Polyploidization And The Origin Of Species
Polyploidization, the duplication of the whole genome, is prevalent in plants . Polyploid individuals have multiple copies of each set of chromosomes. Humans are diploid with two sets of chromosomes, while taxa with three sets are called triploids, four sets are tetraploids, and so forth. An exceptionally high ploidy level occurs in the adders tongue fern, Ophioglossum reticulatum, which is a 96-ploid with 1440 chromosomes .
These polyploids can be categorized to indicate the history of the polyloidy event. Allopolyploids form between two different species , whereas autopolyploids occur within the same species . Therefore species are described by the ploidy level as well as the category of polyploidy. For example, Polypodium hesperium is tetraploid, and is the result of a cross between the diploids Polypodium amorphum and Polypodium glycyrrhiza. Thus it is called an allotetraploid .
C.J. Rothfels, S.P. Otto, in, 2016
How Might Changes To Cell Size Produce Functional Novelty
Ginzberg et al. cite older literature showing that the rate of lipid synthesis by adipocytes is higher in larger cells, even when normalized to cell size, meaning that large cells are disproportionally more productive than small cells. They also discuss in some detail one well-studied mammalian cell type, the pancreatic beta cell, for which size and function are intimately connected. They highlight the complexity of interactions among the various gene regulatory pathways that not only affect cell size but have a wide range of pleiotropic effects and conclude that it is hard to know whether a change in size actually causes a change in function or is simply correlated with it . Niklas , in a review on cell growth, discusses the challenges shared by all organisms with respect to coordinating physiological function with cell size and, in the case of multicellular eukaryotes, organ and organism size. He notes that cells have to achieve appropriate sizes, geometries, and shapes for their functional roles for an organism to operate properly. Despite the importance of cell size, he concludes that among all the topics treated in this review, this aspect of growth and development is perhaps the least well understood.
Cell Surface-to-Volume Ratio and Its Potential Downstream Effects
Cell Cycle and Its Relationship to Ploidy and Genome Size
Cell Size and Its Effect on the Size, Number, and Morphology of Organelles Other than the Nucleus
Genome Size Correlations With Cell Biology: The Nucleotype
Cavalier-Smith considered the two most fundamental questions about the C-value paradox to be why do larger cells have larger genomes, and why do eukaryotes have such a large variation in cell volume? Muntzing, in his 1936 treatise The Evolutionary Significance of Autopolyploidy, stated, As is well known there is a positive correlation, though not strict proportionality, between chromosome number and cell volume . Fifteen years later, Mirsky and Ris noted that the relationship between DNA and the size or number of genes is obscure, but the relationship between the DNA content of a cell and the size of the cell is clear: in general, when homologous cells are compared the greater the DNA content, the larger the cell. By homologous cells they presumably meant the same cell typean important consideration discussed below. Other positive correlations were also noted with genome size, for example, nuclear volume and mitotic cell cycle duration . These correlations led Bennett to develop the concept of the nucleotype:
My basic argument is that S-DNA has a genetic function , but that it does not do this by coding for proteins and is therefore not divided into discrete genes: it mutates, recombines and is inherited like G-DNA, and its amount is determined by natural selection and not by genetic drift or orthogenesis. The genetic properties of DNA comprise both the protein-coding, genic, function of G-DNA and the joint nucleotypic function of both G- and S-DNA.
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Techniques Of Inducing Polyploidy:
It has been found in various seedlings that if their tip is removed or cut off by a sharp knife the callus is produced which give rise to some polyploids.
2. Graft combinations:
It has been observed that callus formation occurs during the graft combinations i.e., 7% which may lead to some extent polyploidy Winkler, 1916.
Irradiation of vegetative and floral buds with X-rays, gamma rays or ultra-violet rays, polyploidy may be brought in some frequencies.
Application of heat and cold shocks to flowers at or near the time of first division of zygote brings about polyploidy.
It also to some extent brings about polyploidy.
