Polyploids

Polyploid Definition
Polyploids refer to organisms with cells that have contain multiple sets of homologous chromosomes. This generally implies that there are more than two copies for a given chromosome. Polyploidy is most often associated with plants although this phenomenom has also been observed in other life forms.
===Life Forms and Polyploidy===
Animals: Polyploid mammals have not yet been observed. In 1999, researchers had thought they found the first polyploid mammal, the South American red vizcacha rat, Tympanoctomys barrerae. However, further investigation demonstrated that the animal's DNA was diploid - not polyploid. (1) Despite not finding a polyploid mammal, this concept still retains its relevance as a variety of tissues within mammals display polyploidy.  These cells include: liver parenchyma, heart muscle cells, bone marrow megakaryocytes, and trophoblast of the placenta. (2,3)
Polyploidy has also been observed in a variety of fish, amphibians, and reptiles. Some polyploid fish include: snakehead fish, spinyfin fish, catfish, carps, minnows, lungfish, carpet sharks, goldfish, salmon, as well as others. The figure to the left shows some of the polyploid fish families that have been observed (4).
Polyploidy has also been seen in amphibians (frogs, toads, and salamanders) and reptiles (lizard, turtles).
Plants: Polyploidy is widespread in plants. While it is prevalent in ferns and flowering plants, polyploidy is also observed in crops such as cotton, tobacco, soybean.  Some of the many other plants include shrubs and trees (Sequoia sempervirens- California Redwood).
===Breif Description of Terminology within the Polyploid Field===
Ploidy Nomenclature:
*Triploid (3 homologous sets of DNA),
*Tetraploid (4 homologous sets of DNA),
*Pentaploid (5 sets of homologous DNA),
*Hexaploid (6 sets of homologous DNA)
Abberant Euploidy: changes that result in the alteration of whole chromosome sets. (Example a diploid organism somehow ends up with two extra sets of DNA to become tetraploid)  Interestingly, the higher the ploidy number the bigger the organism is. 
Autopolyploids: multiple copies of genomes from parents of the same species.
Allopolyploids: multiple copies of genomes from parents of different species (although often closely related).  The chromosome sets are defined as homeologous (partially homologous) as they are not completely homologous.
Neopolyploids: a polyploid that has been produced by artificially inducing chromosome doubling.
Diploidization: polyploid species shifting to become diploid species.
Mechanism of Polyploidy
Various deviant mitotic and meiotic events can induce the formation of a polyploid.  For instance, a diploid gamete can form from nondisjunction (the failure of the DNA to segregate into separate cells during the first or second meiotic division) and fuse with a haploid gamete to produce a triploid organism.   The formation of a triploid organism is characteristically unstable resulting in either death, sterility, or the ability to pass on the mutation depending on the species.  
In general, current reviews attribute basic polyploid formation to nondisjunction.  However, it should be mentioned that a fair amount of literature also comments on the mechanism of polyploidy in the context of evolution and speciation. One concept is the formation of autopolyploids. Autopolyploids are a ploidy organism that has formed from nondisjunction events in one or both parents of the same species.  For instance, if we were to consider this idea with a diploid organism, a autotetraploids forms from the nondisjunction of both diploid parents of the same species.  The progeny forms from the fusion of two diploid gametes.  A autotriploid would form from nondisjunction in one parent, so the offspring result from the fusion of a haploid gamete with a diploid gamete of the same species. (See Figure)
Allopolyploids are another term, that is used when describing the formation of a polyploid organism.  The difference between allopolyploids and autopolyploids is primarily due to the fact that allopolyploids have parents from two different, but often closely related species, while autopolyploids have parents of the same species.  There is some ambiguity regarding the exact mechanism.  However, one idea suggests that diploid gametes in the two different species fuse.  Another, as represented in the figure, suggests that haploid gametes of a species with 2 chromosomes fuse with the haploid gametes of a different species with 3 chromosomes.  At some unspecified time following fusion of the haploid gametes, the genome will duplicate and a allopolyploid will result.   (Note: the figure to the right says these cells are allotetraploid and I am not sure how they are reaching this conclusion, I think that the cell should be an allopentaploid.)  
In the figure to the right b and c are other errors that can occur: "laggard chromosomes" or "multiple unbalanced spindle fibers."
It should also be mentioned, a 2005 paper proposed that many presently diploid species originated from polyploid ancestors.  Other researchers have suggested multiple transitions in the genome number/ploidy over time. 
===Advantages of being Polyploid===
1. Heterosis. Polyploid offspring are more healthy and robust than their parents.  There has been a correlation with ploidy number and size.  The higher the ploidy number the bigger the size of the organism compared to others of the same species with a lower ploidy number. 
2. Gene Redundancy.  In the event, that one set of chromosomes contained lethal or deleterious mutations, the other set of chromosomes could mask the effect or presence of those genes.
3.  In some cases, the ability to reproduce through self-fertilization or asexual means. 
===Disadvantages of being Polyploid===
1. Changes in cell architecture, and regulatory implications.  This especially could pose problems in terms of metabolism and absorption of materials.
2. Mitosis and Meiosis.  Additional chromosomes challenge highly ordered cell division process.
3. Gene Expression becomes increasingly complicated with additional chromosomes.
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Citations===
1. Molecular cytogenetics discards polyploidy in mammals. Svartman M, Stone G, Stanyon R. Genomics. 2005 April.  
2. Significance of polyploidy in megakaryocytes and other cells in health and tumor disease. Winkelmann M, Pfitzer P, Schneider W. Klin Wochenschr 1987 Dec 1
====3. Endoreplication: polyploidy with purpose.  Hyun Lee, Jean Davidson, and Robert Duronio. Genes Dev. 2009 November 1.====
4. Occurrence of polyploidy in the fishes. Rosalind Leggatt and George Iwama. Reviews in Fish Biology and Fisheries. 13: 237–246, 2003.
5. Introduction to Genomics.  (Second Edition)  Arthur Lesk, 2012
6. The advantages and disadvantages of being polyploid.  Luca Comai. Nature Reviews Genetics 6, 836-846.  
7. From polyploidy to aneuploidy, genome instability and cancer. Zuzana Storchova & David Pellman. Nature Reviews Molecular Cell Biology 5, 45-54 (January 2004)