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Introduction The phenomenon of change in chromosome number is called heteroploidy, which includes aneuploidy and euploidy. The change in chromosome number may involve one or a few chromosomes of a genome, and the phenomenon is called aneuploidy, or the change may involve one or more complete sets of chromosomes or genomes and the phenomenon is called euploidy. The chromosome number of euploids is an exact multiple of the basic chromosome number of the species. The basic chromosome number of a species is represented by ‘X” whereas ‘n’ represents the gametic chromosome number and ‘2n’ represents the somatic chromosome number of a species. Euploidy is designated by ‘X’, ‘2X’, ‘3X’, ‘4X’, ‘6X’, etc. depending upon the number of genomes involved. Diploidy is considered as the normal condition in plants and animals. A diploid individual contains two sets of chromosomes (i.e. each chromosome is in a pair form), one set contributed by each parent during sexual reproduction. Thus diploids are 2X with two sets of chromosomes. Any deviation from this 2X condition involving whole set of chromosomes results in euploidy. The level of euploidy in an organism is, however, determined by the tolerance power of the nucleus, mitotic and meiotic behaviour of chromosomes, and gene expression and interaction. Most of the euploids are tetraploids (4X), some are hexaploids (6X) and rarely octaploids (8X) are found. Classification of Euploidy When one or more complete sets of genomes are involved, the condition is called euploidy.Euploidy is classified as: 1. Monoploidy / Haploidy 2. Diploidy 3. Polyploidy • Autopolyploidy • Allopolyploidy Heteroploidy Euploidy Aneuploidy Monoploidy Diploidy (2X) Hyperploidy Haploidy (X) (2n + 1) Tetrasomy (2n + 2) Hypoploidy Polyploidy (3X, 4X, 6X) Monosomy Trisomy Nullisomy (2n – 2) (2n – 1) Autopolyploidy Allopolyploidy (AAA, AAAA) (AABB, AABBCC) 1. Monoploidy / Haploidy Monoploids have a single basic set of chromosomes represented by ‘X’, whereas haploids have half the somatic chromosome number of an individual, represented by ‘n’, e.g. in diploid barley with 2n = 14; both monoploid and haploid chromosome number = 7. On the other hand in tetraploid barley with 2n = 28; Monoploid number = 7 but haploid number = 14. Haploids of polyploids are called polyhaploids, whereas those of aneuploids are called aneuhaploids. In general, the term haploidy is more commonly used than monoploidy. Origin and production of haploids Haploids are produced by many spontaneously or induced artificially, e.g. ways, either • Parthenogenetic development: Haploids can originate from any gametophytic cell without fertilization, e.g. egg, synergids, antipodal cells, pollen tube nucleus, etc. All these cells are haploid produced after meiosis of megaspore mother cell. These cells occasionally give rise to embryo directly. The first case of haploidy was reported in Datura by Blakeslee et al (1922), which was due to parthenogenetic development of egg. In Hymenoptera insects queen and drones are diploid females, while males are haploid. The male insects are produced parthenogenetically. Similarly in flowering plants, haploids originate due to parthenogenetic development of egg (e.g. tomato, cotton). Haploids originating from pollen tube nucleus are called androgenic haploids. • Delayed pollination: A delay in pollination induces parthenogenetic development of egg into haploid embryo, e.g. in maize. • Distant hybridization: Commonly called as Chromosome elimination technique or Bulbosum technique. This technique was developed by Kasha and Kao 1970 for the production of haploids in barley. When Hordeum vulgare and H. bulbosum are crossed, the chromosomes of H. bulbosum are eliminated in early zygotic divisions, and the embryos produced are thus haploids. These embryos can be cultured to get haploid barley plants. Bulbosum technique has been widely used to produce haploids in wheat, barley and maize etc. According to Subrahmanyam and Kasha (1973), normal fertilization between H. vulgare and H. bulbosum is follwed by gradual elimination of bulbosum chromosomes from embryo to endosperm development. Bennet et al (1976) concluded that sudden shortage of proteins in the developing embryo and endosperm and the