or more. specific terms are triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), octoploid (8 sets), nonaploid (9 sets), decaploid (10 sets), undecaploid (11 sets), dodecaploid (12 sets), tridecaploid (13 sets), tetradecaploid (14 sets), etc. some higher ploidies include hexadecaploid (16 sets), dotriacontaploid (32 sets), and tetrahexacontaploid (64 sets), though greek terminology may be set aside for readability in cases of higher ploidy (such as "16-ploid"). polytene chromosomes of plants and fruit flies can be 1024-ploid. ploidy of systems such as the salivary gland, elaiosome, endosperm, and trophoblast can exceed this, up to 1048576-ploid in the silk glands of the commercial silkworm bombyx mori. the chromosome sets may be from the same species or from closely related species. in the latter case, these are known as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). allopolyploids are formed from the hybridization of two separate species. in plants, this probably most often occurs from the pairing of meiotically unreduced gametes, and not by diploid–diploid hybridization followed by chromosome doubling. the so-called brassica triangle is an example of allopolyploidy, where three different parent species have hybridized in all possible pair combinations to produce three new species. polyploidy occurs commonly in plants, but rarely in animals. even in diploid organisms, many somatic cells are polyploid due to a process called endoreduplication, where duplication of the genome occurs without mitosis (cell division). the extreme in polyploidy occurs in the fern genus ophioglossum, the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over one thousand. it is possible for polyploid organisms to revert to lower ploidy by haploidisation. in bacteria and archaea polyploidy is a characteristic of the bacterium deinococcus radiodurans and of the archaeon halobacterium salinarum. these two species are highly resistant to ionizing radiation and desiccation, conditions that induce dna double-strand breaks. this resistance appears to be due to efficient homologous recombinational repair. variable or indefinite ploidy depending on growth conditions, prokaryotes such as bacteria may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. this is because under exponential growth conditions the cells are able to replicate their dna faster than they can divide. in ciliates, the macronucleus is called ampliploid, because only part of the genome is amplified. mixoploidy mixoploidy is the case where two cell lines, one diploid and one polyploid, coexist within the same organism. though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. there are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69, and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. it is a major topic of cytology. dihaploidy and polyhaploidy dihaploid and polyhaploid cells are formed by haploidisation of polyploids, i.e., by halving the chromosome constitution. dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. tetraploids can be reconstituted from the diploids, for example by somatic fusion. the term "dihaploid" was coined by bender to combine in one word the number of genome copies (diploid) and their origin (haploid). the term is well established in this original sense, but it has also been used for doubled monoploids or doubled haploids, which are homozygous and used for genetic research. euploidy and aneuploidy euploidy (greek eu, "true" or "even") is the state of a cell or organism having one or more than one set of the same set of chromosomes, possibly excluding the sex-determining chromosomes. for example, most human cells have 2 of each of the 23 homologous monoploid chromosomes, for a total of 46 chromosomes. a human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid. euploid karyotypes would consequentially be a multiple of the haploid number, which in humans is 23. aneuploidy is the state where one or more individual chromosomes of a normal set are absent or present in more than their usual number of copies (excluding the absence or presence of complete sets, which is considered euploidy). unlike euploidy, aneuploid karyotypes will not be a multiple of the haploid number. in humans, examples of aneuploidy include having a single extra chromosome (as in down syndrome, where affected individuals have three copies of chromosome 21) or missing a chromosome (as in turner syndrome, where affected individuals have only one sex chromosome). aneuploid karyotypes are given names with the suffix -somy (rather than -ploidy, used for euploid karyotypes), such as trisomy and monosomy. homoploid homoploid means "at the same ploidy level", i.e. having the same number of homologous chromosomes. for example, homoploid hybridization is hybridization where the offspring have the same ploidy level as the two parental species. this contrasts with a common situation in plants where chromosome doubling accompanies or occurs soon after hybridization. similarly, homoploid speciation contrasts with polyploid speciation. zygoidy and azygoidy zygoidy is the state in which the chromosomes are paired and can undergo meiosis. the zygoid state of a species may be diploid or polyploid. in the azygoid state the chromosomes are unpaired. it may be the natural state of some asexual species or may occur after meiosis. in diploid organisms the azygoid state is monoploid. (see below for dihaploidy.) special cases more than one nucleus per cell in the strictest sense, ploidy refers to the number of sets of chromosomes in a single nucleus rather than in the cell as a whole. because in most situations there is only one nucleus per cell, it is commonplace to speak of the ploidy of a cell, but in cases in which there is more than one nucleus per cell, more specific definitions are required when ploidy is discussed. authors may at times report the total combined ploidy of all nuclei present within the cell membrane of a syncytium, though usually the ploidy of each nucleus is described individually. for example, a fungal dikaryon with two separate haploid nuclei is distinguished from a diploid cell in which the chromosomes share a nucleus and can be shuffled together. ancestral ploidy levels it