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Polyploid - auto and allopolyploids, their characters; meaning of genome; evolution of wheat, Triticale, cotton, tobacco, Brassica
Polyploidy:
The organisms with more than two genomes are called polyploids. Among plants, polyploidy occurs in multiple series of 3, 4, 5, 6, 7, 8 etc. of the basic chromosome number and thus resulting in triploids, tetraploids, pentaploid, hexaploids, heptaploids, octaploids etc., respectively.
Generally, ploidy levels higher than tetraploid are not commonly encountered in the natural population. However, there are some exceptions. E.g.: hexaploid (6x) wheat, octaploids (8x) straw berries, many commercial fruits and ornamental plants, liver cells of man etc.
Origin of polyploidy:
Different degrees of ploidy are originated by different means. However, two basic irregular processes have been discovered by which polyploids may evolve from diploid plants and become established in nature.
Somatic doubling: Cells sometimes undergo irregularities in mitosis and give rise to meristematic cells that perpetuate these irregularities in new generations of plants.
Reproductive process: Reproductive cells may have an irregular reduction division in which the sets of chromosomes fail to separate completely to the poles at anaphase.
Both the sets thus become incorporated in the same nucleus resulting in the doubling of chromosome number in the gamete.
Thus, a triploid originates by irregularities during meiosis (i.e. by union of diploid gamete with haploid gamete) Likewise a tetraploid may originate by the somatic doubling of the chromosome number or by union of unreduced diploid gametes.
Induction of polyploidy: Polyploidy can be induced by two methods.
By physical agents
By chemical agents
By physical agents:
Temperature shocks: Extreme changes in temperature results in a higher frequency of polyploid cells.
Centrifugation: Centrifugation of seedlings or plants causes polyploidy in their cells.
X-rays: X- rays can also induce polyploidy
By chemical agents:
Some chemicals like colchicine, chlorohydrate, mercuric chloride have been found to induce polyploidy in plants. Colchicine treatment is the most effective and most widely used treatment for chromosome doubling.
The chromosome doubling effect of colchicine was first described by Blakeslee and Nebel independently.
Colchicine interferes or disturbs the formation of spindle fibres during cell division and thus inhibits the movement of sister chromatids to the opposite poles.
Colchicine is a poisonous chemical isolated from the seeds and bulbs of autumn crocus (Colchicum autumnale). Pure colchicine is C22H25O6N.
Kinds of polyploids: Polyploids are distinguished on the basis of source of chromosomes into three main kinds.
Auto polyploids,
Allopolyploids,
Segmental allopolyploids.
Autopolyploids:
In a plant, when same set of chromosomes of a genome are increased in number, autopolyploid are obtained. The prefix “auto” indicates that the ploidy involves homologous chromosome sets.
For example, if a diploid species has two similar sets of chromosomes / genomes designated as AA, an auto triploid will have three similar (AAA) genomes and autotetraploid will have four similar (AAAA) genomes.
Genetical and morphological characters expressed by autopolyploid depend on the genetic constitution of parent plant.
In general, expression is exaggerated either in positive / negative direction. For example: vegetative growth may be more vigorous. Leaves may be broader or dark green in colour. The floral parts, fruits and seeds may be bigger. However, autopolyploid occur rarely in natural populations. E.g.: Auto triploids – Cynodon dactylon (doob grass).
Auto triploids: Auto triploids have three complete sets of genomes of the same species in somatic cell. Triploids can arise in several ways.
Generally, in nature they originate by the fusion of a haploid gamete with a diploid gamete (unreduced gamete).
Diploid gametes occur sporadically as unreduced germ cell in a diploid organism. They are also produced by meiosis in tetraploid organism or in segments of otherwise diploid organisms where doubling of the somatic chromosome number has taken place.
Triploids can be produced artificially by crossing between autotetraploid and diploid species. Triploids are generally highly sterile.
In an auto triploid, there are three sets of homologous chromosomes. If these sets are normally paired, trivalent (as observed in primary trisomics) will be observed.
The trivalent cannot disjoin normally and will either disjoin 2:1 chromosome to two poles or will disjoin 1:1 leaving one chromosome as a laggard.