better ability of vulgare chromosomes to form spindle attachments relative to bulbosum chromosomes may be responsible for chromosome elimination. In short, chromosome elimination is under genetic control. • Anther or pollen culture: It is the most important technique of haploid production and was developed by Guha and Maheshwari (1970). Pollen grains or anthers are cultured on suitable medium to produce pure haploid plants. This technique has been successfully employed in several crop plants for haploid production. • Haploids can also be produced by X – rays, colchicine treatment or through transfer of haploid inducing genes. Morphology and Cytology of Haploids Haploids are generally weak and show reduction in the size of all vegetative and floral parts. The size of seed, pollen, stomata, etc is also smaller in haploids as compared to diploids. Due to the expression of recessive lethal genes and other harmful genes at various stages of growth and irregular meiotic behaviour, the haploids generally die, or do not produce seeds for future generations. Haploids are maintained by vegetative propagation only. Since haploids have no homologous chromosome pairs, therefore all chromosomes are seen as univalents at metaphase –I of meiosis. The anaphase-I segregation is irregular resulting in the formation of defective gametes. Consequently, a considerable amount of sterility and almost zero percent seed set is found in haploids. Seeds can, however, be produced in haploids through apomixis and parthenogenesis. In polyploids like autotetraploids, haploids behave like diploids during meiosis. Uses of Haploids Haploids are used for the production of homozygous diploid lines (purelines) which may be directly used as cultivars or may be used in breeding programmes. Generally, it takes 8-10 years to develop a homozygous diploid line (pureline) in conventional breeding programmes. However, pollen of a superior plant can be cultured, followed by chromosome doubling of the haploid plant, to produce homozygous diploid line also called as pureline. Haploids are used for hybrid sorting in hybrid breeding: One of the essential steps in haploid breeding involves selection of superior plants among haploids derived from F1 hybrids through anther culture. It is popularly described as hybrid sorting, and virtually means selection of recombinant superior gametes. Haploids are very useful in basic and applied research on induced mutations. Due to the presence of only one set of chromosomes, even a recessive mutation is expressed. After diploidization, the useful recessive mutations can be made stable in homozygous condition. Haploids are used in cytogenetic research for production of aneuploids, alien addition and substitution lines, and determination of basic chromosome number. Haploids are currently used in transgenic technology. This helps in stability and better expression of the gene transferred during pollen or anther culture followed by diploidizing the transgenic haploid. 2. Polyploidy Polyploids have more than two sets of genomes. There are two main types of polyploids. Autopolyploids Allopolyploids A. Autopolyploidy Autopolyploids are those where the same basic set of chromosomes is multiplied. i.e. they contain more than two sets of genomes from the same source. For example if a diploid species contains two similar sets of chromosomes or genomes designated as AA, different series of autopolyploids obtained from this diploid set would be: 14) AA = 2X (diploid) – normal, e.g. X = 7, 2n = AAA = 3X (autotriploid, e.g. X = 7, 2n = 21) 28) 42) etc. AAAA = 4X (autotetraploid, e.g. X = 7, 2n = AAAAAA = (autohexaploid, e.g. X = 7, 2n = Autotriploids and Autotetraploids are found more commonly in nature than the higher polyploids. This is due to the fact that the cell of a species can accommodate a certain limit of chromosome number in its nucleus, beyond which it may affect the cell functioning or even lead to cell death. Autotriploids have been reported in grapes, watermelons, sugar beet, tomato, banana, lemon, apple, etc., where as autotetraploids have been reported in rye, corn, berseem, marigold, flox, snapdragon, grapes, apples and several vegetables. Origin and production of Autopolyploids Autopolyploids are generally produced by chromosome doubling. This