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The below mentioned article provides a study note on the Artificial Induction of Polyploidy.
In recent years, a number of methods have been worked out to induce polyploidy in plants.
Some of them are described below:
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1. By Radiation:
Polyploidy can be induced in plants by exposing their certain parts, such as vegetative buds and flower buds, to radiations of shorter wavelengths, ultraviolet rays, x-rays, gamma-rays. Irradiation increases the rate of cell division and also causes the multiplication of chromosome number (somatic doubling of chromosomes).
2. By Injury:
When the meristematic zones of a plant are injured the cells at the points of injury grow rapidly and form a callus. Callus growth is enhanced by a chemical substance named coumerine which also brings about somatic doubling of chromosomes. Vegetative buds generally developing from callus tissue are polyploid in nature. From injured parts of tomato plants it is possible to produce tetraploids (4n) plants.
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3. Regeneration in vitro:
Polyploidy is a common feature in the cells of cultured tissue in vitro. Some of the plants regenerated from the callus in suspension culture may be found to be polyploids. Polyploids have been developed from callus cultures of Nicotiana, Datura, rice and several other species.
4. By chemical treatment:
A number of chemicals are now known which induce polyploidy in plants. Important among them are colchicine, 8-hydoxyquinolin, acetophenon, nitrous oxide, granosan, chloroform, chloral hydrate, some narcotics and alkaloids, veratin sulphate, acenaphthane, sulphanilamide, ethyl murcury chloride, hexachlorocyclohexane etc. Colchicine (C22O6N) is the best chemical for this purpose.
Colchicine:
It is a chemical compound which was first discovered by Pernice in 1889. Colchicine was first demonstrated by Levan (1938) to be a specific and efficient chemical in creating polyploid restitution nuclei. Colchicine is obtained from the extract of seeds (0.2 – 0.8%) and corms (0.1- 0.5%) of the Colchicum autumnale (member of family Liliaceae).
In India it is obtained from Colchicum luteum and Gloriosa superba.
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Method of Application of Colchicine:
Colchicine treatment is done in one of the following ways:
a. Seed treatment:
The dry or soaked seeds are soaked in aqueous solution of colchicine of different strength in shallow container to facilitate aeration (generally, aqueous solutions of 0.25 to 0.5 per cent concentrations are used). Colchicine treatment is given for a definite period which is different for different seeds.
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After the seeds are soaked in colchicine solution for desired period, they are washed thoroughly in water and then they are sown. Treatment of dry seeds gives better result than soaked seeds in some cases.
b. Seedling treatment:
Seedlings may be treated in young stage. During treatment, the shoot tips are dipped in 0.2% colchicine solution and root tips are covered with cotton soaked in water and the treatment may be given from 3 to 24 hours and in some cases the treatment should be repeated on 2nd and 3rd day also.
c. Treatment of growing buds of shoot:
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In some cases growing points are treated with 0.1 to 0.5% solution of colchicine which is applied by brush or with a dropper. Sometimes cotton wool soaked in the aqueous solution of colchicine is applied over the growing point of plant.
The treatment is repeated once or twice daily for a few days. Alternatively, 0.2 to 0.5% colchicine solution is mixed with lanoline paste and is smeared on the shoot apex. This treatment may be repeated 2-3 times daily for a week.
d. In woody plants, 1.0% colchicine solution may be applied over the growing bud. For proper penetration of colchicine, the solution is applied 2-3 times a day for a week.
At any given time only a small proportion of cells would be in mitotic phase, hence repeated treatment should be given at brief intervals so as to double the chromosome number in good number of cells. There may be a competition between polyploid cells and unaffected diploid cells and some-times diploid cells may compete over the polyploid cells and diploid shoots may arise from the treated growing apex.
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Effect of Colchicine:
When colchicine is applied some irregularities occur in the mitotic division. It has no effect on doubling of chromosomes which proceeds in normal fashion. This chemical is a spindle poison and it simply checks the anaphasic movement of chromosomes to the two poles and consequently mitosis fails to complete.
The doubled chromosomes become bounded by a nuclear membrane, thus a restitution nucleus with double chromosome number is formed.
Application of Allopolyploidy in Crop Breeding:
The allopolyploidy has been exploited by plant breeders for the following three purposes:
1. Utilization as bridging species in transfer of desirable characters from one species to another,
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2. For the production of new crop species, and
3. For widening the genetic base of existing allopolyploid crop species.
Utilization of Allopolyploids as Bridging Species:
An amphidiploid is utilized as bridge in transfer of desirable characters of one species to the other related species (particularly from wild species to cultivated one), when the hybrid between the cultivated species to which the desirable character is to be transferred and the wild species (donor) from which the desirable character is to be transferred is sterile.
In such cases, the chromosome number of F1, sterile hybrid between the recipient species and donor species is doubled to produce amphidiploid which is generally fertile and can be crossed with the recipient species. The progeny of the cross between recipient species and amphidiploid would have 2n chromosomes of recipient species and one genome from donor species.
This situation renders the progeny to be sufficiently fertile and so the progeny can be back crossed with the recipient species. Such a programme may give rise to alien addition and alien substitution lines which are used for transferring genes or groups of genes or small segments of chromosome to the recipient species.
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This can be clear from the following example of synthetic allohexaploid Nicotiana digulata as bridge for the transfer of resistance against tobacco mosaic virus from N. sylvestris to N. tabacum.
The scheme is as follows:
The repeated back cross may result in alien addition and alien substitution lines from which materials may be selected with desirable character.
It should be noted that in the use of a species as bridge, the features of amphidiploids are not important and it should be sufficiently fertile to be back crossed with the recipient species.
Use of Allopolyploidy in Creation of New Species:
The fact that many species of plants seem to be all natural allopolyploids can lead one to think that allopolyploidy would enable a man to create new species at will and the new species would be superior to many of the existing crop species.
But before one thinks on this line one should bear the following facts in mind:
1. Allopolyploidy may not necessarily result into a successful species as the allopolyploids may be undesirable and noxious in many cases,
2. The stability of allopolyploids is attained after a long period and so they could hardly be expected to become successful crops within a short period, and
3. The development of several superior characters in synthetic amphidiploid may take a long time and it is not certain that they would develop in the synthetic material.
These facts present a discouraging picture of the possibility for utilization of allopolyploidy in creating new synthetic allopolyploid species.
Triticale and Raphanobrassica are the examples of the two most successful synthetic species which have been evolved through allopolyploidy.
Widening of the Genetic Base of Natural Allopolyploids:
The genetic bases of some natural allopolyploid species may be narrow and the increase in genetic variabilities by producing new synthetic allopolyploids may be quite useful. Brassica napus, for example, has a narrow genetic base and genetic variability can hardly be seen in nature.
In such a case, the only recourse available is to synthesize new allopolyploid B. napus to widen the genetic base. This can be done by crossing the diploid parents B. campestris with B. oleracea and subsequently doubling the chromosomes.