There are various chemicals like chloral hydrate, acenaphthelene, coumarine, vertanine sulphate , cavadin, vernatrine, ethyl mercury chloride, vitamin sulphate, granosan, hydroxyquinoline and nitrous oxide, colchicine etc. Of these the most effective results have been obtained by colchicine and this is now being widely used on all plant species.
Nebel and Ruttle working on Tradescantia, Petunia, Marigolds & Blakeslee and Avery working on Portulaca, Datura and Cucurbita noticed that the alkaloid colchicine was most effective in doubling the chromosome number.
In many cases partially functional spindle may be formed and incipient anaphase and even telophase may be evidenced but in no case cell wall is formed.
The final effect may be brought about in any one of the following three ways:
Control Of Cell Size Remains A Mystery
As discussed above, the strong correlation between cell size and genome size has led to cell size being considered a nucleotypic effect . Beaulieu et al. found significant correlations between genome size and both guard cell length and epidermal cell area in a set of 101 angiosperm species representing a wide taxonomic and genome size range. But why is this true? Ideally, it would be possible to simply turn to the literature to understand the mechanism by which increased genome size leads to increased cell size. Unfortunately, this is not the case. The state of knowledge is captured by the introduction to a relatively recent review :
For well over 100 years, cell biologists have been wondering what determines the size of cells. In modern times, we know all of the molecules that control the cell cycle and cell division, but we still do not understand how cell size is determined. To check whether modern cell biology has made any inroads on this age-old question, BMC Biology asked several heavyweights in the field to tell us how they think cell size is controlled, drawing on a range of different cell types.
The strong correlation between genome size and cell size strongly suggests that polyploidy itself directly causes increased cell size. Breuer et al. concluded that their study of isogenic diploid and tetraploid Arabidopsis wild-type and bin4 cell size mutants
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Polyploidy And Nucleotypic Effects
The idea that some of the most fundamental effects of genome doubling involve characters influenced if not determined by the nucleotype, specifically cell size, is nearly as old as the study of polyploidy. Referring to studies from the 1920s by von Wettstein and his students, Muntzing stated that it has been demonstrated that the morphological alterations in experimental autopolyploids are ultimately due to alterations in cell size. Turning to physiology, he concluded:
Purely quantitative alterations of the number of genomes influence not only the morphology but to a rather high degree also the physiology of the altered individuals. These react in a new way both with the normal environment and to changes of the normal environmental conditions. Most typical and universal is the altered growth rate, the types with the higher chromosome numbers generally being more slow in development. But also other important physiological properties, such as assimilation energy and winter-hardiness, may be greatly influenced by autopolyploidy. Chemical analyses have in certain cases revealed considerable differences between diploids and autopolyploids, and it has also been demonstrated that the concentration of certain vitamins may be altered. It is probable that many or all of these changes ultimately depend on a change in cell size.
A shorter description appeared nearly 20 years later in Ramsey and Schemskes review Neopolyploidy in Flowering Plants :
Speciation Success Of Polyploid Plants Closely Relates To The Regulation Of Meiotic Recombination
- 1Plant Breeding, Wageningen University & Research, Wageningen, Netherlands
- 2Institut de Génétique, Environnement et Protection des Plantes, Institut National de la Recherche Agronomique, Agrocampus Ouest, Université de Rennes 1, Rennes, France
Polyploidization is a widespread phenomenon, especially in flowering plants that have all undergone at least one event of whole genome duplication during their evolutionary history. Consequently, a large range of plants, including many of the worlds crops, combines more than two sets of chromosomes originating from the same or related species . Depending on the polyploid formation pathway, different patterns of recombination will be promoted, conditioning the level of heterozygosity. A polyploid population harboring a high level of heterozygosity will produce more genetically diverse progenies. Some of these individuals may show a better adaptability to different ecological niches, increasing their chance for successful establishment through natural selection. Another condition for young polyploids to survive corresponds to the formation of well-balanced gametes, assuring a sufficient level of fertility. In this review, we discuss the consequences of polyploid formation pathways, meiotic behavior and recombination regulation on the speciation success and maintenance of polyploid species.
