different species with as many as 12 sets of chromosomes (dodecaploid). polyploid lizards are also quite common. most are sterile and reproduce by parthenogenesis; others, like liolaemus chiliensis, maintain sexual reproduction. polyploid mole salamanders (mostly triploids) are all female and reproduce by kleptogenesis, "stealing" spermatophores from diploid males of related species to trigger egg development but not incorporating the males' dna into the offspring. while mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most often result in prenatal death. an octodontid rodent of argentina's harsh desert regions, known as the plains viscacha rat (tympanoctomys barrerae) has been reported as an exception to this 'rule'. however, careful analysis using chromosome paints shows that there are only two copies of each chromosome in t. barrerae, not the four expected if it were truly a tetraploid. this rodent is not a rat, but kin to guinea pigs and chinchillas. its "new" diploid (2n) number is 102 and so its cells are roughly twice normal size. its closest living relation is octomys mimax, the andean viscacha-rat of the same family, whose 2n = 56. it was therefore surmised that an octomys-like ancestor produced tetraploid (i.e., 2n = 4x = 112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents. polyploidy was induced in fish by har swarup (1956) using a cold-shock treatment of the eggs close to the time of fertilization, which produced triploid embryos that successfully matured. cold or heat shock has also been shown to result in unreduced amphibian gametes, though this occurs more commonly in eggs than in sperm. john gurdon (1958) transplanted intact nuclei from somatic cells to produce diploid eggs in the frog, xenopus (an extension of the work of briggs and king in 1952) that were able to develop to the tadpole stage. the british scientist j. b. s. haldane hailed the work for its potential medical applications and, in describing the results, became one of the first to use the word "clone" in reference to animals. later work by shinya yamanaka showed how mature cells can be reprogrammed to become pluripotent, extending the possibilities to non-stem cells. gurdon and yamanaka were jointly awarded the nobel prize in 2012 for this work. humans schematic karyogram of a human, showing the normal diploid (that is, non-polyploid) karyotype. it shows 22 homologous chromosomes, both the female (xx) and male (xy) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (to scale at bottom left).further information: karyotype further information: triploid syndrome true polyploidy rarely occurs in humans, although polyploid cells occur in highly differentiated tissue, such as liver parenchyma, heart muscle, placenta and in bone marrow. aneuploidy is more common. polyploidy occurs in humans in the form of triploidy, with 69 chromosomes (sometimes called 69, xxx), and tetraploidy with 92 chromosomes (sometimes called 92, xxxx). triploidy, usually due to polyspermy, occurs in about 2–3% of all human pregnancies and ~15% of miscarriages. the vast majority of triploid conceptions end as a miscarriage; those that do survive to term typically die shortly after birth. in some cases, survival past birth may be extended if there is mixoploidy with both a diploid and a triploid cell population present. there has been one report of a child surviving to the age of seven months with complete triploidy syndrome. he failed to exhibit normal mental or physical neonatal development, and died from a pneumocystis carinii infection, which indicates a weak immune system. triploidy may be the result of either digyny (the extra haploid set is from the mother) or diandry (the extra haploid set is from the father). diandry is mostly caused by reduplication of the paternal haploid set from a single sperm, but may also be the consequence of dispermic (two sperm) fertilization of the egg. digyny is most commonly caused by either failure of one meiotic division during oogenesis leading to a diploid oocyte or failure to extrude one polar body from the oocyte. diandry appears to predominate among early miscarriages, while digyny predominates among triploid zygotes that survive into the fetal period. however, among early miscarriages, digyny is also more common in those cases less than 8+1⁄2 weeks gestational age or those in which an embryo is present. there are also two distinct phenotypes in triploid placentas and fetuses that are dependent on the origin of the extra haploid set. in digyny, there is typically an asymmetric poorly grown fetus, with marked adrenal hypoplasia and a very small placenta. in diandry, a partial hydatidiform mole develops. these parent-of-origin effects reflect the effects of genomic imprinting. complete tetraploidy is more rarely diagnosed than triploidy, but is observed in 1–2% of early miscarriages. however, some tetraploid cells are commonly found in chromosome analysis at prenatal diagnosis and these are generally considered 'harmless'. it is not clear whether these tetraploid cells simply tend to arise during in vitro cell culture or whether they are also present in placental cells in vivo. there are, at any rate, very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism. mixoploidy is quite commonly observed in human preimplantation embryos and includes haploid/diploid as well as diploid/tetraploid mixed cell populations. it is unknown whether these embryos fail to implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process favoring the diploid cells. fish a polyploidy event occurred within the stem lineage of the teleost fish. plants speciation via polyploidy: a diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote. polyploidy is frequent in plants, some estimates suggesting that 30–80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes. huge explosions in angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species. it has been established that 15% of angiosperm and 31% of fern speciation events are accompanied by ploidy increase. polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes. both autopolyploids (e.g. potato) and allopolyploids (such as canola, wheat and cotton) can be found among both wild and domesticated plant