of new crop species. The natural gene flow has resulted in formation of new species in various field crops such as wheat, tobacco, Brassica, cotton, oats etc. New species have also been developed through man made gene flow. Such species include Raphanobrassica (Radish x Cabbage), Triticale (Wheat x Rye) and Nicotiana digluta (N. tabacum x N. glutinosa). Several other such examples can be cited.
(iv) Development of Hybrids:
Gene flow permits exploitation of heterosis or hybrid vigour in crop plants. The heterosis can be fully exploited by developing hybrids and partially by developing synthetic and composite varieties. Hybrids for commercial, cultivation have been developed in several crops such as maize, sorghum, pearl-millet, cotton, caster, cucumber, sunflower, pigeon-pea, tomato, rice, brinjal, jute etc. The composite and synthetic varieties have been developed in maize.
(v) Improvement in Adaptation:
Adaptation refers to fitness of a genotype to particular environment and adaptability refers to capacity of a genotype to adjust to environmental changes. The varietal adaptability helps in stabilizing crop production over regions and seasons. Gene flow among plant breeding population leads to improvement in varietal adaptation and adaptability due to gene combination from different sources. In other words, gene flow improves buffering capacity of plant breeding populations.
(vi) Improvement in Genetic Base:
The gene flow increases genetic diversity in a plant breeding population which leads to broadening of genetic base of a population. Broad genetic base provides protection against biotic and abiotic stresses to crop plants.
Man-made gene flow sometimes leads to narrow genetic base as in cultivars developed by backcross, pedigree and SSD methods.
Term Paper # 7. Practical Achievements of Gene Flow:
Remarkable practical achievements have been made in different crop plants through natural and man-made gene flow.
Some important achievements are given below:
i. Several desirable alleles have been combined with cultivated genotypes from un-adopted germplasm and wild species.
ii. Sources of male sterility have been identified through interspecific crosses between cultivated and wild species.
iii. Large number of hybrids has been evolved through intraspecific and interspecific hybridization i.e. by man-made gene flow.
iv. In some crop plants, new species have been developed both by natural gene flow and man-made gene flow. Examples of development of new species are wheat, tobacco, Brassica, cotton and oats. The man- made gene flow resulted in development of three new species viz Raphanobrassica, Triticate and Nicotiana digluta.
v. Man-made gene flow has resulted in development of thousands of cultivars in field crops, vegetable crops and fruit crops.
vi. Gene flow has played key role in improving various agronomic and quality traits in different field crops.
vii. Gene flow has helped in improving varietal adaptation, broadening genetic base and breaking undesirable linkages in plant breeding populations.
Term Paper # 8. Undesirable Effects of Gene Flow:
There are some undesirable effects of gene flow in nature which are briefly discussed below:
i. The gene flow from herbicide resistant genetically engineered (GE) plants to non-GE wild relatives may lead to development of herbicide resistant weeds what is known as super weed.
ii. The gene flow from GE crop to non GE organic crop will lead to contamination of organic crop resulting in financial loss to farmers in terms of price.
iii. The gene flow can make distant populations genetically similar to one another and hence reduce the chance of speciation. Lesser the gene flow between two populations more will be chances of evolving two species from such populations.