Here is a compilation of notes on Polyploidy. After reading these notes you will learn about: 1. Meaning of Polyploidy 2. Origin of Polyploidy 3. Types 4. Induction 5. Effects 6. Complex 7. Roles.
Contents:
- Notes on the Meaning of Polyploidy
- Notes on the Origin of Polyploidy
- Notes on the Types of Polyploidy
- Notes on the Induction of Polyploidy
- Notes on the Effects of Polyploidy
- Notes on the Polyploidy Complex
- Notes on the Roles of Polyploidy
Note # 1. Meaning of Polyploidy:
Plant species with three or more genomes are Polyploidy. The basic set of chromosomes undergoes multiplications. For example, in Chrysanthemum basic set is x = 9. Its species and hybrids show multiple of 9, such as 18, 27, 36, 45. In Nicotiana and Solanum basic set is x = 12 and multiple of somatic chromosome numbers are 24, 48 and 72 and in Triticum it is x = 7 and multiples are 14, 21, 42.
Also among fruit plants, such as banana – Musa sapientum (3x = 33) and mango – Mangifera indica (40), polyploidy is quite frequent. On the other hand, rye, barley and beet, the diploid is maintained (14 in rye and barley, 18 in beet).
In several horticultural species like that of Tradescantia (2n = 12, 24) and Chrysanthemum (2n = 27, 36, 72), polyploidy is well known. Polyploidy occurs in large number of plants, ferns and several mosses, whereas in coniferous trees, the phenomenon is quite rare.
About one half of all known plant genera contain Polyploidy, but polyploidy is rarely seen in animals. This may be because sex balance in animals is much more delicate than that in plants.
Note # 2. Origin of Polyploidy:
Polyploidy may arise either by abnormal mitosis or by abnormal meiosis.
(i) Origin of Polyploidy by abnormal mitosis:
Polyploidy may arise if the chromosomes of a dividing cell fail to separate or cell division stops after the duplication of chromosome. The cell thus produced is with double number of chromosomes than the diploid parent cell.
If such tetraploidy occurs:
(a) In the zygote or
(b) In a group of cells at the shoot apical region, or
(c) In the single apical initial, a tetraploid plant may directly originate in the first case and a tetraploid shoot in the latter two cases.
The tetraploid shoots on maturity may form flowers with diploid gametes and ultimately the seeds produced will develop in tetraploid plants. In case of vegetatively reproducing plants, tetraploid clones may develop into tetraploid plants directly.
(ii) Origin of Polyploidy by abnormal meiosis:
The homologous chromosomes synapse and prepare for normal reduction division, but due to some reasons these may fail to occur. Therefore, daughter cell receives all the chromosomes in the restitution nucleus which undergoes second mitotic division and produces two diploid daughter cells which form diploid gametes.
When these diploid gametes unite with the normal haploid gametes, triploids are produced or if fuse with each other, tetraploids are formed.
Note # 3. Types of Polyploidy:
There are mainly four different types of Polyploidy, namely:
i) Auto-Polyploidy,
ii) Allopolyploids,
iii) Segmental allopolyploids and
iv) Auto-allopolyploids (Fig. 11.7).
(i) Auto-Polyploidy:
Auto-Polyploidy occur when the same genome is duplicated, i.e., the same basic set of chromosomes is multiplied. For instance, if a diploid species has two similar sets of chromosome or genomes (AA), an auto-triploid will have three similar genomes (AAA) and an auto- tetraploid will have four such genomes (AAAA).
Auto-triploids are known in watermelon, banana, sugar beet, tomato, grapes, and auto-tetraploids are common in rye, corn, red clover, snapdragon and Allium tuberosum.
Meiosis in an Autopolyploid:
Meiotic behaviour in an autopolyploid such as autotetraploid is different than in a diploid. This is due to the presence of four homologous chromosomes of each kind.
Assuming that the primary material is a diploid species with 14 chromosomes (AA), these will form seven pairs (bivalents) at meiosis (Fig. 11.8). In the tetraploid (AAAA) there will be four chromosomes of each type, and at meiosis, these seven groups of four chromosomes may form seven quadrivalents.
