Polyploidy: Meaning, Causes and Evolution | Chromosomal Aberration

In this article we will discuss about:- 1. Meaning of Polyploidy 2. Types of Polyploids 3. Causes or Origin of Polyploidy 4. Techniques of Inducing Polyploidy 5. Evolution 6. Practical Value 7. Role in Plant Breeding 8. Some Artificially Produced Polyploids.

Meaning of Polyploidy:

An organism having more than two sets of homologous chromosomes is known as polyploid and the phenomenon polyploidy. It was discovered by Lutz. It is rarely found in animals but is of general occurrence in plants. A survey of the chromosome numbers of the species in a family shows that these species generally fall in to polyploid series e.g., 2n, 3n, 4n, 5n, 6n etc.

The species are grouped together under a taxonomic head due to certain morphological similarities and relationships. They may or may not be crossable at all, with one another. However, the chromosome numbers of the species show a general relationship i.e., they form a multiple of common basic number like 7 in Triticum, Hordeum, Pisum, Lathyrus.

Thus, the different species of Solanaceae form a polyploid series with a basic chromosome number of 12.

The basic chromosome number is usually expressed by the symbol x. Examples of Oryza are:

Oryza, x = 12

Oryza glaberrima = 24

Oryza sativa = 24

Oryza coarchtata = 48

Oryza perennis = 24

Oryza officinales = 24

Types of Polyploids:

A typical diploid is rare in nature. Most of the so called diploids are really polyploids but their behaviour is like diploids. Many of our cultivated plants are considered diploids but actually these are polyploids of two types namely primary and secondary polyploids.

1. Primary Polyploids:

Primary polyploids are of two types:

(i) Autopolyploid

(ii) Allopolyploids which may be distinguished on the basis of the source of the chromosome.

(i) Allopolyploids:

Autopolyploid are those polyploids, which have the same basic set of chromosomes multiplied. For example, if a diploid species has two similar sets of chromosomes or genomes (AA) an auto-tri-polyploid will have three similar genomes (AAA) and an autotetraploid will have four such genomes (AAAA). In most cases tetraploids are normal.

One of the most important effects of auto-polyploidy is that it often reduces fertility. Morphologically the tetraploids are larger and vigorous than the normal diploid plants. Autotetraploid condition occurs when diploid gametes fuse. One of the very common example of natural auto-ploidy relevant to Northern India pertains to ‘doob’ grass (Cynodon dactylon).

Auto-triploids are also known in water lemons, sugar beet, tomato, banana, grapes etc. Likewise, auto-tetraploids are known in rye (Secale cereale), berseem (Trifolium alexandrium), marigolds (Tagetes), corn (Zea mays), apples, Phlox and Oenothera lamarckiana (an American plant, a giant mutant), etc.

(a) Auto-triploids:

These are some times more vigorous than diploids. These are comparatively more leafy and tending to perenniality. In some cases, disturbances may be present in floral parts. The plants usually are highly sterile and rarely any seeds set.

In nature, seeds propagated triploids do not occur. On the other hand, vegetatively propagated plants can exist as triploids. This advantage is taken of in Horticulture specially for ornamental plants.It was first reported by Gates (1908) in Oenothera.

Cytological behaviour of autopolyploid is briefly discussed as below:

(i) No irregularities are noticed in somatic divisions. At telophase 3 nucleoli are formed.

(ii) Due to abnormalities in meiotic cell division, functional gametes are rarely formed in triploids. They are mostly sterile and rarely set seed on selfing or out-crossing.

(b) Autotetraploids:

These usually show greater vigour, increased cell size, mainly in stomata and guard cells. Depending on the genie make up of diploid ‘gigas’ characters may be expressed. The auto-tetraploidy leads the plant to perenniality and may show reduced fertility.

Autotetraploids are slower in growth, have greater adaptability, variability and some times show disease resistance. Because of their greater economic importance and breeding possibility, auto-tetraploids are now induced artificially.

