Polyploidy, Biology tutorial


Polyploidy is a word employed to explain cells and organisms having more than two paired (homologous) sets of chromosomes. Most of the eukaryotic species are diploid, signifying they contain two sets of chromosomes-one set inherited from each and every parent. Though polyploidy is found in some organisms and is particularly common in plants. Moreover, polyploidy as well takes place in some tissues of animals that are or else diploid, such as human muscle tissues. This is termed as endopolyploidy. (Monoploid organisms as well take place; a Monoploid consists of just one set of chromosomes. These comprise the huge majority of prokaryotes.)

Polyploidy signifies to a numerical change in a whole set of chromosomes. Organisms in which a specific chromosome, or chromosome segment, is under-or overrepresented are stated to be aneuploid (from the Greek words signifying 'not', 'good', and 'fold'). Thus the difference between aneuploidy and polyploidy is that aneuploidy signifies to a numerical change in part of the chromosome set, while polyploidy signifies to a numerical change in the entire set of chromosomes.

Types of polyploidy:

The two main modes of origin of the polyploid condition are identified somatic doubling in mitosis, and non-reduction in meiosis (that is, Heilborn, 1934; Grant, 1971). The method of somatic doubling is illustrated through polyploid Primula kewensis, and non-reduction was the mode of origin seen in the polyploid Rhaphobrassica. It is assumed that polyploid made by hybridization followed by chromosome doubling. Though, Harlan and deWet (1975) argued that unreduced gametes played a significant role. Whereas agronomy researchers took notice of this (example: Peloquin, 19XX), textbooks didn't change. In recent times, a lot of theoretical modeling (Rodriguez, 1996; Ramsey and Schemske, 1998, 2002) and fieldwork (Husband 1999, 2000) has contributed to the observe that unreduced gametes and triploid bridges are a main source of polyploid formation. This is as well a method for how diploid and polyploid genomes can interact (therefore, the new polyploid species are not severely sealed off from its diploid progenitors). 

Throughout meiosis, homologous chromosomes pair undergo crossing over resultant in the exchange of portions of their chromosomes. In diploid hybrids derived from crosses of two species, chromosomes from the two species might vary or one of the chromosomes might be absent. This can cause irregularities throughout meiosis and might outcome in cell cycle arrest and subsequent embryo abortion. Though, if the chromosome number is twice in the hybrid, allotetraploids are made, which encompass four sets of chromosomes. This can take place by crossing the autotetraploids of the two species, or more likely in nature, through the fusion of unreduced gametes. Allotetraploids usually will encompass pairing and crossing over only in the two chromosomes of each and every original parent (that is, the homologous chromosomes AA) and only rarely between chromosomes from the two original parents (that is, the homologous chromosomes AA').

This meiotic behavior makes sure proper pairing of the chromosomes and the right assortment into gametes.

Polyploid types are labeled according to the number of chromosome sets in the nucleus:

- Triploid (three sets; 3x), for instance: seedless watermelons, common in the phylum Tardigrada.

- Tetraploid (four sets; 4x), for instance Salmonidae fish.

- Pentaploid (five sets; 5x), for instance Kenai Birch (Betula papyrifera var. kenaica).

- Hexaploid (six sets; 6x), for instance wheat, kiwifruit.

- Octaploid (eight sets; 8x), for instance Acipenser (that is, genus of sturgeon fish), dahlias.

- Decaploid (ten sets; 10x), for instance some strawberries

- Dodecaploid (twelve sets; 12x), for instance the plant Celosia argentea and the amphibian Xenopus ruwenzoriensis

Polyploidy in animals and plants:

Polyploidy takes place in some animals, like salmon, goldfish and salamanders. Though, polyploidy is particularly common among ferns and flowering plants, comprising both wild and cultivated species. Wheat, for illustration, after millennia of hybridization and modification by humans, has strains which are diploid (that is, two sets of chromosomes); tetraploid (that is, four sets of chromosomes), having the common name of durum or macaroni wheat; and Hexaploid (that is, six sets of chromosomes), having the common name of bread wheat.

Most of the agriculturally significant plants of the genus Brassica are as well tetraploids. This genus, termed as cabbages or mustards, comprises turnips, Brussels sprouts, cauliflower, cabbage, broccoli, mustard seed and other significant crops. The Triangle of U is a theory, proposed by a Woo Jang-choon, a Korean botanist who was working in the Japan, that states the genomes of three ancestral species of Brassica joined to form the three common tetraploid species Brassica juncea  (or Indian mustard), Brassica napus  (or Rapeseed, rutabaga), and Brassica carinata (or Ethiopian mustard).

Speciation through polyploidy: A diploid cell undergoes failed meiosis, generating diploid gametes that self-fertilize to generate a tetraploid zygote.

Polyploidy in Plants:

Polyploidy is much common in plants, particularly in angiosperms. From 30% to 70% of nowadays angiosperms are thought to be polyploid. Species of coffee plant having 22, 44, 66, and 88 chromosomes are recognized. This recommends that the ancestral condition was a plant having a haploid (n) number of 11 and a diploid (2n) number of 22, from which evolved the dissimilar polyploid descendants. 