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What Is An Example Of Ecological Speciation
Several investigators have shown that reproductive isolation has evolved as a by-product of adaptive divergence, a process called ‘ecological speciation’. For example, the evolution of mimicry appears to have played an important role in speciation in the butterfly genus Heliconius. The recently split sister species H.
References And Recommended Reading
This article was adapted from Comai, L., The advantages and disadvantages of being polyploid. Nature Reviews Genetics6, 838-845
Adams, K. L., & Wendel, J. F. Polyploidy and genome evolution in plants. Current Opinion in Plant Biology8, 135-141
Comai, L. et al. Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids. Plant Cell12, 1551-1568
Fransz, P., et al. Interphase chromosomes in Arabidopsis are organized as well defined chromocenters from which euchromatin loops emanate. Proceedings of the National Academy of Sciences99, 14584-14589
Guo, M., Davis, D., & Birchler, J. A. Dosage effects on gene expression in a maize ploidy series. Genetics142, 1349-1355
Melaragno, J. E., Mehrotra, B., & Coleman, A. W. Relationship between endopolyploidy and cell size in epidermal tissue of Arabidopsis. Plant Cell5, 1661-1668
Mittelsten Scheid, O., et al. A change of ploidy can modify epigenetic silencing. Proceedings of the National Academy of Sciences93, 7114-7119
Shaked, H., et al. Sequence elimination and cytosine methylation are rapid and reproducible responses of the genome to wide hybridization and allopolyploidy in wheat. Plant Cell13, 1749-1759
Van de Peer, Y., & Meyer, A. Chapter 6: Large-scale gene and ancient genome duplications. In The Evolution of the Genome, ed. T. R. Gregory, 330-363
Wang, J., et al. Genome-wide nonadditive gene regulation in Arabidopsis allotetraploids. Genetics172, 507-517
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Evolutionary Potential Of Polyploid Organisms
At first sight, the epigenetic changes observed in polyploids would seem to be deleterious because of their disruptive effects on regulatory patterns established by selection. However, these epigenetic changes might instead increase diversity and plasticity by allowing for rapid adaptation in polyploids. One example may be the widespread dispersal of the invasiveallopolyploidSpartina angelica. However, it is not clear whether the success of this species can be attributed to fixed heterosis or to the increased variability that results from epigenetic remodeling. Polyploidy is also believed to play a role in the rapid adaptation of some allopolyploid arctic flora, probably because their genomes confer hybrid vigor and buffer against the effects of inbreeding. However, fertility barriers between species often need to be overcome in order to form successful allopolyploids, and these barriers may have an epigenetic basis.
What Is Polyploidy Breeding
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 and rye . It combines sought-after characteristics of the parents, but the initial hybrids are sterile.
What is polyploidy in evolution?
Polyploidy, the condition of possessing more than two complete genomes in a cell, has intrigued biologists for almost a century. Polyploidy is found in many plants and some animal species and today we know that polyploidy has had a role in the evolution of all angiosperms.
What is polyploidy in biology quizlet?
Polyploidy is when an organism has more than two complete sets of chromosomes in its somatic cells.
Are humans diploid?
In humans, cells other than human sex cells, are diploid and have 23 pairs of chromosomes. Human sex cells contain a single set of chromosomes and are known as haploid.
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The Nucleus And Its Response To Genome Size Increase
The way the nucleus is often diagrammed in textbooks bears little resemblance to the reality of its structure . According to Gavrilov and Razin , The eukaryotic cell nucleus is one of the most complex cell organelles. Despite the absence of membranes, the nuclear space is divided into numerous compartments where different processes involved in genome activity take place. There is much that is not understood about these various compartments . It is with this complexity in mind that the possible impact of genome size increase on nuclear size and function should be considered.
Complex internal structure of the eukaryotic nucleus . a = nucleolus b = perinucleolar space c = interchromatin domain d = topologically associated domain e = lamina f = nuclear envelope g = lamina-associated domains h = nucleolus-associated domains i = chromosome territories j = Polycomb body k = insulator body l = promyelocytic leukaemia body m = Cajal body n = nuclear speckles o = nuclear pore complex.