Auto-tetiaploids have been reported in Sorghum, wheat, rice, maize, chilli, red gram, black gram, green gram, bengal gram, cotton, guava, coffee etc. It may arise by somatic doubling and somatic doubling generally happens by the failure of first meiotic division in the zygote.

(c) Auto-pentaploids:

These behave like tetraploids. The phenotype may differ. At meiosis pentavalents, quadrivalents, trivalents, bivalents and univalents may be formed. Gametes produced are fully sterile as meiosis is quite irregular.

(d) Auto-hexaploids:

These are more stable. Meiosis may be more regular and fertility high. Multivalents already occur.

(ii) Allopolyploids:

Polyploidy may also result from doubling of chromosome number of F₁ hybrid which is derived from two distinctly different species. This will bring two sets of chromosomes in F_t hybrid. Let A represent a set of chromosome ( = genome = a complete set of genes present in a haploid set of chromosomes) in species X, and let B represent another genome in a species Y. The F₁ then will have one A genome and another B genome.

The doubling of chromosome in this F₁ hybrid (AB) will give rise to a tetraploid with two A and two B genomes. Such a polyploid is called an allopolyploid or amphidiploid (form derived from a hybrid between two diploids, so that the homologues come from different sources).

An allotetraploid has been produced by the Russian Geneticist G.D. Karpechenko (1927) by crossing Raphanus sativus (2n = 18) and Brassica oleracea (2n = 18). The hybrid formed by crossing these two species is itself a diploid (2n = 18). It contains only one set of radish chromosome (n = 9) and one set of cabbage (n = 9) chromosomes. The hybrid differs from both the parents and showed many characters of both.

It is almost sterile, because radish and cabbage chromosome are so different that they do not pair or fail to pair at meiosis I. But the hybrid forms an occasional gamete which contains one complete set of radish chromosomes and one complete set of cabbage chromosomes.

When such two gametes combine they produce a plant which contains two sets of radish chromosome and two sets of cabbage chromosomes (18+18 = 36). These F₂ progenies were fertile and tetraploids. At meiosis, pairing was regular and the plant was fertile. This plant showed foliage like radish and root like cabbage. The fruit was peculier.

It resembled the cabbage in its lower portion and the radish in its apical portion. The allotetraploid bred true, hence of no practical value. As it combines characters of both radish and cabbage, therefore, has been named Raphanobrassica.

However, the experiment was significant and it demonstrated a method by which fertile interspecific hybrids can be obtained. Attempts to produce a fertile hybrid between potato and tomato, with underground parts like potato and aerial parts like tomato, have also been unsuccessful so far.

Some of the synthetic allotetraploids resemble closely with the existing species. Various species like wheat, cotton, tobacco etc. might have developed by this method. During the recent years a new genus Triticale has been synthesised by combining the chromosome of Triticum duram and Secale cereale (rye). This new genus Triticale is a very useful allopolyploid (2n = 56).

2. Aneuploidy or secondary polyploidy = (heteroploidy):

Another type of difference in the chromosome number is known as aneuploidy (not true ploidy in which the chromosome number is some other or different than the exact multiple of the basic number). This difference may be due to addition or loss of a single chromosome or several chromosomes. Aneuploids are, therefore, unbalanced individuals and show phenotypic differences.

Blakeslee (1910) discovered in Datura stromonium, the first case of aneuploidy. When a diploid individual having one chromosome represented three or four times instead of twice is called as polysomic.

When the chromosome complement is increased by one chromosome, it is called trisomic (2n + 1). These are found in Drosophila and more common in plants. Organisms containing 2n—1 chromosomes are called monosomic but they are neither fertile and nor vigorous.

If the increase is in two or more different chromosomes, each having one extra homologue, then it is called double trisomic (2n + 1 + 1). When both the chromosome of a given pair are missing, the individual is called a nullisomic (2n—2). These are inviable in some species but viable in others.

If there are two homologues added to a chromosome pair, then it is called tetrasomic (2n + 2) and when it is 2n + 2+ 2 and so on, it is called double tetrasomic.