However, the chromosome content of most of the plant groups recommends that the fundamental angiosperm genome comprises of the genes on 7 - 11 chromosomes. Domestic wheat, having its 42 chromosomes, is most likely Hexaploid (6n), where n is the ancestral haploid number was 7.

Origin of Polyploidy:

Polyploidy has occurred frequently in the evolution of plants. The process can start when diploid (2n) gametes are formed. These can occur in at least two ways. 

a) The gametes might be formed through mitosis rather than meiosis. 

b) Plants, in contrary to animals, form germ cells (that is, sperm and eggs) from the somatic tissues.

When the chromosome content of a precursor somatic cell has accidentally twice (example: as an outcome of passing via S phase of the cell cycle without following up by mitosis and cytokinesis), then gametes having 2n chromosomes are made Polyploidy as well takes place naturally in some plant tissues. 

As the endosperm (3n) builds up in corn (maize) kernels (Zea mays), its cells experience successive rounds (as many as 5) of endoreplication generating nuclei which range as high as 96n. 

If rhizobia infect the roots of their legume host, they induce the infected cells to experience endoreplication generating cells which can become 128n.

Polyploidy in animals:

Illustrations in animals are more general in the lower forms like leeches, flatworms and brine shrimp. Polyploid animals are frequently sterile; therefore they often reproduce through parthenogenesis. Polyploid lizards are as well quite common and parthenogenetic. Polyploid mole salamanders (mainly triploids) are all female and reproduce through kleptogenesis, 'stealing' spermatophores from diploid males of associated species to trigger egg growth however not incorporating the males DNA into the offspring. Whereas mammalian liver cells are polyploid, rare examples of polyploid mammals are known; however most often outcome in prenatal death.

An octodontid rodent of Argentina's harsh desert areas, termed as the Plains Viscacha-Rat (that is, Tympanoctomys barrerae) has been reported as an exemption to this rule. Though, careful analysis employing chromosome paints exhibits that there are just two copies of each and every chromosome in T. barrerae not the four expected if it were truly a tetraploid. The rodent is not a rat, however kin to guinea pigs and chinchillas. Its 'new' diploid [2n] number is 102 and therefore its cells are around twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the similar family, whose 2n = 56. It was thus surmised that an Octomys-like ancestor produced tetraploid (that is, 4n = 112) offspring that were, by virtue of their twice chromosomes, reproductively isolated from their parents.

Polyploidy in Man:

Polyploidy takes place in a few animals, like salmon, goldfish and salamanders, however is particularly common among ferns and flowering plants, comprising both wild and cultivated species. Wheat, for illustration, after millennia of hybridization and modification by humans, consists of strains which are diploid (that is, two sets of chromosomes), tetraploid (that is, four sets of chromosomes) having the common name of durum or macaroni wheat, and Hexaploid (that is, six sets of chromosomes) having the common name of bread wheat. Most of the agriculturally significant plants of the genus Brassica are as well tetraploids. Polyploidization is a method of sympatric speciation as polyploid is generally not able to interbreed by their diploid ancestors.

True polyploidy hardly ever takes place in humans; however it takes place in some tissues (particularly in the liver). Aneuploidy is very common. Polyploidy takes place in humans in the form of triploidy, having 69 chromosomes (at times termed as 69,XXX), and tetraploidy having 92 chromosomes (at times termed as 92,XXXX).

Triploidy, generally due to polyspermy, takes place in around 2 to 3% of all the human pregnancies and 15% of miscarriages. The huge majority of triploid conceptions end as miscarriage and those which do survive to term usually die shortly after birth. In certain cases survival past birth might take place longer when there is mixoploidy having both diploid and a triploid cell population present.

Triploidy might be the outcome of either digyny (that is, the extra haploid set is from the mother) or diandry (that is, the additional haploid set is from the father). Diandry is mainly caused due to the reduplication of the paternal haploid set from a single sperm, however might as well be the effect of dispermic (that is, two sperm) fertilization of the egg. Digyny is most generally caused by either failure of one meiotic division throughout oogenesis leading to a diploid oocyte or failure to extrude one polar body from the oocyte. Diandry seems to predominate among early miscarriages whereas digyny predominates among triploidy which survives to the fetal period. Though, among early miscarriages, digyny is as well more common in those cases < 8.5 weeks gestational age or those in which an embryo is present. There are as well two dissimilar phenotypes in triploid placentas and fetuses which are based on the origin of the additional haploid set. In digyny there is usually an asymmetric poorly grown fetus having marked adrenal hypoplasia and a very small placenta. In diandry, a partial hydatidiform mole develops. Such parent-of-origin consequences reflect the effects of genomic imprinting.

Complete tetraploidy is more seldom diagnosed than triploidy, however is observed in 1 to 2% of early miscarriages. Though, some tetraploid cells are generally found in chromosome analysis at prenatal diagnosis and these are usually considered 'harmless'. It is not obvious whether such tetraploid cells simply tend to arise throughout in vitro cell culture or whether they are as well present in placental cells in vivo. There are, at any rate, very few clinical reports of fetuses or infants diagnosed by tetraploidy mosaicism.

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