For the improvement of a crop it is necessary that there is genetic variation, which can be carried over to the next generation. There are several ways of doing this – domestication, germplasm collection, plant introduction, hybridisation, polyploidy, and mutation and genetic variation.
Way # 1. Domestication:
As the human beings started living a settled life they Started domestication of the plants and animals. This helped in improving the variety of already existing species. Domestication provided man with food, fibre, medicine and clothes. As the population increased, need was felt to increase the yield.
This led to technological advancement. Using specific genes from wild variety to improve the cultivated variety is an example of such technological advancement and an important step in improvement of domesticated variety.
Over a period of time, the domesticated variety has become very different from its wild relative. This change can be natural as well as anthropogenic. We are still domesticating wild varieties as well, so that they do not become extinct. USA and Japan have recently started cultivation of latex producing plants of family Euphorbiaceae. The latex will be used for extraction of petroleum products.
Way # 2. Germplasm Collection:
Germplasm is a term used to describe the genetic resources, or more precisely the DNA of an organism and collection of that material. Germplasm (or protoplasm) is a zone found in the cytoplasm of the egg cell of some model organisms (such as Drosophila melanogaster, Xenopus laevis), which contains determinants that will give rise to the germ cell lineage. As the zygote undergoes mitotic divisions the germplasm is ultimately restricted to a few cells of the embryo, these germ cells then migrate to the gonads.
The term ‘germplasm’ was first used by the German biologist August Weismann (1834-1914) to describe a component of germ cells that he proposed were responsible for heredity, roughly equitable to our modern understanding of DNA. The reason of having germplasm collection is that all the required genes are not present in the same plant, i.e. all the desired genes are present in different plants of a species.
The germplasm of a particular species consists of the following characters:
i. Improved and cultivated varieties
ii. Varieties, which are improved but are no more cultivated
iii. Local old and desi varieties
iv. Wild species which are related to the crop species
v. Different lines produced by plant breeders.
For successful plant breeding it is necessary to do intensive germplasm collection. Once we have ample collection of germplasm, it acts as a building material for construction of improved and desirable varieties.
When the modern variety is compared with the old or wild variety it is found that old and wild variety is highly variable and thus is a good source of germplasm, whereas, when we study the modern variety we find that, they are low in their germplasm variety.
The reason being that over a period of time, the characters are narrowed down by the process of selection. Secondly, the human intervention and industrialisation have also taken toll on the wild variety. This has also led to the reduction of germplasm, so it becomes more important to collect and conserve this germplasm.
A collection of genotypes of a particular species, from different sources and geographic sites, used as source materials in plant breeding is called germplasm collection. It means collecting different varieties, lines and related wild species. This can be done indigenously and also by procuring the germplasm from other organisations, agencies, etc. In India, it is done by National Bureau of Plant Genetic Resource (NBPGR), New Delhi which has a large collection of various crop species. This kind of collection is called gene bank.
Once the germplasm is collected, it is evaluated, classified and multiplied. After this it is preserved and part of it is utilised.
Germplasm can be conserved in-situ or ex-situ. In in-situ preservation the germplasm is conserved in the natural habitat. Protecting the varieties in natural parks, gene sanctuaries or natural biospheres can do this. It helps in saving the biodiversity and maintaining the natural process of evolution. The only difficulty in this method is maintenance of large areas.
Ex-situ conservation overcomes this problem. In this method, the different parts such as seed, shoot tips, genes, DNA, etc. can be stored at -196°C in liquid nitrogen which is known as cryopreservation. By this method the material can be preserved from 50 to 100 years.
Seeds, which can be cryopreserved, are called as orthodox seeds. There are seeds such as rubber, litchi, coconut, tea, cocoa, etc. which cannot be cryopreserved as they get killed and dried by freezing. These are known as recalcitrant seeds.
DNA banks have been established to save the endangered species from being extinct. These banks store the DNA of complete genome, which can be cloned. These DNA segments can be used to generate transgenic plants or organisms.