Polysomics form trivalents, quadrivalents according to the duplications. Trisomy has led to pollen sterility.

A summary of the terms used to describe polyploidy/heteroploidy (variation in chromosome number).

Different kinds of numerical changes in chromosome (x= basic chromosome number; 2n = somatic chromosome number).

Euploidy means that the organism possess one or more full sets of chromosomes where as aneuploidy means presence of chromosome number which is different than a multiple of basic chromosome number. Let us suppose that 7 is the basic chromosome number (x) in a particular class of individuals where diploid number (2n) is 14.

In this case, chromosome number 2n = 13 and 2n = 15 would be aneuploids, while those having 2n = 7, 21, 28, 35, 42 would be euploids. A classification of different kinds of numerical changes in chromosomes is given above.

Causes or Origin of Polyploidy:

Polyploids might originate by one of the several methods given below:

(i) Doubling of the chromosomes during early stage of development of the embryo.

(ii) Union or fusion of the gametes, one or both of which due to some reason may be unreduced in chromosome number.

(iii) Two male gametes occasionally or sometimes unite with a single egg cell thus forming a triploid.

Despite above, it has been already described that aberration in cell division may lead to polyploidy. Sometimes it is caused by a large number of mutations which occur in the nature. It may also arise by manipulation of the breeder. Irregularities in cell division which occur in mitosis and meiosis, are given as below-

Mitosis:

(i) Failure of formation of cell wall under cytokinesis after the division of nucleus (karyokinesis).

(ii) At anaphase, the spindle mechanism may fail to separate the sister chromatids or daughter chromosomes to the different poles, consequently the chromosome number is doubled.

Meiosis:

Polyploid gametes may result by aberrations during meiosis:

(i) If the first, second or both the divisions fail, leading to the formation of restitution nucleus (Anucleus) which contains all the chromosomes present in a cell following the failure of cell to undergo a complete division.

(ii) By the failure of mitotic division immediately preceding meiosis, pollen mother cell giving rise to the formation of two nuclei.

(iii) If the fusion of homotypic spindle in second mitotic or equational division takes place.

(iv) If double division of chromosomes occur during both the division of meiosis.

Techniques of Inducing Polyploidy:

1. Decapitation:

It has been found in various seedlings that if their tip is removed or cut off by a sharp knife the callus is produced which give rise to some polyploids.

2. Graft combinations:

It has been observed that callus formation occurs during the graft combinations i.e., 7% (fusion of stock and scion) which may lead to some extent polyploidy – Winkler, 1916.

3. Radiations:

Irradiation of vegetative and floral buds with X-rays, gamma rays or ultra-violet rays, polyploidy may be brought in some frequencies.

4. Temperature:

Application of heat and cold shocks to flowers at or near the time of first division of zygote brings about polyploidy.

5. Hybridization:

It also to some extent brings about polyploidy.

6. Chemicals:

There are various chemicals like chloral hydrate, acenaphthelene, coumarine, vertanine sulphate (Whitkins and Berger 1944), cavadin, vernatrine, ethyl mercury chloride, vitamin sulphate, granosan, hydroxyquinoline and nitrous oxide, colchicine etc. Of these the most effective results have been obtained by colchicine and this is now being widely used on all plant species.

Nebel and Ruttle (1938) working on Tradescantia, Petunia, Marigolds & Blakeslee and Avery (1937) working on Portulaca, Datura and Cucurbita noticed that the alkaloid colchicine was most effective in doubling the chromosome number.

This alkaloid is extracted from the plant Colchicum autmnale (Family: Liliaceae) which is available in temperate regions. Colchicum luteum is an allied species found in the western Himalayans. Gloriosa superba (Liliaceae) is reported by Parthasarthy (1941) to contain the alkaloid colchicine.

In many cases partially functional spindle may be formed and incipient anaphase and even telophase may be evidenced but in no case cell wall is formed.

The final effect may be brought about in any one of the following three ways:

(i) Complete failure of spindle formation.

(ii) Fusion of nuclei after pseudo-anaphase.

