Polyploidy

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Polyploidy is the condition of some biological cells and organisms of containing more than two homologous sets of chromosomes. Polyploid types are termed according 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. Where an organism is normally diploid, some spontaneous aberrations may occur which are usually caused by a hampered cell division.

[edit] Examples

Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, 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.

Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote.

Examples in animals are more common in the ‘lower’ forms such as flatworms, leeches, and brine shrimp. Polyploid animals are often sterile, so they often reproduce by parthenogenesis. Polyploid salamanders and lizards are also quite common and parthenogenetic. While mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most often result in prenatal death.

The only known exception to this rule is an octodontid rodent of Argentina's harsh desert regions, known as the Red Viscacha-Rat (Tympanoctomys barrerae), discovered by Milton Gallardo Narcisi, member of Universidad Austral de Chile. This rodent is not a rat, but kin to guinea pigs and chinchillas. Its "new" diploid [2n] number is 102 and so its cells are roughly twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n=56. It is surmised that an Octomys-like ancestor produced tetraploid (i.e., 4n=112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents; but that these likely survived the ordinarily catastrophic effects of polyploidy in mammals by shedding (via translocation or some similar mechanism) the "extra" set of sex chromosomes gained at this doubling.

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 or "goatsbeard" is another example of polyploidy resulting in new species.

[edit] Polyploidy in humans

Polyploidy also occurs in humans, although much less frequently. The polyploid conditions that have been observed in humans are triploidy (69,XXX ) and tetraploidy (92,XXXX). Additional chromosomes encode a large amount of surplus gene product, causing multiple anomalies such as defects of the heart and central nervous system. Most polyploid conceptions are spontaneously aborted, and many are incompatible with long-term survival.

Triploidy is seen infrequently, but it accounts for an estimated 15% of the chromosome abnormalities occurring at conception. Thus, the vast majority of triploid conceptions are spontaneously aborted, and many that survive to term typically die shortly after birth. The most common cause of triploidy is the fertilization of an egg by two sperm (dispermy). Triploidy can also be caused by the fusion of an ovum and a polar body, each containing 23 chromosomes, and subsequent fertilization by a sperm cell. Meiotic failure, in which a diploid sperm or egg cell is produced, can also produce a triploid zygote. If the third set of chromosomes came from the father, in dispermy or diploid sperm, the result is a molar pregnancy. If the extra set is maternal then the result is an overgrown, very abnormal embryo. Neither is viable, but occasionally an individual has mixoploidy with both a diploid and a ployploid cell line.

Tetraploidy is rarer than triploidy, both at conception and among live births. Tetraploidy can be caused by a mitotic failure in the early embryo in which all of the duplicated chromosomes migrate to one of the two daughter cells. It can also result from the fusion of two diploid zygotes.

[edit] Polyploid crops

Polyploid plants in general are more robust and 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, ginger
• 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 are commonly found as both diploid and as triploid. Daylilies (Hemerocallis) cultivars are available as either diploid or tetraploid.

[edit] Terminology

[edit] Autopolyploidy

Autopolyploids are polyploids with chromosomes derived from a single species. Autopolyploids can arise from a spontaneous, naturally-occurring genome doubling (for example, the potato). Bananas and apples can be found as triploid autopolyploids.

[edit] Allopolyploidy

Allopolyploids are polyploids with chromosomes derived from different species. 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). Amphidiploid is another word for an allopolyploid.

The giant tree Sequoia sempervirens or Coast Redwood has a hexaploid (6n) genome, and is also thought to be autoallopolyploid (AAAABB).

[edit] Paleopolyploidy

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 cytogenetically 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 have experienced a polyploidy event at some point in their evolutionary history.

[edit] See also

• Speciation
• Polyploid complex

[edit] References

• Griffiths, A. J. et al. 2000. An introduction to genetic analysis, 7th ed. W. H. Freeman, New York ISBN 0-7167-3520-2

[edit] Further reading

• Arabidopsis Genome Initiative (2000). Analysis of the genome sequence of the flowering plant Arabidopsis thaliana. Nature 408: 796-815.
• Eakin, G.S. & Behringer, R.R. (2003). Tetraploid development in the mouse. Developmental Dynamics 228: 751-766.
• Gregory, T.R. & Mable, B.K. (2005). Polyploidy in animals. In The Evolution of the Genome (edited by T.R. Gregory). Elsevier, San Diego, pp. 427-517.
• Jaillon, O. et al. (2004). Genome duplication in the teleost fish Tetraodon nigroviridis reveals the early vertebrate proto-karyotype. Nature 431: 946-957.
• Paterson, A.H., Bowers, J. E., Van de Peer, Y. & Vandepoele, K. (2005). Ancient duplication of cereal genomes. New Phytologist 165: 658-661.
• Raes, J., Vandepoele, K., Saeys, Y., Simillion, C. & Van de Peer, Y. (2003). Investigating ancient duplication events in the Arabidopsis genome. Journal of Structural and Functional Genomics 3: 117-129.
• Simillion, C., Vandepoele, K., Van Montagu, M., Zabeau, M. & Van de Peer, Y. (2002). The hidden duplication past of Arabidopsis thaliana. Proceedings of the National Academy of Science of the USA 99: 13627-13632.
• Taylor, J.S., Braasch, I., Frickey, T., Meyer, A. & Van de Peer, Y. (2003). Genome duplication, a trait shared by 22,000 species of ray-finned fish. Genome Research 13: 382-390.
• Tate, J.A., Soltis, D.E., & Soltis, P.S. (2005). Polyploidy in plants. In The Evolution of the Genome (edited by T.R. Gregory). Elsevier, San Diego, pp.371-426.
• Van de Peer, Y., Taylor, J.S. & Meyer, A. (2003). Are all fishes ancient polyploids? Journal of Structural and Functional Genomics 3: 65-73.
• Van de Peer, Y. (2004). Tetraodon genome confirms Takifugu findings: most fish are ancient polyploids. Genome Biology 5(12):250.
• Van de Peer, Y. and Meyer, A. (2005). Large-scale gene and ancient genome duplications. In The Evolution of the Genome (edited by T.R. Gregory). Elsevier, San Diego, pp.329-368
• Wolfe, K.H. & Shields, D.C. (1997). Molecular evidence for an ancient duplication of the entire yeast genome. Nature 387: 708-713.
• Wolfe, K.H. (2001). Yesterday's polyploids and the mystery of diploidization. Nature Reviews Genetics 2: 333-341.

[edit] External links

• Polyploidy on Kimball's Biology Pages
• Animation of Inversion