chromosome doubling may occur spontaneously in nature, or may be induced artificially by physical and chemical agents. • Spontaneous production Autopolyploids are spontaneously produced by the formation of unreduced gametes followed by their fertilization. They can also be produced by chromosome doubling followed by failure of cell division or failure of 2nd meiotic division. Similarly, autopolyploids are also produced during tissue culture. Plants with various ploidy levels are regenerated from the callus in tissue culture. Naturally produced autopolyploids can be inferred by their mitotic and meiotic behaviour, e.g. karyotype analysis during mitosis, formation of multivalent (tri, tetra and hexavalents, etc.). Fig. Chromosome duplication due to fertilization between unreduced gametes (Downloaded from internet) • Induced Autopolyploids (Colchicine Treatment) The best method of inducing polyploidy is by treatment with colchicine. Colchicine is an alkaloid obtained from seeds and corms of Colchicum autumnale. It has been used with great success in a large number of crop plants belonging to both dicots and monocots. The successful doubling of chromosomes by colchicine was for the first time described by Levan, 1938 and Eigsti, 1940. Colchicine has the property of arresting and breaking the spindle or preventing spindle formation during cell division, so that chromosome duplication, followed by failure of movement of chromosomes to anaphase and failure of cell division leading to doubling of chromosome number, in the same cell occurs. Colchicine treatment is given to seeds, meristematic tips or axillary buds, etc. The concentration and duration of treatment may vary from plant to plant. Autotriploids are generally produced by crossing diploid and tetraploid genotypes of the concerned species. Figs. Chromosome doubling by colchicine treatment (Downloaded from internet). Different types of autopolyploids can be produced as follows. Diploid genome AA (2n = 20, X = 10) AA 2n = 2X = 20 Colchicine treatment AAAA 2n = 4x = 40 (Autotetraploid) X AA 2n = 20 AAA 2n = 3X = 30 (Autotriploid) treatment Colchicine AAAAAA 2n = 6X = 60 (Autohexaploid) Fig. showing various ploidy levels (Downloaded from internet) Cytology and breeding behaviour Autopolyploids generally show multivalent associations at metaphase. In case of autotriploids, trivalents of various shapes, or a bivalent plus one univalent, are observed at metaphase. The anaphase segregation of each trivalent is irregular. For example the trivalent can segregate in 2:1 or 1:1 (with one chromosome lagging). The overall chromosome segregation in triploids is highly irregular, leading to the formation of defective or sterile gametes, which fail to take part in fertilization. Thus triploids do not produce seeds due to their irregular meiotic behaviour. Triploids can, however, be maintained by vegetative propagation, or by crossing diploids and tetraploids. On the other hand, autotetraploids show tetravalents of various shapes at metaphase. Occasionally, there can be two bivalents or a trivalent plus a univalent at metaphase. At anaphase, generally there is 2:2 segregation of a tetravalent producing balanced 2X gametes which on fertilization result in the production of tetraploid progeny. Sometimes, there can be 1:3 or 2:1 (one chromosome lagging) segregation at anaphase. Thus, autotetraploids mostly produce fertile gametes; however, some sterility is also found. (Fig. downloaded from internet) Morphological features and applications of Autopolyploids: • One of the important features of autopolyploids is their gigantism. The fruits, flowers, seeds, leaves etc. are generally larger in size than in the normal diploids. • Polyploids have larger cell size than diploids. Guard cells of stomata are larger and number of stomata per unit area is lower in polyploids than in diploids. • Pollen grains of poyploids are generally larger than those of the corresponding diploids. • Polyploids are generally slower in growth and show delayed flowering and maturity. • Polyploids generally show reduced fertility due to irregularities during meiosis and due to genotypic imbalance leading to physiological disturbances. • Different species have different tolerance levels in ploidy. Some species survive up to 3X level