(iii) Formation of sticky chromosome bridges.

Reports from Russian work show that the chemical ‘GRANOSAN’ is as effective as colchicine. Colchicine has been used with great success in a large number of crop species belonging to both dicot and monocot groups.

Colchicine is a poisonous chemical isolated from seeds (0.2-0.8%) and bulbs (0.1-0.5%). It is readily soluble in alcohol, cold water and chloroform, but is relatively less soluble in hot water.

Pure colchicine is C₂₂ H₂₅ O₆ N. It blocks spindle formation and thus prevents the movement of sister chromatids or daughter chromosomes to the opposite poles. The resulting restitution nucleus includes all the chromatids; as a result, the chromosome number is just doubled.

Colchicine affects only meristematic or dividing cells. The method of colchicine application varies considerably. It is usually applied in aqueous solution, dilute alcohol, appropriate emulsion, lanoline paste, agar solution, glycerine, cold water. It may be used up in the range of 0.0006% —1% concentration.

It is used in three ways:

(i) Seed treatment:

The seeds are immersed in the solution of 0.2 -1.6% concentration and are sown before germination has initiated.

(ii) Seedling treatment:

Here the radicle is immersed in the solution of 0.02%-0.1 concentration for 3-24 hours. The after-effect is visible subsequently and then sown in the field.

(iii) Treatment of Young shoots and buds:

The buds or tips of shoots, when are treated with colchicine (.5-1% concentration) the satisfactory results come out. Injection of colchicine solution even is effective in buds. It has already exhibited most significant result.

Evolution of Polyploidy:

Polyploidy is believed to be a method for the origin of new species. As a general rule, species with a higher chromosome number are regarded more advanced than those with lower number from evolutionary point.

Plants with a higher chromosome number are considered to have released from either by direct increase of the lower number or by crossing with the other species, hence the three species of wheat, Triticum monococcum (n = 7), Triticum duram (n = 14) and Triticum vulgare (n = 21), have been evolved by polyploidy. Several species which already remain in nature have been synthesised in the laboratory by inducing polyploidy.

According to Danish geneticist O.Winje (1917) 1/2 to 1/3 species of angiosperms are polyploids. The maximum number (about 70%) is found in Gramineae family. In Cruciferae 42% and in Leguminosae 23% are available as polyploids. Despite it, other families where polyploids are found in plenty are notable e.g., Malvaceae, Cyperaceae etc. All these species are produced by hybridization.

Any organism where the chromosome number is highly increased, the genetic differences also are increased. This is due to increase in number of genes on the chromosomes. Hence, polyploidy is of great importance in the development of taxonomic series. The varieties which are produced in this way, are very important for production of crops.

An example for clear understanding of polyploidy role in evolution is given below:

It has been found that completive ability is more in polyploids. It has got more adaptability and their geographical distribution is found maximum.

Practical Value of Polyploidy:

Induced polyploidy has important practical application. It is possible to produce disease resistant plants having desirable qualities by induced polyploidy. Polyploids are bigger in size, produce large seeds and fruits, exhibit gigas characters. These are also more vigorous. Blakeslee points out that tetraploids are of horticultural value because of the unusually large size of flowers and of the plants.

The most important use of polyploidy is to overcome or remove the sterility of hybrids which are borne by hybridization or distant cross, interspecific or inter-generic because no pairing occurs in F₁ and the gametes which are formed are abnormal. By doubling the number of chromosomes in F₁ allotetraploids (= amphidiploids) are produced which are now fertile.

Due to hybridization species of new characters may be produced. Raphanobrassica, Triticale are inter-generic cross, Inter-varietal and interspecific crosses have become most successful. The plants which are propagated asexually, gigas characters may be developed in them by application of chemicals like colchicine etc.

Under gigas character plants show larger cells, leaves, stomata, flowers, pollen grains and seeds. These exhibit slower growth, late flowering and maturity etc. These also show the size larger than diploids. The best example is sugarcane and many ornamental plants.