The number of chromosomes in the gametes of a triploid organism therefore, will vary from n to 2n. Most of these gametes are unbalanced leading to high degree of sterility.
Examples of triploids in animals are rather rare. Triploids are useful only in those plant species which propagate asexually like banana, sugarcane, apple, Sugar beet, watermelon, tomato, Cynodon dactylon (doob grass).
Seedless watermelons are grown commercially in Japan. They are produced by crossing tetraploid (4x, used as female) and diploid (2x, used as male) lines, since the reciprocal cross (diploid female X tetraploid male) is not successful.
The triploid plants do not produce true seeds. Almost all the seeds are small, white rudimentary structures like cucumber seeds. For good fruit setting, diploid plants are planted in the ratio of 1 diploid: 5 triploid plants.
Auto tetraploids: In autotetraploid, four copies of the genome of same species (AAAA or BBBB) are present.
They may arise spontaneously or can be induced artificially by doubling the chromosomes of a diploid species with colchicine treatment.
In autotetraploid, since there are four sets of chromosomes, quadrivalents are formed, which disjoin in a normal 2:2 manner giving diploid gametes.
Rarely, a quadrivalent may disjoin in 3:1 or may leave one or more chromosomes as laggards at anaphase I.
Therefore, autotetraploid also have a certain degree of sterility, although it will not be as high as in auto triploids.
Autotetraploid are usually larger and more vigorous than the diploid species. E.g.: Rye, alfalfa, grapes, groundnut, potato, coffee, Oenothera lamarckiana.
In an autotetraploid, four chromosomes are homologous to each other, hence each gene has four copies. A simplex individual has one
dominant and three recessive alleles (Aaaa), a duplex has two dominant and two recessive alleles (AAaa), a triplex has three dominant and one recessive allele (AAAa), a quadruplex has all dominant alleles (AAAA), while a nulliplex has no dominant alleles (aaaa).
Allopolyploids: A polyploid containing genetically different chromosome sets from two or more species is known as allopolyploid. The prefix “allo” indicates the involvement of non-homologous sets of chromosomes.
Origin of allopolyploids: Natural allopolyploids most likely originate through chromosome doubling of F1 hybrid produced by chance through natural hybridization between two distinct species of the same genus or from different genera.
Experimental production of allopolyploids is achieved through chromosome doubling of F1 hybrid with the help of colchicine. Such allopolyploids are often called synthetic allopolyploids.
The synthesis of allopolyploids involves two steps.
Production of F1 hybrids by crossing two distinct species
Chromosome doubling of such F1 hybrids.
The man-made cereal Triticale is an example of synthetic allopolyploid.
Amphidiploid: It is an allopolyploid (allotetraploid) which arises by combining genomes of two different species.
The amphidiploids are fertile due to the presence of homologous chromosomes and behave as a diploid during meiosis. The term amphidiploid was proposed by Nawashin.
Natural allopolyploids: Inter-specific crossing followed by chromosome doubling in nature have resulted in origin of some natural allopolyploid crops like cotton, tobacco, mustard, wheat, etc. The origin of some natural allopolyploid crops is briefly presented below:
Cotton: The new world cotton (Gossypium hirsutum) is an interesting example of allopolyploid.
J.O. Beasley crossed old world cotton (Gossypium herbaceum) with American cotton (Gossypium raimondii) and doubled the chromosome number in F1 hybrids.
The allopolyploid thus produced resembled the cultivated new world cotton (Gossypium hirsutum) and when crossed with it gave fertile F1 hybrids.
These results thus suggested that tetraploid cotton (Gossypium hirsutum) originated from two diploid species namely Gossypium herbaceum (2n = 26) and Gossypium raimondii (2n = 26).
Tobacco: There are two cultivated species of tobacco. i. e. Nicotiana tabacum and Nicotiana rustica.
Nicotiana tabacum is an allotetraploid and available evidence suggests that it is derived from a cross between Nicotiana sylvestris x Nicotiana tomentosa.
Nicotiana rustica is believed to be an amphidiploid obtained from a cross between Nicotiana paniculata and Nicotiana undulata.
