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Polyploid cells and organisms are those containing more than two paired (homologous) sets of chromosomes. Most species whose cells have nuclei (Eukaryotes) are diploid, meaning they have two sets of chromosomes—one set inherited from each parent. However, polyploidy is found in some organisms and is especially common in plants. In addition, polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues. This is known as endopolyploidy. Species whose cells do not have nuclei, that is, Prokaryotes, may be polyploid organisms, as seen in the large bacterium ''Epulopiscium fishelsoni'' (). Hence ploidy is defined with respect to a cell. 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. Male 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. Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not", "good", and "fold"). Therefore the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes. Polyploidy may occur due to abnormal cell division, either during mitosis, or commonly during metaphase I in meiosis. Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, but is especially 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. Polyploidy 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. == Types == Polyploid types are labeled according to the number of chromosome sets in the nucleus. The letter ''x'' is used to represent the number of chromosomes in a single set. *triploid (three sets; 3''x''), for example seedless watermelons, common in the phylum Tardigrada *tetraploid (four sets; 4''x''), for example Salmonidae fish, the cotton ''Gossypium hirsutum '' *pentaploid (five sets; 5''x''), for example Kenai Birch (''Betula papyrifera'' var. ''kenaica'') *hexaploid (six sets; 6''x''), for example wheat, kiwifruit *heptaploid or septaploid (seven sets; 7''x'') *octaploid or octoploid, (eight sets; 8''x''), for example ''Acipenser'' (genus of sturgeon fish), dahlias *decaploid (ten sets; 10''x''), for example certain strawberries *dodecaploid (twelve sets; 12''x''), for example the plant ''Celosia argentea'' or the invasive one ''Spartina anglica'' or the amphibian ''Xenopus ruwenzoriensis''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「polyploid」の詳細全文を読む スポンサード リンク
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