while others can tolerate up to tetraploid (4X) and hexaploid (6X) levels. • Autotetraploids are produced in those plants where large size of seeds and fruits, increased flower and leaf size is the economic requirement, e.g. several fruit crops, vegetables, ornamentals and forage crops. • Autotriploids are produced in those plants where seed is not the economic product, e.g. banana, oranges, lemon, watermelon, grapes, apple, tomato, etc. • Autotetraploidy is used to overcome selfincompatibility in some plants, viz. Nicotiana, clover, and petunia. • Some distant crosses fail at diploid level, but are successful at tetraploid level because the hybrid produced is stable here. e.g. 4x Brassica. oleracea X 4x B. chinensis • Some examples of autopolyploids are Potato (Solanum tuberosum) 4x = 48 Coffee (Coffea arabica) 4x = 44 Alfalfa (Medicago sativa) 4x = 32 Pea nut (Arachis hypogea) 4x = 40 Sweet potato (Ipomoea batatus) 6x = 90 Banana (Musa sapientum) 3x = 33 Apple (Malus domestica) 3x = 51 Fig. Seedless fruits (Autotriploids) 2n and 4n grapes Fig. Comparison of stomata size between diploids and polyploids (Downloaded from internet) B. Allopolyploidy Allopolyploids are formed by hybridization between two or more than two different species (interspecific or intergeneric) followed by doubling of chromosomes. The two or more species can hybridize naturally, or can also be crossed artificially. Similarly, chromosome doubling of the hybrid can occur spontaneously, or can be induced by colchicine treatment. Genomes of two different species cannot survive in diploid condition in a hybrid due to lack of chromosome pairing and genetic disharmony. That is why evolution has favoured chromosome doubling so that they survive at tetraploid or hexaploid level. Schematic representation of Allopolyploidy AA x BB production of AB F1 hybrid sterile colchicine Chromosome doubling / AABB X Allotetraploid fertile CC ABC Allotriploid (sterile) doubling / Chromosome colchicine AABBCC Allohexaploid (fertile) Since allopolyploids (AABB, AABBCC) have two or more different genomes, they behave like diploids during meiosis and are thus also called amphidiploids, as the genomes of two different species behave like diploids. The production of allopolyploids has attracted considerable attention as they result into the evolution of new species. Sometimes, the genomes of two or more than two different species have some homology so that multivalents are occasionally formed during meiosis. Such polyploids are called segmental allopolyploids. It is believed that natural allopolyploids might have been produced due to following reasons. • Failure of chromosome segregation at anaphase after zygote formation. • Irregular mitotic cell division in apical meristem, leading to the formation of allopolyploid branch. • Fertilization between unreduced gametes Fig. One possible mechanism of origin allopolyploid (Downloaded from internet) of Schematic representation of origin of allopolyploidy (unreduced gametes) AA x 2n egg (AA) BB 2n pollen (BB) 4n zygote (AABB) Allotetraploid Synthetic alloplyploids can be produced artificially by hybridization between two diploid species followed by colchicine treatment or between two autotetraploid species. AAAA x BBBB Gametes AA x BB AA BB F1 sterile AABB treatment Allotetraploid (fertile) Important features of allopolyploids AB hybrid Colchicine AABB Allotetraploid Allopolyploids generally show the characteristics of both parents. However, one cannot predict whether all desirable characters of the two species would come into the hybrid or not, e.g. Raphanobrassica was produced to combine the root of radish (Raphanus sativus) and leaves of cabbage (Brassica oleracea). But the hybrid showed undesirable characters, like root of cabbage and leaves of raddish. On the other hand the first man made cereal, Triticale developed from a cross between rye (Secale cereal) and wheat (Triticum aestivum), showed all desirable characters i.e. the hardiness of rye and yielding ability of wheat. In general allopolyploids are more vigorous than diploids. Allopolyploids have better adaptability than diploids. Many allopolyploids are apomictic in nature, i.e. produce seeds without fertilization, e.g. Poa, Taraxacum, Parthenium, Rubus, Fritillaria, Tulipa etc. Allopolyploids