In humans, polyploidy can be caused by at least two mechanisms: dispermy and unreduced gametes. Dispermy is the term used to describe the process by which two sperm fertilize a single egg to produce a triploid zygote. Unreduced gametes are diploid rather than haploid. The union of an unreduced egg and a haploid sperm would result in a triploid zygote, while the union of an unreduced egg and an unreduced sperm would result in a tetraploid zygote.
SC formation could cause inappropriate associations of homologs in mitosis that may not be properly resolved if other meiotic proteins are lacking. Meiotic HORMA proteins form linear structures along unsynapsed sets of sister chromatids, help mediate crossing over and the synapsis of homologs Hollingsworth et al.
The latter is apparently due to its usual meiotic role in promoting the use of the homolog as a double-strand break repair template rather than the sister chromatid by suppressing Radmediated double-strand break repair. In the absence of the meiotic recombination machinery, when the Rad51 pathway is blocked, double-strand break repair is instead shuttled to the error-prone nonhomologous end-joining NHEJ pathway, resulting in genome instability Watkins et al.
Recent work with the yeast homolog Hop1 suggests that purified Hop1 protein can self-associate to form rigid rod-like structures that tightly unite DNA molecules independent of homology Khan et al. This finding supports the idea that when these proteins are aberrantly expressed in cells that lack the proteins necessary to subsequently remove them e.
Recently, we found that a unique allele of ASY1 , the A. Much remains to be learned about somatic reduction. For example, what meiotic genes minimally suffice to drive somatic reduction? Does stability of these divisions correlate with the number of meiosis genes expressed? Are somatic reduction divisions aberrations, or can they be important in normal development? Does somatic reduction ever provide a reliable remedy for the normally irreversible fate of somatic endopolyploidy?
Results from a wide range of eukaryotes clearly show that WGD often provides adaptive opportunities. However, in those cases where polyploid cells continue to divide, they face substantial challenges, especially for the regular segregation of chromosomes. This can lead to chromosome instability and aneuploidy, which can sometimes be adaptive at the level of cell lineages but appears in most cases to be deleterious or, at best, neutral for the organism at large.
Thus, the regulated management of cellular genome content plays important and beneficial roles in development, tissue repair, and stress responses, while its mismanagement can lead to genome instability and contribute to tissue aging and pathologic states, including cancer progression.
Viewing proliferating polyploid cell lineages from an evolutionary and comparative perspective may yield novel insights into the role that the double-edged sword of polyploidy plays in the biology of organisms and their evolution.
Many open questions remain, such as understanding the mysterious process and developmental role if any of somatic reduction divisions, the role that aneuploidy may play in normal development or stress resilience if any , and the causes and consequences of expressing partial meiotic programs in somatic cells. Furthermore, many potentially interesting effects are currently only correlated with polyploidy, and more work is required to test causality. Learning which effects are direct outcomes of polyploidization itself and their mechanistic basis has the potential to provide important insights.
Where similar correlates are observed across kingdoms, deeper investigation of the underlying causes for the apparent similarities may yield novel insights into the most fundamental effects that polyploidy has on the biology of cells, both individually and in the context of the multicellular organisms in which they are found.
We thank Nancy Kleckner and members of the Bomblies and Yant groups for helpful discussions. View all Previous Section Next Section. Figure 1. Big cells and rapid growth—developmental roles of somatic polyploidy Polyploid cells often arise in diploids as a normal and regulated part of development. Are there costs of endopolyploidy? Fungi Natural variation for ploidy exists in Saccharomyces cerevisiae , with haploids, diploids, and tetraploids endemic to the same microsite, suggesting that ploidy variation could play an adaptive role Ezov et al.
Mammals In mammals, the anarchic proliferation that characterizes within-host cancer evolution commonly includes a high diversity of aneuploid cell lineages associated with disease progression, and at least some of these are thought to arise via chromosome missegregation from tetraploid intermediates for review, see Ganem et al. Box 1. Engines of genome diversification Aggressive aneuploid cancers, some of which may arise via tetraploid intermediates, exhibit striking genomic modifications, some of which are recognized in many systems, while others are specific to the cancer literature.
Several dramatic examples of the types of genome modification that have been reported in cancer evolution include the following: Chromoplexy from the Greek pleco, to weave or braid : A phenomenon of complex genome restructuring in which DNA translocations and deletions emerge in a highly interdependent manner; observed first in prostate cancers, where it frequently accounts for dysregulation of important cancer loci Baca et al.
Polyploid cell lineages as evolving populations Genome duplication increases the number of available alleles for mutations to accumulate and upon which selection can then act Otto Polyploid meiosis Segregating additional chromosomes in meiosis is a vexing challenge for newly formed autopolyploids.