Way # 3. Plant Introduction:
This is the easiest and the oldest method of creating genetic variation in a population. Plant introduction means bringing a variety, strain, line or population from their native area to a new area. In early days, the traders or travellers used to bring the new variety to the places where they travelled.
Nowadays organisations such as Indian Agricultural Research Institute (IARI) New Delhi, and National Bureau of Plant Genetic Resource are responsible for plant introduction. Some of the plants introduced are tomato, cauliflower, potato, grapes, guava, etc. Recently introduced crop is soyabean.
Some crop varieties are introduced and released directly for commercial purpose such as dwarf Mexican wheat – Sonara 64, semi dwarf rice like TNI and IR 8. These are the varieties, which were used in green revolution in India. Coffee was introduced in South America from Africa to save it from leaf rust disease.
Introduced crop varieties may also be used to create hybrid variety of local crop. One thing that needs to be strictly observed while introducing a variety is that it does not bring any new weed, pest, insect or disease. Argemone mexicana is one such example, which entered India because of plant introduction.
To keep a check on this the varieties which are introduced should be carefully examined for the presence of weeds, pests, insect or disease. This is known as Quarantine. Quarantine is also applicable on animals and even human beings e.g., when SARS spread, quarantine was applicable on human beings. This reduced the risk of entry and spread of a pathogen from one country to another.
Way # 4. Hybridisation:
Crossing over or mating between two genetically different individuals is called as hybridisation. This is the most common and cheap method of creating genetic variation. The first plant hybridisation was done by Thomas Fair Child between Dianthus (sweet Williams) and Dianthus caryophyllus (carnation). Mendel’s laws of heredity were also based on hybridisation experiments on Pisum sativum.
The individuals related by descent are called as line. The lines or individuals used in hybridisation are called as parents. Choosing the parents is a critical job, as the success of hybridisation will be based on this selection. Once the parents are chosen one is considered as male and the other as female. The parent, which is to play the part of female, is emasculated, i.e. the anthers are removed before the pollen grains mature.
This will help in preventing self-pollination. Pollens are taken from the plant, which is considered as male and transferred to the female flower. The seeds so produced by these female flowers are the hybrid or are called F1 seeds. These will follow Mendel’s Law as segregation, independent assortment, and recombination in F2 and later generations. The coming generations show genetic variations depending on the amount of variation in the parents.
Hybridisation is of two types:
i. Intervarietal hybridisation and
ii. Distant/interspecific/intergeneric hybridisation.
i. Intervarietal Hybridisation:
Here different varieties of the same species are crossed to produce a hybrid. It can be simple – when two parents are crossed to produce F1. e.g., crossing of different varieties of wheat.
A x B → F1
It can be complex if we use more than two parents to create a hybrid. This is called convergent cross.
(A x B) x (C x D) → F1
ii. Distant/Interspecific/Intergeneric Hybridisation:
Crossing between different species of same genera (interspecific) or of different genera (inergeneric) is called as distant hybridisation. It is a different process but is used in creating disease resistance. All varieties of sugarcane cultivated today are result of interspecific hybridisation. The long fibre of cotton is the result of cross between Indian cotton, Gossoypium arboreum and American cotton, Gossypium hirsutum.
Man-made cereal Triticale is a result of intergeneric hybridisation between Triticum sp. (wheat) and Secale sp. (Rye).
Hybrids produced shows superiority over their parents and this is called as heterosis or vigour.
Way # 5. Polyploidy – In Crop Improvement:
Studies have shown that eukaryotes are either diploid or polyploid. Diploid cells have two copies of a single genome. Chromosomes which belong to the same genome differ from each other in their gene content as well as morphology. In polyploidy cells have more than two sets of chromosomes. Polyploidy is of two types – autopolyploidy and allopolyploidy.
Autopolyploids consists of more than two sets of same genome. In allopolyploids, genome is derived from different species and each genome has two copies. This phenomenon may occur due to any of the following reasons – spontaneous, regeneration in vitro, colchicines treatment, and treatment by physical agents.
i. Autopolyploidy:
Out of the above, treatment with colchicines is the most extensively used method. Colchicines inhibits the spindle formation during mitosis. Due to the absence of spindle fibres the chromatid movement does not take place during anaphase.