show meiotic behaviour like diploids, unlike auto-polyploids which show multivalent associations during meiosis. Allopolyploidy helps in creation of new species with characteristics from two or more parents. Role of allopolyploidy in evolution Allopolyploids have been found more successful as crop species than autopolyploids due to their better adaptability. Many of our present day crop species are allopolyploids, and have thus contributed to a great extent in the evolution of plants. It is estimated that about 1/3rd of the present angiosperms are polyploids, and that majority of them are allopolyploids. Some of our major crops, such as wheat, brassica, cotton, tobacco, etc. are all allopolyploids. Natural allopolyploids are identified by karyotype analysis and meiotic behaviour. It has been possible to trace back the evolutionary history of many allopolyploid crop species and heir diploid progenitors have been identified with some degree of certaintity. The identification of diploid parents is primarily based on pairing between the chromosomes of diploid progenitor and the allopolyploid species. The homology between some of the chromosomes suggests that diploid species may be one of the parental species of the allopolyploid. Additional evidence on the parental diploid species is obtained by synthesising the naturally occuring allopolyploid from parental diploid species. The resemblance between synthetic and natural allopolyploid species gives an evidence of possible progenitors of the allopolyploid crop. Protein and enzyme analysis and chromosome banding provide the further confirmatory evidence. The evolutionary history of some important crop species has been worked out, some of which are discussed below. 1. Evolution of bread wheat (Triticum aestivum) The common bread wheat is an important example of alloplyploidy. It is hexaploid with 2n=42 and has evolved from three diploid species during the course of evolution. These are: I. Triticum monococcum: AA (2n=14) II. Aegilops speltoides: BB (2n=14) III. Aegilops squarrosa: DD (2n=14) The hexaploid wheat is designated as AABBDD. A cross between Triticum monococcum and Aegilops speltoides produced an unstable hybrid in which spontaneous chromosome doubling has occurred resulting in an allotetraploid Triticum diccocoides with 2n=28 (AABB). This was followed by hybridization between tetraploid Triticum diccocoides and Aegilops squarrosa. The chromosome doubling of the hybrid has finally resulted into Triticum aestivum with 2n=42(AABBDD). The present evidence suggests that A, B and D genomes from three diploid species as shown above are not much different from one another. For this reason common bread wheat is now considered as a segmental allopolyploid. Triticum monococcum X 2n=14(AA) Aegilops speltoides 2n=14(BB) F1 – 2n= 7+7 (AB) Chromosome doubling Aegilops squarrosa 2n=14 (DD) X Triticum diccocoides 2n=28 (AABB) F1 – 2n= 7+7+7 (ABD) Chromosome doubling Triticum aestivum 2n=42 (AABBDD) (Allohexaploid) Evolution of bread wheat 2. Evolution of Triticale: Triticale is the first man made cereal. Tritical was derived from a cross between wheat (Triticum sp.) and Rye (Secale cereale). Depending upon whether the tetraploid Triticum durum (2n=28) or hexaploid Triticum aestivum (2n=42) is used in the cross, Triticale is hexaploid or octaploid, respectively. a. Triticum durum 2n=28 (AABB) X Secale cereale 2n=14 (RR) F1 – 2n=3x=21(ABR) Chromosome doubling 2n=6x=42 (Hexaploid Triticale) AABBRR b. cereale Triticum aestivum X Secale 2n=42 (AABBDD) 2n=14 (RR) F1 – 2n=4x=28(ABDR) Chromosome doubling 2n=8x=56 (Octaploid Triticale) AABBDDRR 3. Evolution of Raphanobrassica (Raphanus sativus x Brassica oleracea) G.D. Karpechenko (1927) made a cross between Raphanus sativus (2n=18) and Brassica oleracea (2n=18). The F1 hybrid with 2n=18 was completely sterile due to lack of paring of chromosomes. However, among these sterile plants, he found some fertile plants. On cytological examination, these fertile plants were found to have 2n=36 chromosomes, indicating that chromosome doubling has occurred leading to normal pairing of chromosomes. (Table: Chromosome numbers of different autopolyploids - downloaded from internet) (Table: Chromosome numbers of different allopolyploids - downloaded from internet)