is possible on rare occasions for ploidy to increase in the germline, which can result in polyploid offspring and ultimately polyploid species. this is an important evolutionary mechanism in both plants and animals and is known as a primary driver of speciation. as a result, it may become desirable to distinguish between the ploidy of a species or variety as it presently breeds and that of an ancestor. the number of chromosomes in the ancestral (non-homologous) set is called the monoploid number (x), and is distinct from the haploid number (n) in the organism as it now reproduces. common wheat (triticum aestivum) is an organism in which x and n differ. each plant has a total of six sets of chromosomes (with two sets likely having been obtained from each of three different diploid species that are its distant ancestors). the somatic cells are hexaploid, 2n = 6x = 42 (where the monoploid number x = 7 and the haploid number n = 21). the gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor, einkorn wheat. tetraploidy (four sets of chromosomes, 2n = 4x) is common in many plant species, and also occurs in amphibians, reptiles, and insects. for example, species of xenopus (african toads) form a ploidy series, featuring diploid (x. tropicalis, 2n=20), tetraploid (x. laevis, 4n=36), octaploid (x. wittei, 8n=72), and dodecaploid (x. ruwenzoriensis, 12n=108) species. over evolutionary time scales in which chromosomal polymorphisms accumulate, these changes become less apparent by karyotype – for example, humans are generally regarded as diploid, but the 2r hypothesis has confirmed two rounds of whole genome duplication in early vertebrate ancestors. haplodiploidy ploidy can also vary between individuals of the same species or at different stages of the life cycle. in some insects it differs by caste. in humans, only the gametes are haploid, but in many of the social insects, including ants, bees, and termites, males develop from unfertilized eggs, making them haploid for their entire lives, even as adults. in the australian bulldog ant, myrmecia pilosula, a haplodiploid species, haploid individuals of this species have a single chromosome and diploid individuals have two chromosomes. in entamoeba, the ploidy level varies from 4n to 40n in a single population. alternation of generations occurs in most plants, with individuals "alternating" ploidy level between different stages of their sexual life cycle. tissue-specific polyploidy in large multicellular organisms, variations in ploidy level between different tissues, organs, or cell lineages are common. because the chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote by mitosis. however, in many situations somatic cells double their copy number by means of endoreduplication as an aspect of cellular differentiation. for example, the hearts of two-year-old human children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age the proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei. adaptive and ecological significance of variation in ploidy there is continued study and debate regarding the fitness advantages or disadvantages conferred by different ploidy levels. a study comparing the karyotypes of endangered or invasive plants with those of their relatives found that being polyploid as opposed to diploid is associated with a 14% lower risk of being endangered, and a 20% greater chance of being invasive. polyploidy may be associated with increased vigor and adaptability. some studies suggest that selection is more likely to favor diploidy in host species and haploidy in parasite species. when a germ cell with an uneven number of chromosomes undergoes meiosis, the chromosomes cannot be evenly divided between the daughter cells, resulting in aneuploid gametes. triploid organisms, for instance, are usually sterile. because of this, triploidy is commonly exploited in agriculture to produce seedless fruit such as bananas and watermelons. if the fertilization of human gametes results in three sets of chromosomes, the condition is called triploid syndrome. in unicellular organisms the ploidy nutrient limitation hypothesis suggests that nutrient limitation should encourage haploidy in preference to higher ploidies. this hypothesis is due to the higher surface-to-volume ratio of haploids, which eases nutrient uptake, thereby increasing the internal nutrient-to-demand ratio. mable 2001 finds saccharomyces cerevisiae to be somewhat inconsistent with this hypothesis however, as haploid growth is faster than diploid under high nutrient conditions. the nlh is also tested in haploid, diploid, and polyploid fungi by gerstein et al 2017. this result is also more complex: on the one hand, under phosphorus and other nutrient limitation, lower ploidy is selected as expected. however under normal nutrient levels or under limitation of only nitrogen, higher ploidy was selected. thus the nlh – and more generally, the idea that haploidy is selected by harsher conditions – is cast into doubt by these results. older wgds have also been investigated. only as recently as 2015 was the ancient whole genome duplication in baker's yeast proven to be allopolyploid, by marcet-houben and gabaldón 2015. it still remains to be explained why there are not more polyploid events in fungi, and the place of neopolyploidy and mesopolyploidy in fungal history. glossary of ploidy numbers termdescription ploidy numbernumber of chromosome sets monoploid number (x)number of chromosomes found in a single complete set chromosome numbertotal number of chromosomes in all sets combined zygotic numbernumber of chromosomes in zygotic cells haploid or gametic number (n)number of chromosomes found in gametes diploid numberchromosome number of a diploid organism tetraploid numberchromosome number of a tetraploid organism the common potato (solanum tuberosum) is an example of a tetraploid organism, carrying four sets of chromosomes. during sexual reproduction, each potato plant inherits two sets of 12 chromosomes from the pollen parent, and two sets of 12 chromosomes from the ovule parent. the four sets combined provide a full complement of 48 chromosomes. the haploid number (half of 48) is 24. the monoploid number equals the total chromosome number