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Polyploid (in Greek: πολλαπλόν - multiple) cells or organisms contain more than two copies (ploidy) of their chromosomes. The polyploid types are termed triploid (3x), tetraploid (4x), pentaploid (5x), hexaploid (6x) and so on. Where an organism is normally diploid, a monoploid (1x) may arise as a spontaneous aberration; monoploidy may also occur as a normal stage in an organism's life cycle.
Autopolyploids are composed of multiple sets of chromosomes from within one species, while allopolyploids are composed of chromosome sets from different species. Allopolyploids usually only form between closely related species, the chromosome of allopolyploids is described as homeologus since they are only partially homologous. Amphidiploid and allotetraploid mean having two chromosome sets from one species and two chromosome sets from another species. These are formed from the hybridisation of two separate species followed by their subsequent chromosome doubling.
Polyploidy occurs in animals but is especially common among 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 plants from the genus Brassica also show interesting inter-specific allotetraploids; the relationship is described by the Triangle of U.
Examples in animals are more common in the ‘lower’ forms such as flatworms, leeches, and brine shrimps. Reproduction is often by parthenogenesis (asexual reproduction by a female) since the polyploids are often sterile. Polyploid salamanders and lizards are also quite common and parthenogenetic.
Triploids are always sterile since one third of the chromosomes cannot pair. This trait has been exploited commercially in both animals and plants. The popular seedless watermelons are triploids and the advantage of sterility in this case is clear. Farmed oysters are also triploid and have a less clear advantage over the diploid wild species. The diploids have a spawning season and during this time they taste bad, however, since the triploids are sterile they do not spawn and are never out of season.
Polyploidy can be induced in cell culture by some chemicals: the best known is colchicine, which causes chromosome doubling.
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