A quadrivalent is an association of four homologous chromosomes (Fig. 11.9). Quadrivalents may be of different appearances. Sometimes, the homologous chromosomes are represented by an association of three chromosomes, called a trivalent and a univalent (Fig. 11.9) or by two bivalents.
As a rule, the average number of quadrivalents per cell is, therefore, lower than the medium possible number. Autotetraploids of different species behave differently in this respect. Some of them have a very high frequency of quadrivalents as in A. tuberosum (Fig. 11.9), in some cases bivalents are formed.
The occurrence of trivaients and univalents at meiosis in an autotetraploid leads to disturbances in chromosome distribution and to the formation of gametes with deviating chromosome numbers. This is the principal cause for the high degree of sterility in an autotetraploid.
Segregation of Genes in Autopolyploids:
The number of alleles of each gene is represented according to the ploidy level of the Polyploidy individual and gametes containing more than one allele of each gene (homo- or heterozygotic) may be produced.
According to the number of dominant and recessive alleles at a particular locus, the genotype of an autotetraploid may be quadriplex (AAAA or A4), triplex (AAAa or A3a), duplex (AAaa or A2a2), monoplex or simplex (Aaaa or Aa3) and nulliplex (aaaa or a4).
Auto-Polyploidy such as tetraploids show the so- called tetrasomic inheritance. The segregation of genes in auto-Polyploidy is affected by factors which play no essential role in diploid.
Among such factors are the number and position of chiasmata in the multivalents, the distance between particular locus and centromere, the behaviour of homologues in multivalent associations during anaphase I and the presence of univalents.
In auto-tetraploids, if it is assumed that the four homologous chromosomes are distributed to poles in 2:2 during anaphase I, theoretical segregation ratios for various autotetraploid genotypes of a locus may be calculated (Table 11.3).
(ii) Allopolyploids:
Polyploidy may also result from doubling of chromosome number in hybrid which is derived from two or more distinctly different species. This brings two (or more) different sets of chromosome in hybrid. The doubling of chromosomes in the hybrid, which gives rise to a Polyploidy, is called an allopolyploid.
An allopolyploid in which a sterile hybrid (AB) originating out of the combination of two different species, undergoes duplication of chromosome set, is known as amphidiploid (AABB) (Fig. 11.10).
Raphanobrassica is a classical example of amphidiploidy. In 1927, Karpechenko, a Russian scientist, reported a cross between Raphanus sativus (2n = 18) and Brassica oleracea (2n = 18) to produce F2 hybrids which were completely sterile.
This sterility was due to lack of chromosome pairing, since there is no homology between genomes from Raphanus sativus and Brassica oleracea. Among these sterile hybrids certain fertile plants were found. On cytological examination, these fertile plants were found to have 2n = 36 chromosomes, which showed normal pairing into 18 bivalents (Fig. 11.11).
Thus in allopolyploids the paring is of autosyndesis type (paternal-paternal or maternal- maternal pairing) in contrast to allosyndesis (paternal-maternal pairing) in diploids and autopolyploid.
Of the allopolyploids, amphidiploid hybrids containing two sets of each species are of special importance because they are usually fertile, occur rather widely among angiosperms in nature, afford clues to the relationship of certain species, and open a new path to the improvement of cultivated plants.
One of the earliest known amphidiploid hybrids was the fertile Primula kewensis, with 36 somatic chromosomes. A cross between P. floribunda (2n = 18) and P. verticillata (2n = 18) had yielded the sterile diploid P. kewensis (2n = 18) with one genome from each parent species.
From a lateral bud on this plant there arose spontaneously a tetraploid shoot with two genomes from each parent, and this proved to be fertile. The numerical changes may be represented as (9 + 9) X 2 = 36. Some amphidiploids have arisen from crosses of species differing in chromosome number.
The chromosome complement, for example, of Nicotiana digJuta arose from a cross of N. giutinosa (24 somatic chromosomes) and N. tabacum (48 somatic chromosomes): (12 + 24) x 2 = 72.
The species N. tabacum is again a tetraploid with 2 genomes from two different species. N. digluta, in terms of the basic number for the genus (12), would be allohexaploid with 4 genomes from one species and 2 from the other. The general formula for such cases would be (x + 2x) x 2 = 6x. Further examples of this type of hybrid include Gossypium sp. hybrid (26 + 13) x 2 = 78.
