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Polyploidy refers to cells or organisms that contain more than two copies of each of their chromosomes. Where an organism is normally diploid, some spontaneous aberrations may occur which are usually caused by a hampered cell division. Polyploid types are termed corresponding to the number of chromosome sets in the nucleus: triploid (three sets; 3n), tetraploid (four sets; 4n), pentaploid (five sets; 5n), hexaploid (six sets; 6n) and so on. A haploid (n) only has one set of chromosomes. Haploidy may also occur as a normal stage in an organism's life cycle as in ferns and fungi. In some instances not all the chromosomes are duplicated and the condition is called aneuploidy.
Polyploidy occurs in some animals, such as goldfish, but is especially common among ferns and flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids; their relationship is described by the Triangle of U.
Examples in animals are more common in the ‘lower’ forms such as flatworms, leeches, and brine shrimp. Reproduction is often by parthenogenesis since polyploid animals are often sterile. Polyploid salamanders and lizards are also quite common and parthenogenetic. Rare instances of polyploid mammals are known, but most often result in prenatal death.
Polyploidy can be induced in cell culture by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well.
In plant breeding, the induction of polyploids is a common technique to overcome the sterility of a hybrid species. Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale.
Salsify is another example of polyploidy resulting in new species.
Polyploid plants in general are more robust and more sturdy than diploids. In the breeding of crops, those plants that are stronger and tougher are selected. Thus many crops have unintentionally been bred to a higher level of ploidy:
- Triploid crops: banana, apple
- Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek, tobacco, peanut, Pelargonium
- Hexaploid crops: chrysanthemum, bread wheat, triticale, oat
- Octaploid crops: strawberry, dahlia, pansies, sugar cane
Some crops are found in a variety of ploidy. Apples, tulips and lilies for example are commonly found as both diploid and triploid.
Autopolyploids are polyploids with chromosomes derived from a single species. Autopolyploids can arise from a naturally occurring spontaneous genome doubling (for example potato). In general, autopolyploids are sterile. Bananas and apples can be found as triploid autopolyploids.
Allopolyploids are polyploids with chromosomes derived from different species. Allopolyploids are fertile. Triticale is an example of an allopolyploid, having six chromosome sets, four from wheat (Triticum turgidum) and two from rye (Secale cereale). Cabbage is a very interesting example of a fertile allotetraploid crop (see Triangle of U). Amphidiploidy is another word for an allopolyploid.
Ancient genome duplications probably characterize all life. Duplication events that occurred long ago in the history of various evolutionary lineages can be difficult to detect because of subsequent diploidization (such that a polyploid starts to behave cytogentically as a diploid over time) as mutations and gene translations gradually make one copy of each chromosome unlike its other copy. In many cases, these events can be inferred only through comparing sequenced genomes. Examples of unexpected but recently confirmed ancient genome duplications include the baker's yeast (Saccharomyces cerevisiae), mustard weed/thale cress (Arabidopsis thaliana), rice (Oryza sativa), and an early evolutionary ancestor of the vertebrates (which includes the human lineage) and another near the origin of the teleost fishes. Angiosperms (flowering plants) may have paleopolyploidy in their ancestry. All eukaryotes probably experienced a polyploidy event at some point in their evolutionary history, as likeliest the first living organisms had only one chromosome.
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Histone (H1, H2A, H2B, H3, H4)
Centromere (A, B, C1, C2, E, F, H, I, J, K, M, N, O, P, Q, T]
|Modes of speciation:||
Allopatric • Peripatric • Parapatric • Sympatric • Polyploidy • Paleopolyploidy
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