Polyploid cells and organisms are those containing more than two paired sets of
chromosomes. Most species whose cells have nuclei 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 Epulopicium fishelsoni [1]. Hence ploidy
is defined with respect to a cell. Most eukaryotes have diploid somatic cells,
but produce haploid gametes 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. 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, including both wild and cultivated species. Wheat, for
example, after millennia of hybridization and modification by
humans, has strains that are diploid, tetraploid with the common name of durum
or macaroni wheat, and hexaploid 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, for example seedless
watermelons, common in the phylum Tardigrada
tetraploid, for example Salmonidae fish, the cotton Gossypium hirsutum
pentaploid, for example Kenai Birch hexaploid, for example wheat, kiwifruit
heptaploid or septaploid octaploid or octoploid,, for example
Acipenser, dahlias decaploid, for example certain
strawberries dodecaploid, for example the plant
Celosia argentea or the invasive one Spartina anglica or the amphibian
Xenopus ruwenzoriensis. Animals
Examples in animals are more common in non-vertebrates such as flatworms,
leeches, and brine shrimp. Within vertebrates, examples of stable
polyploidy include the salmonids and many cyprinids. Some fish have as many
as 400 chromosomes. Polyploidy also occurs commonly in amphibians; for
example the biomedically-important Xenopus genus contains many different
species with as many as 12 sets of chromosomes. Polyploid lizards are also
quite common, but are sterile and must reproduce by parthenogenesis. Polyploid
mole salamanders 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 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. The 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 offspring that were, by virtue of their doubled
chromosomes, reproductively isolated from their parents.
Polyploidy was induced in fish by Har Swarup 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 transplanted intact nuclei from somatic cells to
produce diploid eggs in the frog, Xenopus 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 = True polyploidy rarely occurs in humans,
although polyploid cells occur in highly differentiated tissue, such as liver
parenchyma and heart muscle, and in bone marrow. Aneuploidy is more common.
Polyploidy occurs in humans in the form of triploidy, with 69 chromosomes, and
tetraploidy with 92 chromosomes. 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 extend longer if there is mixoploidy with both a diploid
and a triploid cell population present. Triploidy may be the result of either
digyny or diandry. Diandry is mostly caused by reduplication of the paternal
haploid set from a single sperm, but may also be the consequence of dispermic
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 triploidy that
survives into the fetal period. However, among early miscarriages, digyny is also
more common in those cases