Polyploidy(redirected from Hexaploid)
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The occurrence of related forms possessing chromosome numbers which are multiples of a basic number (n), the haploid number. Forms having 3n chromosomes are triploids; 4n, tetraploids; 5n, pentaploids, and so on. Autopolyploids are forms derived by the multiplication of chromosomes from a single diploid organism. As a result the homologous chromosomes come from the same source. These are distinguished from allopolyploids, which are forms derived from a hybrid between two diploid organisms. As a result, the homologous chromosomes come from different sources. About one-third of the species of vascular plants have originated at least partly by polyploidy, and as many more appear to have ancestries which involve ancient occurrences of polyploidy. The condition can be induced artificially with the drug colchicine and the production of polyploid individuals has become a valuable tool for plant breeding.
In animals, most examples of polyploidy occur in groups which are parthenogenetic, or in species which reproduce asexually by fission. See Breeding (plant), Chromosome aberration, Gene, Genetics, Plant evolution, Speciation
In addition to polyploid organisms in which all of the body cells contain multiples of the basic chromosome number, most plants and animals contain particular tissues that are polyploid or polytene. Both polyploid and polytene cells contain extra copies of DNA, but they differ in the physical appearance of the chromosomes. In polytene cells the replicated copies of the DNA remain physically associated to produce giant chromosomes that are continuously visible and have a banded pattern. The term polyploid has been applied to several types of cells: multinucleate cells; cells in which the chromosomes cyclically condense but do not undergo nuclear or cellular division (this process is termed endomitosis); and cells in which the chromosomes appear to be continually in interphase, yet the replicated chromosomes are not associated in visible polytene chromosomes. See Chromosome, Chromosome aberration, Genetics, Mitosis
a multiple increase in the number of chromosomes in the cells of plants or animals. Polyploidy is widely observed among plants. Rarely encountered among dioecious animals, it does characterize ascarids and some amphibians.
In the somatic cells of plants and animals, each type of chromosome is diploid, that is, each chromosome is represented twice (2N). One is descended from the maternal organism, and the other from the paternal organism. The germ cells are hap-loid, that is, they have half the number of chromosomes (N) characteristic of somatic cells. In haploid cells each chromosome is single, without a homologous partner. The haploid number of chromosomes in the cells of organisms of the same species is referred to as the basic number; the sum total of genes contained in such a haploid set is called a genome. A haploid number of chromosomes in the germ cells originates as a result of reduction in half of the number of chromosomes during meiosis. The diploid number is restored during fertilization. (Quite often the diploid cells of plants contain supernumerary b-chromosomes. Their role has not been studied extensively, even though such chromosomes are present in such common plants as corn.)
The number of chromosomes in various species of plants is extremely diverse. For example, the fern Ophioglosum reticulata has 1,260 chromosomes per diploid set, whereas the composite Haplopappus gracilus, which is in the phylogenetically most developed family, has only two chromosomes per haploid set.
With polyploidy, deviations from the diploid number of chromosomes are observed in the somatic cells, and deviations from the haploid number in the germ cells. Cells may arise in which each chromosome set is triploid (3N), tetraploid (4N), or pentaploid (5N). Organisms having cells marked by ploidy, or multiple increases in sets of chromosomes, are called triploids, tetraploids, pentaploids. In general, they are referred to as polyploids.
A multiple increase in the number of chromosomes in the cells may be effected by extreme temperatures, ionizing radiation, chemical substances, and changes in the physiological state of the cell. These factors disrupt the separation of chromosomes in mitosis or meiosis, causing the formation of cells with multiple increases in the number of chromosomes in comparison with the original cell. The chemical agent that most effectively disrupts the normal separation of chromosomes is the alkaloid colchicine, which inhibits formation of the spindle fibers during cell division. (By treating seeds and buds with a dilute solution of colchicine, experimental polyploids are easily obtained.)
