Triploidy

Polyploid (in Greek: πολλαπλόν - multiple) cells or organisms that contain more than two copies of each of their chromosomes. Polyploid types are termed triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n) and so on. Where an organism is normally diploid, a haploid (n) may arise as a spontaneous aberration; haploidy may also occur as a normal stage in an organism's life cycle. more...

Polyploids are defined relative to the behavior of their chromosomes at meiosis. Autopolyploids (resulting from one species doubling its chromosome number to become tetraploid, which may self-fertilize or mate with other tetraploids) exhibit multisomic inheritance, and are often the result of intraspecific hybridization, while allopolyploids (resulting from two different species interbreeding and combining their chromosomes) exhibit disomic inheritance (much like a diploid), and are often a result of interspecific hybridization. In reality these are two ends of an extreme, and most polyploids exhibit some level of multisomic inheritance, even if formed from two distinct species.

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 polyploids 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.

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. It has also been suggested that all angiosperms (flowering plants) may have paleopolyploidy in their ancestry. Technically, all living organisms 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).

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