Common cultivated wheat is another important example of allopolyploidy. There are three different chromosome numbers in the genus Triticum, namely, 2n = 14, 2n = 28, 2n = 42. The common wheat is hexaploid with 2n = 42 and is derived by Kihara, Sears from three diploid species, Triticum monococcum, Aegilops speltoides and Aegilops squarrosa (Fig. 11.12).
Allopolyploids thus may be artificially synthesized. The tetraploid cotton (Fig. 11.13) is another example of artificially synthesized allopolypbids. Origin of some allopolyploid species flowering plants has been represented in Table 11.4.
Amphidiploids sometimes arise in ways other than by somatic chromosome doubling. Diploid spores, and, therefore, diploid gametes may appear on failure of meiosis and union of two diploid gametes gives rise to tetraploid. Although the chance of obtaining such plants in this manner seems to be relatively small.
(iii) Segmental Allopolyploids:
In some allopolyploids the different genomes that are present are not quite different from one another, i.e., having partial homology with each other (616,8282). Consequently, in these Polyploidy , chromosomes from different genomes do pair together to some extent and multivalents are formed. This means that segments of chromosomes and not the whole chromosome is homologous.
Such allopolyploids are called segmental allopolyploids (Stebbins). These chromosomes which are partially homologous and not completely homologous with each other are sometimes also described as homologous chromosomes. It is also believed that most of the naturally occurring Polyploidy are neither true auto-Polyploidy nor true allopolyploids.
Soianum tuberosum is the best example of segmental allopolyploid.
(iv) Auto-Allopolyploids:
When autopolyploidy is combined with allopolyploidy, autoallopolyploids are produced (AAAA6B). Polyploidy of this type are possible from hexaploid level upward as observed in Nicotiana tabacum and Soianum nigrum. Autoallopolyploids have importance in the evolution of certain plant species.
Note # 4. Induction of Polyploidy:
For induction of polyploidy two basic strategies are adopted:
(i) Prevention of the halving of the chromosome number at meiosis and
(ii) Suppression of chromosome separation at mitosis.
Both methods have yielded positive results. Under the influence of various agents the chromosomes may divide, but the daughter halves fail to separate and remain in the same cell.
By different external agents, especially treatment with narcotics and high or low temperatures, meiosis may be disturbed and the normal halving of the chromosome number does not occur. In this way unreduced gametes are formed. In auto-triploids this happens spontaneously, because meiosis is always irregular.
(a) Temperature treatment:
An important means to double the chromosome number is the treatment of ordinary vegetative cell or zygote by various external agents. One method is to expose the fertilized egg cell to a heat shock (40-45°C) at the time of its first division. A low but regular percentage of the seeds obtained in this way give rise to auto-tetraploids.
(b) Radiation:
Polyploidy may 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).
(c) 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 coumarin which also brings about somatic doubling of chromosomes. Vegetative buds generally developing from callus tissue are Polyploidy in nature. From injured parts of tomato plants, it is possible to produce tetraploid plants.
(d) Regeneration in vitro:
Polyploidy is a common feature in the cells of cultured tissue in vitro. Some of the plants regenerated from the callus or suspension culture may be found to be Polyploidy. Polyploidy have been developed from callus cultures of Nicotiana, Datura, rice and several other species.
(e) Chemical treatment:
A number of chemicals are now known which induce polyploidy in plants. Important among them are colchicine, 8-hydroxyquinoline, nitrous oxide, chloral hydrate, some narcotics and alkaloids, veratin sulphate, acenaphthane, and gammexane (hexachlorocyclohexane). Colchicine (C22O6N) is the best chemical for this purpose.
Colchicine was first demonstrated to be a specific and efficient chemical in creating Polyploidy restitution nuclei by Eigsti and Dustin in 1955. Colchicine is obtained from the extract of seeds and corms of Colchicum autumnale, Colchicum luteum and Gloriosa superba of family Liliaceae.
Method of Application of Colchicine:
Colchicine treatment is done in one of the following ways:
i. Seed treatment:
The dry or soaked seeds are soaked in aqueous solution of colchicine of different strength in shallow container to facilitate aeration (generally, solutions of 0.05 to 0.5% concentrations are used).Colchicine treatment is given for a definite period which is different for different seeds.
