Polyploidy

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See Polyploidy in Wikipedia, the free encyclopedia.

Polyploidy is the condition where cells have more than one set of chromosomes, as opposed to haploidy. In most organisms which reproduce by sexual reproduction the usual state of cells is diploidy. Some plants (especially flowering plants) exhibit triploidy and tetraploidy, or even octoploidy e.g. in strawberry plants and sugar cane.

Plants are more likely than animals to exhibit polyploidy because their gametes are produced from somatic cells rather than germ-line cells, and therefore go through more cell cycles, but polyploidy can also occur in some animals. Polyploidy has been observed in insects, fish, amphibians, reptiles, and once in mammals (a rat), though it was once thought polyploidy would be fatal to mammals.

Polyploidy is a route by which plants could increase their chromosome numbers, and the size of their genome. Over time changes in the duplicated chromosomes are liable to build up until they can be distinguished as different chromosomes, and the organism can once again be described as diploid ('diploidisation', during which the haploid chromosome number of the organism becomes difficult to define). Polyploidy will duplicate the genes on each chromosome, and during the subsequent changes many copies will be 'turned off', increading pseudogenes and the proportion of 'Junk DNA' in the genome.

Polyploid plants are often larger and have other accentuated characteristics, making them useful in agriculture and horticulture. Polyploidy can be artificially induced in plants by botanists and horticulturalists, e.g. with the chemical colchicine, which prevents chromosomes from separating during mitosis.

1 Polyploidy and speciation
2 See also
3 References
4 External links

Polyploidy and speciation

Polyploidy in plants has been observed to alter morphology and is believed to contribute to speciation. Polyploidy in ancient plants appears to coincide with the appearance of new species.

When tetraploid (4n) plants cross with diploid (2n) plants they form a triploid (3n) offspring. A triploid plant is sterile because cell division can not produce a haploid gamete. Tetraploid individuals can mate with other tetraploid individuals, however, so a new species will have formed. A tetraploid individual can also be formed when the diploid genomes of two different species come together in the same cell.