The interspecific variability in root tip nuclear crowdedness is only one piece of evidence indicating that a model in which DNA content directly determines nuclear volume through crowding constraints is overly simplistic. A recent review in a section on Genome Size and Ploidy notes:
Is Polyploidy Sympatric Speciation
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Sympatric speciation occurs when populations of a species that share the same habitat become reproductively isolated from each other. This speciation phenomenon most commonly occurs through polyploidy, in which an offspring or group of offspring will be produced with twice the normal number of chromosomes.
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Mechanisms Of Hybrid Incompatibility
To understand the extraordinary contribution of polyploids to diversity, it will be necessary to elucidate the mechanisms that lead to phenotypic variation and how they are modified to achieve adaptation. Among novel hybrid phenotypes, sterility and lethality are deleterious and produce reproductive barriers. Other consequences, such as heterosis or hybrid vigor, can be advantageous. Heterosis makes hybrids perform better than their parents in terms of increased biomass, size, yield, fertility, resistance to disease, and so on. Hybrids that survive lethality during embryogenesis can display vigorous growth during their later life. Remarkably, heterosis is reinforced by polyploidy: tetraploid hybrids show stronger heterosis than the corresponding diploid hybrids, which helps explain the remarkable success of polyploid plants in evolution .
The range of hybrid effects is puzzling and their molecular basis is not understood. All effects, however, must result from genetic variation that has accumulated in the parental lines since their divergence from a common ancestor. So, both favorable and unfavorable effects may derive from fundamentally similar mechanisms.
How Might Changes To Nuclear Size And Volume Produce Novelty
Assuming that genome doubling will affect at least some cell types by changing nuclear size, internal crowding, or both, would phenomena such as transcription be affected, and if so, in predictable ways? This certainly seems likely to quote Vukovic et al. again, How nuclear size and shape affect cell physiology is still unclear, but it is certainly possible that nuclear morphology impacts chromatin organization and gene expression. Elucidating the functional significance of nuclear size necessitates an understanding of the mechanisms that control nuclear size. As with other aspects of the cell, changes to nuclear surface-to-volume ratio are implicated in changes resulting from a larger nucleus. Pandit et al. suggest that such changes could make transport between the nucleus and cytoplasm less efficient. For example, as Almassalha et al. suggest, A greater surface of chromatin interface facilitates gene transcription due to, among other effects, the better access of transcription factors to DNA. A reduction in this chromatin interface could therefore explain the pattern described above of a less than doubling of transcriptome size following genome duplication .
Sugawara and Kimura provide a succinct overview of this contention:
Movement and Transport of RNAs
Chromatin Structure and Its Emerging Role in Gene Regulation
Chromosome Structure and Positioning
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Polyploidy Alters The Genome
It has long been understood that polyploid genomes are often not uniform 2N-fold duplications of diploid genomes. Recent genome sequencing efforts and new tissue models underscore the ability of polyploid cells to drastically manipulate their genome . Such alteration creates genomes that diploid cells are unlikely to tolerate, which can increase cellular heterogeneity.
One long-appreciated polyploid genome modification is underreplication of specific genome regions . Underreplication often occurs in polyploid cells with giant polytene chromosomes, such as in dipteran salivary glands or mammalian placental giant trophoblast cells . Interestingly, underreplicated regions include gene-containing regions .
Yet another recurring form of polyploid genome alteration occurs in endopolyploid cells capable of mitosis. The ability of some endopolyploid cells to divide shows that polyploidization cannot be universally characterized as a means of tissue growth for nonproliferative tissues. During these polyploid divisions, instead of evenly partitioning the genome, daughter cells are frequently created with chromosome number imbalances, or aneuploidy. Although the association between polyploidy and aneuploidy was originally appreciated in cases of aberrant polyploidy , its now known that naturally occurring mouse liver hepatocytes and Drosophila and Culex pipiens rectal papillar cells generate aneuploidy during polyploid divisions .