Polyploids are better adjusted to tolerate unfavorable climates. Some of the most important polyploid (allopolyploid) crops are wheat, cotton, oats, tobacco, Brassica species etc. Allopolyploidy has played an important role in the evolution of plant species e.g., in allotetraploid cabbage and tomato ascorbic acid content is high.

Role of Polyploidy in Plant Breeding:

The first new polyploid variety which came in the light was Galeopsis tetraliita, 2n = 32, produced from the genomes of Galeopsis spiciosa (2n = 16) and Galeopsis pudescence (2n = 16).

Similarly, there are several auto-polyploids like orchard grass (Dactylis glomerate), Alfa-alfa (Medicago sativa), potato (Solanum tuberosum), Barley (Hordeum bulbosum), Agropyron crisialum, Phleum pratense, Biscutella laevigata, Tradescantia species etc. These are all allopolyploids and perennials asexually propagated plants. Due to irregularities in the meiosis, auto-polyploids survive least in the nature.

Polyploidy is believed to be a means for the origin of new species, especially in the plants in which this phenomenon is more common. As a general rule, species with a higher chromosome number are regarded more advanced from an evolutionary point than those with lower number.

Plants with a higher chromosome number are thought to have evolved either by the direct increase of the lower number, or by crossing with the other species. Thus, as described earlier the three species of wheat, Triticum monococcum (n = 7), T. durum(n = 14), and T. vulgare (n = 21) have been evolved by polyploidy. Several species which already exist in nature have been synthesised in the laboratory by inducing polyploidy.

Induced polyploidy has important practical application. By artificially induced polyploidy, it is possible to produce disease resistant plants having desirable qualities. Polyploids are larger in size and produce large seeds and fruits. They are more vigorous too.

Some Artificially Produced Polyploids:

1. Wheat:

2. Cotton:

3. Tobacco:

4. Primula:

5. Pentaploid:

The variety B.C. 201 of Gossypium was evolved by crossing American and Asian cotton at Surat Agriculture Research Centre which chromosome number is 65 (2n).

6. Hexaploid:

In 1944, a variety of Gossypium which was triploid was made hexaploid after doubling chromosome number. It was found more stable and fertile and exhibited many irregularities in meiosis. The artificial induction of chromosome doubling by the colchicine technique has opened new fields for the production of polyploid plants.

More successful results are obtained in cross fertilized plants and those in which the chromosome number is high or that have fewer chromosomes than self fertilized plants. Superior auto-polyploids have been obtained in sugar beet, mustard, rye, turnip, red clover etc.

On account of the larger number of genes, polyploidy possess a greater potential for variability arising from mutation. Structural changes in the chromosomes are also likely to be more frequent in polyploids than in diploids and a greater variability is likely to result from crossing with different species.

Tetraploids grapes, produced in somatic tissues are now cultivated in U.S.A., England and Japan. Although the number of fruits in a cluster was low in the tetraploids but these far exceeded in weight and size than diploid and contained fewer seeds.

The first example of synthetic crop made by man with commercially valuable characters in Triticale (rye wheat) produced by Muntzing by crossing Secale cereale (rye) and Triticum aestivum (wheat) and subsequent chromosome doubling in the hybrid. Further breeding of Triticale and selection by Borlaugh in Mexico and at the IARI, Delhi have resulted in forms that are promising from an agronomic points of view.

The Russian scientist Pissorev developed day-neutral and semi-dwarf varieties of this new species. A new variety of this rye-wheat hybrid has been evolved at the IARI, New Delhi for cultivation in the desert area of Rajasthan. This new cereal is more drought resistant than other varieties of barley and wheat and resembles in production and resistance to disease and grain character. It also grows well in hill and rain-fed areas.

It is also rich in protein, but its glutein content is low that is why the chapattis are not prepared good. The research work is going on at Pantnagar to evolve commercial useful variety of this synthetic crop which may be acceptable as a food grain. Borlaugh is making the possibilities of hybridising wheat and rice cells at the cellular level to produce ‘whrice’, a combination of wheat & rice.