Brassica: Several of Brassica species like Brassica juncea, Brassica napus and Brassica carinata are allotetraploids (amphidiploids).
It is believed that Brassica juncea is an amphidiploid derived from a cross between Brassica nigra and Brassica campestris; Brassica napus is an amphidiploid derived from a cross between Brassica oleracea and Brassica campestris and Brassica carinata is an amphidiploid derived from a cross between Brassica nigra and Brassica oleracea.
Wheat: The common or bread wheat, Triticum aestivum (formerly Triticum spelta) is an allohexaploid. It was artificially synthesized in 1946 by Mc Fadden and Sears.
It has two copies each of the genomes A, B and D and its somatic complement is represented as AA BB DD.
The sources of A and D genomes are more or less unanimously accepted as Triticum monococcum (AA) and Triticum tauschii (DD) (formerly Aegilops squarrosa –goat grass), respectively.
There is considerable doubt about the source of B genome. According to one hypothesis, Aegilops speltoides may be the source of this genome.
But recent evidences do not support this idea. Most likely, the donor of B genome is now extinct and its identity is still not clear.
Most likely, the amphidiploid AABB was produced initially. This gave rise to a tetraploid wheat, Triticum turgidum (formerly, Triticum dicoccum – emmer wheat).
This amphidiploid (AABB) was subsequently outcrossed with Triticum tauschii (formerly Aegilops squarrosa goat grass) to ultimately yield the hexaploid wheat, Triticum aestivum (AABBDD).
Artificial allopolyploids: Artificial allopolyploids have been synthesized in some crops either to study the origin of naturally available allopolyploids or to explore the possibilities of creating new species. Some examples of artificial allopolyploids are given below:
Triticale: Triticale, a man made cereal, is first produced by Muntzing. Triticale is a new crop species synthesized by crossing wheat and rye (Secale cereale).
Some triticales are hexaploids and are developed from a cross between tetraploid wheat (Triticum turgidum) and rye.
Octaploid triticales are produced from a cross between hexaploid wheat (Triticum aestivum) and rye.
Raphanobrassica: Russian geneticist G.D. Karpechenko in 1927 synthesized Raphanobrassica, which is an allopolyploid resulting from a cross between Radish and cabbage.
He wanted to develop a fertile hybrid between these two species with roots of radish and leaves of cabbage.
However, the F1 hybrids, he got, were diploid having roots of cabbage and shoots of radish.
They were highly sterile because of failure of each set of chromosomes to provide sufficient genetic homology to effect pairing.
Among these sterile F1 hybrids, he found certain fertile allotetraploids which contained 36 chromosomes due to spontaneous doubling and were named as Raphanobrassica.
Segmental allopolyploids: In some allopolyploids, different genomes present are not quite different from one another.
Consequently, in these polyploids, chromosomes from different genomes do pair together to some extent and multivalent are formed.
This means that certain segments of chromosomes and not the entire chromosomes are homologous (Homeologous chromosomes). Therefore, such allopolyploids are called segmental allopolyploids according to Stebbins (1943 – 1950).
These are intermediate between autopolyploid and allopolyploids and can be identified by their meiotic behaviour. The common hexaploidy bread wheat is also regarded as a segmental allopolyploid, because the three diploid genomes i.e. A, B and D are related (homoeologous) to each other.
Effects of polyploidy:
Genetical effects: The polyploidy often results in sterility. For example, an extra set of chromosomes in case of triploids is distributed in various combinations resulting in genetically imbalanced gametes.
Phenotypic effects: Most usual phenotypic effect of polyploidy is gigantism in morphology of plants. Eg: The tetraploid plants may have large sized pollen grains, cells of leaves, stomata, xylem etc. than a normal diploid plant. They are also more vigorous. As a result, large sized fruits, seeds and flowers are obtained from economically important plants.
Physiological effects: The ascorbic acid content has been reported to be higher in tetraploid cabbage and tomato than in corresponding normal diploid species. Corn flour of tetraploid maize has been found to contain 40% more vitamin A than that of normal diploid species.
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