species. most polyploids display novel variation or morphologies relative to their parental species, that may contribute to the processes of speciation and eco-niche exploitation. the mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and/or expression levels. many of these rapid changes may contribute to reproductive isolation and speciation. however, seed generated from interploidy crosses, such as between polyploids and their parent species, usually have aberrant endosperm development which impairs their viability, thus contributing to polyploid speciation. polyploids may also interbreed with diploids and produce polyploid seeds, as observed in the agamic complexes of crepis. some plants are triploid. as meiosis is disturbed, these plants are sterile, with all plants having the same genetic constitution: among them, the exclusively vegetatively propagated saffron crocus (crocus sativus). also, the extremely rare tasmanian shrub lomatia tasmanica is a triploid sterile species. there are few naturally occurring polyploid conifers. one example is the coast redwood sequoia sempervirens, which is a hexaploid (6x) with 66 chromosomes (2n = 6x = 66), although the origin is unclear. aquatic plants, especially the monocotyledons, include a large number of polyploids. crops the induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. for example, triticale is the hybrid of wheat (triticum turgidum) and rye (secale cereale). it combines sought-after characteristics of the parents, but the initial hybrids are sterile. after polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale. in some situations, polyploid crops are preferred because they are sterile. for example, many seedless fruit varieties are seedless as a result of polyploidy. such crops are propagated using asexual techniques, such as grafting. polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine. examples triploid crops: some apple varieties (such as belle de boskoop, jonagold, mutsu, ribston pippin), banana, citrus, ginger, watermelon, saffron crocus, white pulp of coconut tetraploid crops: very few apple varieties, durum or macaroni wheat, cotton, potato, canola/rapeseed, leek, tobacco, peanut, kinnow, pelargonium hexaploid crops: chrysanthemum, bread wheat, triticale, oat, kiwifruit octaploid crops: strawberry, dahlia, pansies, sugar cane, oca (oxalis tuberosa) dodecaploid crops: some sugar cane hybrids some crops are found in a variety of ploidies: tulips and lilies are commonly found as both diploid and triploid; daylilies (hemerocallis cultivars) are available as either diploid or tetraploid; apples and kinnow mandarins can be diploid, triploid, or tetraploid. fungi besides plants and animals, the evolutionary history of various fungal species is dotted by past and recent whole-genome duplication events (see albertin and marullo 2012 for review). several examples of polyploids are known: autopolyploid: the aquatic fungi of genus allomyces, some saccharomyces cerevisiae strains used in bakery, etc. allopolyploid: the widespread cyathus stercoreus, the allotetraploid lager yeast saccharomyces pastorianus, the allotriploid wine spoilage yeast dekkera bruxellensis, etc. paleopolyploid: the human pathogen rhizopus oryzae, the genus saccharomyces, etc. in addition, polyploidy is frequently associated with hybridization and reticulate evolution that appear to be highly prevalent in several fungal taxa. indeed, homoploid speciation (hybrid speciation without a change in chromosome number) has been evidenced for some fungal species (such as the basidiomycota microbotryum violaceum). schematic phylogeny of the chromalveolata. red circles indicate polyploidy, blue squares indicate hybridization. from albertin and marullo, 2012 as for plants and animals, fungal hybrids and polyploids display structural and functional modifications compared to their progenitors and diploid counterparts. in particular, the structural and functional outcomes of polyploid saccharomyces genomes strikingly reflect the evolutionary fate of plant polyploid ones. large chromosomal rearrangements leading to chimeric chromosomes have been described, as well as more punctual genetic modifications such as gene loss. the homoealleles of the allotetraploid yeast s. pastorianus show unequal contribution to the transcriptome. phenotypic diversification is also observed following polyploidization and/or hybridization in fungi, producing the fuel for natural selection and subsequent adaptation and speciation. chromalveolata other eukaryotic taxa have experienced one or more polyploidization events during their evolutionary history (see albertin and marullo, 2012 for review). the oomycetes, which are non-true fungi members, contain several examples of paleopolyploid and polyploid species, such as within the genus phytophthora. some species of brown algae (fucales, laminariales and diatoms) contain apparent polyploid genomes. in the alveolata group, the remarkable species paramecium tetraurelia underwent three successive rounds of whole-genome duplication and established itself as a major model for paleopolyploid studies. bacteria each deinococcus radiodurans bacterium contains 4-8 copies of its chromosome. exposure of d. radiodurans to x-ray irradiation or desiccation can shatter its genomes into hundred of short random fragments. nevertheless, d. radiodurans is highly resistant to such exposures. the mechanism by which the genome is accurately restored involves reca-mediated homologous recombination and a process referred to as extended synthesis-dependent strand annealing (sdsa). azotobacter vinelandii can contain up to 80 chromosome copies per cell. however this is only observed in fast growing cultures, whereas cultures grown in synthetic minimal media are not polyploid. archaea the archaeon halobacterium salinarium is polyploid and, like deinococcus radiodurans, is highly resistant to x-ray irradiation and desiccation, conditions that induce dna double-strand breaks. although chromosomes are shattered into many fragments, complete chromosomes can be regenerated by making use of overlapping fragments. the mechanism employs single-stranded dna binding protein and is likely homologous recombinational repair.