As a result, all the chromatids remain in the same nucleus. This leads to autopolyploidy. Colchicine is a poisonous alkaloid extracted from the bulb and seeds of Colchicum autmnale (autumn crocus) member of family Liliaceae.
Crop produced by autopolyploidy shows increase in vigour and plant size. It also produces large flowers, fruits and leaves. It also shows delayed flowering and slow growth rate.
A few examples of plants which are the result of autopolyploidy and available for commercial uses are:
1. Triploid watermelons – seedless
2. Triploid sugar beet – more sugar
3. Autotetraploid variety of vetiver (Sugandha) – gives 11% more oil
4. Autotetraploid turnip and cabbage – large in size
5. Autotetraploid ornamental plants have large flower size and longer flower duration
6. Tetraploid maize — 43% more carotenoid pigment and vitamin A.
The disadvantage of autopolyploid is seen in seed crops where the sterility rate is high and genetic instability is also seen.
ii. Allopolyploidy:
In allopolyploidy the first step is to select two parents of different species which will be hybridised to create F1 generation. The F1 generation is highly sterile. To overcome this, the chromosome number of F1 generation is doubled by treating with colchicines. After this the resulting allopolyploid is partially fertile. Allopolyploidy occurs in nature and a few of them are successful as crop plants.
Examples of naturally occurring polyploids are wheat, oat, tobacco, etc. Triticale is an allopolyploid produced by man. Man-made allopolyploids are called as synthetic allopolyploids (Fig. 4) Triticale has derived characters of two parents, i.e. high yield of wheat – Triticum turgidum and hardness of rye Secale cereale.
Sometimes we may not get the desired characters. One such example is an allopolyploid Raphanobrassica. This was a cross between radish and cabbage. The expected result was root of radish and leaves of cabbage. But the actual result was root of cabbage and leaves of radish. So this experiment was a failure.
iii. Haploidy:
A cell with single set of chromosomes is called haploid. They are highly sterile. Haploidy is found in maize. When the chromosome number of such plants is doubled we get all the characters in homozygous state. Culturing anthers or ovaries can produce haploid plants. Haploid plants are formed in nature but in low frequency. Haploidy finds application in isolation of mutants expressing itself in both dominant and recessive condition.
Way # 6. Mutation:
Mutation is a sudden, heritable and permanent change in the genetic constitution of an organism. This change can be in chromosome number, chromosome structure and molecular structure of DNA.
Most mutations are recessive and harmful. Only 0.1% mutations are useful. Though most of the mutations are harmful, still they are considered as a source of variation. Example of natural mutation is origin of Ancon – dwarf sheep. Mutation has played an important part in causing plant variation. Sweden has used this technique extensively to create variation in crops.
Mutation can be induced artificially by chemical and physical agents and are known as mutagens. Chemical mutagens are – Ethylmethan Sulphonate (EMS), Methylmethane Sulphonate (MMS), Diethyle Sulphate (DES), Acridine dyes, Base analogues, etc. Physical mutagens can be – UV rays, X rays, Gamma rays, etc. Mutations produced due to mutagens is called induced mutation.
Induced mutation in plant breeding which is used to develop improved variety is known as mutation breeding. This causes improvement in morphological and physiological characters. Mutation in plants may cause – disease resistance, increased yield, etc.
In mutation breeding, seeds are generally treated with mutagens. These are then grown in fields and self-pollinated. Progeny of this generation is grown in the next season. After this the desired mutant line is selected and released as a new variety. In India there are around 200 varieties of such crops.
Various applications of mutation breeding are:
i. Desired mutant genes can be induced in the crop plants
ii. Specific characters can be improved by this technique
iii. Quantitative traits of economic value can be improved
iv. Translocations are produced in interspecific hybrids.
Following are the examples of mutant breeding:
i. Semi dwarf high yielding variety of rice – Jagan Nath
ii. Gama rays mutation in sugarcane – CO 8152
iii. Physical mutation in cotton — MCU 7, MCU 10.
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