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**Terminology and Types of Polyploidy:**

– Autopolyploids have multiple chromosome sets from a single taxon.
– Examples include Tolmiea menzisii and Acipenser transmontanum.
– Autopolyploidy often results from fusion of unreduced gametes, leading to triploid or tetraploid offspring.
– Triploid offspring are typically sterile but may aid in the formation of tetraploids.
– Autotriploidy in plants is linked to apomictic mating systems and can result in seedlessness in crops like watermelons and bananas.
– Allopolyploids have chromosomes from two or more diverged taxa.
– Fusion of unreduced gametes from different taxa leads to allopolyploidy.
– Allopolyploidy can occur before or after hybridization.
– Hybridization followed by genome duplication is a common path to allopolyploidy.
– Allopolyploids may display intermediate levels of polysomic inheritance between autopolyploids and allopolyploids.

**Causes and Effects of Polyploidy:**

Polyploidy can result from abnormal cell division during mitosis or meiosis.
– Chemicals like colchicine and oryzalin can induce polyploidy in plants and cell cultures.
– Mammals like goldfish, salmon, and salamanders may have polyploid cells in certain organs.
Polyploidy is common in ferns, flowering plants, and agriculturally important crops like wheat and Brassica.
– Polyploidization can lead to sympatric speciation by preventing interbreeding with diploid ancestors.
– Autopolyploids possess at least three homologous chromosome sets, affecting meiosis and fertility.
– Selection for fertility can stabilize meiosis in autopolyploids.
– Autopolyploids display polysomic inheritance due to high homology among duplicated chromosomes.
– Allopolyploids may display disomic inheritance and can be distinguished from autopolyploids based on inheritance patterns.
– About half of all polyploids are estimated to result from autopolyploidy.

**Examples and Benefits of Polyploidy:**

Polyploidy occurs in plants, mammals, and other organisms.
Wheat has diploid, tetraploid, and hexaploid strains due to hybridization.
Sugarcane can have ploidy levels higher than octaploid.
– Polyploidization can lead to speciation, as seen in Erythranthe peregrina.
Polyploidy can enable gene flow between lineages, as observed in Arabidopsis arenosa and Arabidopsis lyrata.
– Heterozygosity can have beneficial effects on fitness or desirable traits in allopolyploids.
– Allopolyploidy is prevalent in crop species like bread wheat and triticale.
Cotton, peanut, and quinoa are examples of allotetraploids with multiple origins.
– The Triangle of U describes relationships in Brassicaceous crops.
– Complex patterns of allopolyploid evolution also observed in animals like Xenopus frogs.

**Unreduced Gamete Formation and Aneuploidy:**

– Hybrids between taxa often have high rates of unreduced gamete formation.
– Divergence between genomes of taxa leads to abnormal chromosome pairing or meiotic nondisjunction.
– Allopolyploidy can restore normal meiotic pairing by providing homologues for each homoeologous chromosome.
– Multivalent pairing is common in newly formed allopolyploids.
– Established allopolyploids benefit from fixed heterozygosity of homoeologous alleles.
– Aneuploidy occurs when a chromosome or segment is under- or over-represented.
– It refers to a numerical change in part of the chromosome set.

**Endopolyploidy, Monoploidy, and Other Polyploidy Variations:**

Polyploidy can occur in specific tissues of diploid animals like human muscle tissues, known as endopolyploidy.
– Prokaryotes like Epulopiscium fishelsoni can exhibit polyploidy.
Ploidy is defined in relation to a cell.
– Monoploidy involves having only one set of chromosomes, typically used for cells or organisms that are usually diploid.

Polyploidy (Wikipedia)

Polyploidy is a condition in which the cells of an organism have more than one pair of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one from each of two parents; each set contains the same number of chromosomes, and the chromosomes are joined in pairs of homologous chromosomes. However, some organisms are polyploid. Polyploidy is especially common in plants. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Males of bees and other Hymenoptera, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations. The gametophyte generation is haploid, and produces gametes by mitosis; the sporophyte generation is diploid and produces spores by meiosis.

This image shows haploid (single), diploid (double), triploid (triple), and tetraploid (quadruple) sets of chromosomes. Triploid and tetraploid chromosomes are examples of polyploidy.

Polyploidy may occur due to abnormal cell division, either during mitosis, or more commonly from the failure of chromosomes to separate during meiosis or from the fertilization of an egg by more than one sperm. In addition, it can be induced in plants and cell cultures 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. Oryzalin will also double the existing chromosome content.

Among mammals, a high frequency of polyploid cells is found in organs such as the brain, liver, heart, and bone marrow. It also occurs in the somatic cells of other animals, such as goldfish, salmon, and salamanders. It is common among ferns and flowering plants (see Hibiscus rosa-sinensis), 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. Sugarcane can have ploidy levels higher than octaploid.

Polyploidization can be a mechanism of sympatric speciation because polyploids are usually unable to interbreed with their diploid ancestors. An example is the plant Erythranthe peregrina. Sequencing confirmed that this species originated from E. × robertsii, a sterile triploid hybrid between E. guttata and E. lutea, both of which have been introduced and naturalised in the United Kingdom. New populations of E. peregrina arose on the Scottish mainland and the Orkney Islands via genome duplication from local populations of E. × robertsii. Because of a rare genetic mutation, E. peregrina is not sterile.

On the other hand, polyploidization can also be a mechanism for a kind of 'reverse speciation', whereby gene flow is enabled following the polyploidy event, even between lineages that previously experienced no gene flow as diploids. This has been detailed at the genomic level in Arabidopsis arenosa and Arabidopsis lyrata. Each of these species experienced independent autopolyploidy events (within-species polyploidy, described below), which then enabled subsequent interspecies gene flow of adaptive alleles, in this case stabilising each young polyploid lineage. Such polyploidy-enabled adative introgression may allow polyploids at act as 'allelic sponges', whereby they accumulate cryptic genomic variation that may be recruited upon encountering later environmental challenges.

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