Polyploidy
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.
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Examples
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.
Polyploid crops
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 unintentially 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.
Terminology
Autopolyploidy
Autopolyploids are polyploids with chromosomes derived from a single species. Autopolyploids can arise from a naturally occuring spontaneous genome doubling (for example potato). In general, autopolyploids are sterile. Bananas and apples can be found as triploid autopolyploids.
Allopolyploidy
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
Amphidiploids refers to the process of doubling the chromosomes of an organism. It refers to the organism created from autoreduplication in an early cell division of a sterile hybrid from two different species, leading to allopolyploidy. During meiosis in the amphidiploid, the chromosomes from the parental species pair up as if the organism were diploid (hence the name), allowing for normal meiosis.
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 cytogentically as a diploid over time). In many cases, it is only through comparisons of sequenced genomes that these events can be inferred. 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. All angiosperms (flowering plants) may have paleopolyploidy in their ancestry. All eukaryotes probably experienced a polyploidy event at some point in their evolutionary history, as it's unlikely that the first living organisms had more than one stretch of DNA (i.e., one chromosome).
References
- Griffiths, A. J. et al. 2000. An introduction to genetic analysis, 7th ed. W. H. Freeman, New York ISBN 0716735202
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.
External links
| Evolution of chromosomes | (edit) |
|---|---|
| Basic topics: Chromosome | Karyotype | Ploidy | Meiosis | |
| Classification: Autosome | Sex chromosome | |
| Evolution: Chromosomal inversion | Chromosomal translocation | Polyploidy |
| Speciation guide | (edit) |
|---|---|
| Basic concepts: species | chronospecies | speciation | |
| Modes of speciation: allopatric | parapatric | sympatric | polyploidy | |
| Auxiliary mechanisms: sexual selection | punctuated equilibrium | |
| Intermediate stages: hybrid | Haldane's rule |