After the seeds are soaked in colchicine solution for a desired period, they are washed thoroughly in water and then sown. Treatment of dry seeds gives better result than soaked seeds in some cases.
ii. 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. The treatment may be given from 3 to 24 hours and in some cases the treatment should be repeated on 2nd and 3rd days.
iii. Treatment of growing buds of shoot:
In some cases, growing points are treated with 0.1 to 0.5% solution of colchicine which is applied with a brush or a dropper. Sometimes cotton 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.
C-mitosis:
C-mitosis or Stathmokinesis and C-tumour formation is so named because it was first observed with colchicine. It takes place through the breakdown of the spindle after the chromatids have separated at the end of metaphase, so that they lie within the same cell without subsequent cell plate formation.
When the tissue is allowed to recover, the chromosome number is doubled resulting in polyploidy. Prolonged treatment may lead to high degrees of polyploidy as observed with gammexane.
The C-mitotic activity is inversely proportional to its solubility in water in case of most chemicals. Colchicine is, however, an exception. It is highly soluble in water but even at very low concentrations (0.5 per cent) is capable of causing spindle inhibition and arresting metaphase.
As a result, a large number of metaphase can be obtained. C-tumour formation results in the formation of bead-like swellings in the root-tips. The cells, due to loss of polarity, result in disorganized division.
This effect may occur independent of C-mitosis though it usually accompanies the latter.
Gavauden divided C-mitotic chemicals into two groups:
(a) Those in which the threshold follows the physical property of the chemical, e.g., solubility, showing that the effect depends on a physical action, and
(b) Those in which a large margin is observed between reaction threshold and water solubility, indicating involvement of chemical reactions.
An example is colchicine, in which one exchange of methoxy and aldehyde groups in the C rings, forms iso- colchicine. The latter does not have C-mitotic activity.
Endopolyploidy:
The process of chromosome duplication without cell division is called endopolyploidy. In this process a cell with successive S phases without entering into divisional phase subjected to endomitosis. This resulted in polytene chromosome as found typically in the salivary gland of Drosophila as well as in the tapetum, endosperm and suspensor of many plants.
They arise due to repeated longitudinal spitting’s of chromatids and consequent non-separation of split portions.
Note # 5. Effects of Polyploidy:
External properties mode of reproduction and Physiological changes in Polyploidy: With regard to external characters, auto-tetraploids are characterized by a certain degree of giganticism – stems, leaves, flowers and seeds having greater dimensions than in the original diploids. Moreover, stomatal size shows an increase.
These changes, which are often very striking and therefore, of great importance for the production of new types of ornamental plants, are primarily due to the fact that the cells are considerably larger in the tetraploids. In general, doubling of the chromosome number leads to an increase in the size of the various organs and in many cases, but certainly not always, to an increase in the size of the entire plant.
It should be stressed as well, that primarily the tetraploids are often weaker and more disharmonious. Moreover, meiotic behaviour in polyploids, due to sudden increase in chromosome number leading to dis-balance in nucleocytoplasmic ratio, is very irregular at the initial stage.
The general outcome is the high gametic sterility. At the initial stage, polyploids often resort to apomictic type of reproduction without undergoing fertilization. In this way the problem of gametic imbalance leading to sterility is avoided at the formative stage.
Gradually, in evolution, through selective pressure, the nucleocytoplasmic balance is restored, regular segregation comes in and Polyploidy survive with fertile seeds. The tetraploids which ultimately have been derived from the primary tetraploids after a period of gene recombination and selection are thus stable and behave normally.
Chromosome doubling also has physiological consequences. Auto-Polyploidy often have a lower osmotic pressure, a retarded rate of cell division, and a longer vegetative period than the corresponding diploids. The lower osmotic pressure often leads to reduced frost hardiness, in several cases, differences in the contents of vitamins and in the chemical composition of the cells have also been found.
The physiological effects also lead to the fact that the number of flowers that are embryo logically formed and developed are often lower in the tetraploids than in the original diploid maternal. In general, the Polyploidy are more resistant to temperatures and climatic stress than diploids.
Note # 6. Polyploidy Complex:
In many groups of plants, different types of Polyploidy exist together with their diploid progenitors. Diploids may develop auto-Polyploidy by the increase of the same genome. Mixing of genomes of two or more diploids may give rise to allopolyploids.