Polyploidy may arise as a result of endomitosis—chromosomal replication within a cell nucleus that does not divide. If the chromosomes do not separate during mitosis, polyploid somatic cells are formed; if they do not separate during meiosis, polyploid germ cells are formed with a modified, most often diploid, number of chromosomes. The merging of such gametes yields a polyploid zygote: two diploid gametes yield a tetraploid zygote (4N), and an unreduced gamete and a normal haploid gamete yield a triploid zygote (3N). The appearance of cells with a chromosome number three, four, five or more times the haploid number is called a genome mutation, and the forms obtained are called euploids. Along with euploidy, one often finds aneuploidy, in which cells appear with a changed number of certain chromosomes in the genome (for example, in sugar cane and in wheat-rye hybrids). A distinction is made between autopolyploidy, which is the multiple increase in the number of chromosomes of the same species, and allopolyploidy, which is the multiple increase in the number of chromosomes in hybrids when there is crossbreeding of different species (interspecific and intergeneric hybridization).
Polyploid plant forms often exhibit gigantism (increase in the size of cells and such organs as leaves, flowers, fruits), an increase in the content of a number of chemical substances, and changes in the periods of flowering and fruiting. These characteristics are observed more often in cross-pollinated forms than in self-pollinators.
The economically beneficial qualities of polyploids have long attracted the attention of breeders. This has led to much work with artificially produced polyploids, which provide an important source of variation and may be used as parent material in selective breeding (for example, triploid sugarbeet, tetraploid clover, tetraploid radish). The common drawback of autopoly-ploids is low fertility. However, after prolonged selection, lines with quite high fertility may be obtained. Good results are obtained from the creation of artificial synthetic populations composed of the more fertile lines of certain cross-pollinating autopolyploids, for example, rye.
Allopolyploids are no less important in selective breeding. The chromosome sets that compose allopolyploids are not identical, but differ in the set of genes contained in them and, sometimes, in the form and number of chromosomes. By crossbreeding plants of different genera, such as rye and wheat, a hybrid arises with a haploid set of rye chromosomes and a haploid set of wheat chromosomes. Such a hybrid is sterile, and only by doubling the number of chromosomes in each plant, that is, obtaining am-phidiploids, can meiosis be normalized and fertility restored.
Allopolyploidy can be a method for synthesizing new forms on the basis of hybridization. A classical example of such synthesis is the production by G. D. Karpenchenko of Raphanobrassica—a hybrid of radish and cabbage having 36 chromosomes (18 from the radish and 18 from the cabbage). Breeders, including V. E. Pisarev, N. V. Tsitsin, A. I. Derzhavin, and A. R. Zhe-brak in the USSR, have obtained allopolyploids in a substantial number of plant species. The majority of crops cultivated by man are polyploids.
Polyploidy has had enormous significance in the evolution of wild and cultivated plants. It is conjectured that about one-third of all plant species originated as a result of polyploidy, although in some groups, such as conifers and fungi, this phenomenon is rarely observed. Some groups of animals, predominantly those that are parthenogenetic, arose as a result of polyploidy. Proof of the role of polyploidy in evolution is given by the polyploid series, in which species of a single genus or family form a euploid series with increases in chromosome number that are multiples of the basic haploid number. For example, the wheat Triticum monococcum has 2N = 14 chromosomes, T. turgidum has 4N = 28, and T. aestivum 6N = 42. The polyploid series of Solatium species is represented by a series of forms with 12, 24, 36,48, 60, and 72 chromosomes.
Polyploid species are no less frequent among parthenogenetic animals than among apomictic plants. The Soviet scientist B. L. Astaurov was the first to obtain artificially a fertile polyploid form (tetraploid) from the hybrids of two species of the silkworm: Bombyx mort and B. mandarina. On the basis of his work he proposed the hypothesis of the indirect origin (through parthenogenesis and hybridization) of dioecious polyploid species of animals in nature.
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M. E. LOBASHEV