Polyploid mitosis Unlike with meiosis, it is not immediately obvious, when considering only genome duplication, why mitosis should be problematic for autopolyploids; it is, as discussed above, nevertheless consistently linked with aneuploidy, showing that polyploid cells clearly do face problems in mitosis. Reduction divisions in somatic cells A mysterious process that highlights the sometimes fluid boundaries between mitosis and meiosis is somatic reduction, which refers to the observation that some somatic polyploid cells undergo meiosis-like reduction divisions e.
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It is quite common in plants, for example many crops like wheat or Brassica forms. It seems to be rarer in animals but still it is present among some amphibian species like Xenopus. As I know in mammals polyploidy is lethal I don't mean tissue - limited polyploidy.
I understand that triploidy is harmful due to stronger influence of maternal or paternal epigenetic traits that cause abnormal development of placenta, but why there is no tetraploid mammals? Great question, and one about which there has historically been a lot of speculation, and there is currently a lot of misinformation.
I will first address the two answers given by other users, which are both incorrect but have been historically suggested by scientists.
Then I will try to explain the current understanding which is not simple or complete. My answer is derived directly from the literature, and in particular from Mable , which in turn is part of the special issue of the Biological Journal of the Linnean Society tackling the subject.
In HJ Muller addressed this question in a famous paper, "Why polyploidy is rarer in animals than in plants" Muller, Muller briefly described the phenomenon that polyploidy was frequently observed in plants, but rarely in animals. The explanation, he said, was simple and is approximate to that described in Matthew Piziak's answer :. Unfortunately, whilst the first two points are valid facts about polyploids, the third point is incorrect.
A major flaw with Muller's explanation is that it only applies to animals with chromosomal ratio-based sex determination, which we have since discovered is actually relatively few animals. In there was comparatively little systematic study of life, so we really didn't know what proportion of plant or animal taxa showed polyploidy.
Muller's answer doesn't explain why most animals, e. Another line of evidence disproving Muller's answer is that, in fact, polyploidy is very common among dioecious plants those with separate male and female plants; e. Westergaard, , while Muller's theory predicts that prevalence in this group should be as low as in animals. Another answer with some historical clout is the one given by Daniel Standage in his answer, and has been given by various scientists over the years e.
Stebbins, This answer states that animals are more complex than plants, so complex that their molecular machinery is much more finely balanced and is disturbed by having multiple genome copies. This answer has been soundly rejected e.
Firstly, whilst polyploidy is unusual in animals, it does occur. Various animals with hermaphroditic or parthenogenetic modes of reproduction frequently show polyploidy. There are also examples of Mammalian polyploidy e. Gallardo et al. In addition, polyploidy can be artificially induced in a wide range of animal species, with no deleterious effects in fact it often causes something akin to hybrid vigour; Jackson, It's also worth noting here that since the s Susumo Ohno e.
Ohno et al. If true, it further highlights that animals being more complex itself a large, and in my view false, assumption does not preclude polyploidy. In addition, there are now several new suspected factors involved in ploidy which are currently being investigated:. Plants have a simpler anatomical structure than mammals is anatomical the right word, or would physiological be more appropriate?
Mammals on average don't have more genes than plants, so my understanding is that this additional complexity is the result of finer and more complex regulatory mechanisms.
When you remove or duplicate an individual gene in an organism, that organism must compensate somehow. The more complex the regulatory system, more likely that even small perturbations will cause severe defects or even lethality.
Extend this to the scale of a whole genome, and it shouldn't be surprising that polyploidy is lethal for a lot of organisms. It find it extremely fascinating that it's not lethal for some organisms, but it makes sense that organisms with simpler regulatory mechanisms would be more successful handling a genome duplication event through gene subfunctionalization, neofunctionalization, etc.
In animals, polyploidy is not tolerated and very few polyploid species are known to exist. Those that do exist are usually asexual, parthenogenetic, or hermaphroditic. Most of the problems resulting from polyploidy occur during synapsis of homologues during prophase I.
As plants do not have a chromosomal mechanism for sex determination, synapsis and subsequent disjunction is not as great a problem.
In fact, most plants are monoecious. These paragraphs are taken from these lecture notes from an Emporia State University genetics course. Muller, The American Naturalist , , but I didn't get a chance to access the article. My point in stating what is below is to emphasize even when polyploidy is present in closely related species there is a question of why an organism can survive one ploidy event but not another and why tolerance of polyploidy varies among similar organisms?
Several species of deciduous azaleas are tetraploid. Closely related diploid deciduous azaleas are more than happy to accept pollen and produce seedpods. We have attempted such crosses many times and they almost always success in terms of seedpod development. Tetraploid deciduous azaleas on the other hand are very reluctant to accept pollen from diploid deciduous azaleas.
We have attempted such crosses nearly times without success. However producing seedpods and producing viable seeds are of course different.
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