Closely related diploid species can produce segmental allopolyploids; auto-allopolyploids may develop involving two or more genomes. By these means can arise the type of variation pattern designated by Babcock and Stebbins the Polyploidy complex. Such a complex constitutes a series of diploid forms with a great numbers of intermediate Polyploidy (Fig. 11.7).
The species of Crepis form a Polyploidy series having chromosome numbers 33, 44, 55, 77, 88, based on the haploid number x = 11.
There are seven diploid species — C. pleurocarpa, C. monticola, C. bakeri, C. occidental is, C. modocensis, C. atribarba, C. acuminata. Polyploidy species show in their external morphology various combinations of characteristics of two or more diploid species. Intermediate Polyploidy species appear to be allopolyploids.
In a Polyploidy complex, the Polyploidy species can acquire greater ecological amplitude than diploid species which gives them a high degree of buffering against environmental changes over long periods of time. This lead to entirely different evolutionary patterns among the Polyploidy members as compared to the diploid representatives of any particular Polyploidy complex.
As the Polyploidy complex becomes older, diploid members become progressively more restricted in geographic distribution and finally extinct. The Polyploidy members on the other hand, enlarge their gene pools and geographic distributions.
Note # 7. Role of Polyploidy:
Some of the important roles played by polyploidy are described below:
i. Role of Polyploidy in Plant Breeding:
When the techniques for artificial chromosome doubling became established, investigations on the origin of many of our economic plants were resumed. Many important crop plants like wheat, oat, sugarcane, cotton, tobacco as well as many fruits and vegetables are the Polyploidy of various degrees.
One of the important effects of polyploidy is the changes in the blooming season of the induced Polyploidy . As such, interspecific hybrids can be obtained of such species which otherwise remain isolated by seasonal isolation and different blooming season.
By artificial polyploidy induction, disease resistance and other desirable characters have been incorporated into some commercial crop plants. For example, Nicotiana tabacum is susceptible to TMV whereas N. glutinosa appears to be resistant.
The two tobacco species when crossed, the hybrids were found to be resistant but totally sterile. When the chromosomes were doubled it was possible to secure a fertile Polyploidy resistant to the virus. Many Polyploidy are selected and cultivated because of their larger size, vigour and ornamental values. Several varieties of apples, pears and grapes have produced giant fruits which are of much economic value.
ii. Role of Polyploidy in Evolution:
Polyploidy combined with interspecific hybridization provides a mechanism by which new species may arise in nature and play a role in evolution. Allopolyploidy can produce new species by combining new characters and stable in evolution. It has already been discussed under amphidiploidy how different types of new species may be evolved.
Among the interspecific hybridization, the most important are Primula kewensis (n = 18) obtained by crossing P. floribunda (n = 9) and P. verticillata (n = 9), Digitalis mertqnensis (n = 56) obtained by crossing D. purpurea (n = 28) and D. ambigua (n = 28) and Spartina townsendii (n = 63) obtained from cross of S. stricta (n = 28) and S. alterniflora (n = 35).
The above observations have substantiated the importance of polyploidy in evolution.
Origin of some of the economically important plants like rice, wheat, cotton, tobacco is important in this aspect. The chromosome number of rice (Oryza sativa) is 2n = 24. It is an example of typical secondary allopolyploids with basic chromosome number x = 5.
The present cultivated variety of rice is actually produced by hybridization followed by aneuploidy and euploidy. The origin of wheat, cotton, tobacco, etc. have been discussed earlier.
iii. Media of Conservation of Characters:
Polyploidy plays an important role in conserving the characters. A recessive mutation in order to be expressed in an autotetraploid, all four genes must be in recessive condition which is a time requiring process. Thus the characters in a Polyploidy plant could be conserved.
iv. Polyploidy and Geographical Distribution:
The Polyploidy plants can cope with diverse geographical areas than a diploid. Hence, the geographical distributions of Polyploidy plants are greater than diploids. Auto- Polyploidy cannot produce new species, but they can colonize a new environment easily. As allopolyploids contain different genomes, they can withstand different environmental condition.
Both these power of colonization and coping with a diverse environment of the Polyploidy plants, help their wide geographical distribution.