Is Polyploidy More Common In Plants Or Animals

Polyploids are common among plants, also as amidst certain groups of fish and amphibians. How does this interesting condition crop up, and what advantages and disadvantages does information technology impart?

Introduction

Polyploidy is the heritable condition of possessing more than two consummate sets of chromosomes. Polyploids are common among plants, as well as amongst certain groups of fish and amphibians. For instance, some salamanders, frogs, and leeches are polyploids. Many of these polyploid organisms are fit and well-adapted to their environments. In fact, recent findings in genome research indicate that many species that are currently diploid, including humans, were derived from polyploid ancestors (Van de Peer & Meyer, 2005). These species that have experienced ancient genome duplications and so genome reduction are referred to every bit paleopolyploids. This article discusses the mechanisms underlying polyploidy, and both the advantages and disadvantages of having multiple sets of chromosomes.

Mechanisms of Polyploidy

How does an organism become polyploid? Polyploids arise when a rare mitotic or meiotic ending, such every bit nondisjunction, causes the formation of gametes that take a consummate prepare of indistinguishable chromosomes. Diploid gametes are frequently formed in this way. When a diploid gamete fuses with a haploid gamete, a triploid zygote forms, although these triploids are generally unstable and can frequently be sterile. If a diploid gamete fuses with another diploid gamete, withal, this gives rise to a tetraploid zygote, which is potentially stable. Many types of polyploids are found in nature, including tetraploids (four sets of chromosomes), hexaploids (6 sets of chromosomes), and other chromosome-pair multiples (Figure 1).

Researchers usually make a distinction betwixt polyploids that ascend within a species and those that arise due to the hybridization of two distinct species. The sometime are known every bit autopolyploids, while the latter are referred to as allopolyploids. Autopolyploids are substantially homozygous at every locus in the genome. However, allopolyploids may have varying degrees of heterozygosity depending on the divergence of the parental genomes. Heterozygosity is credible in the gametes that polyploids produce. Allopolyploids can more often than not exist distinguished from autopolyploids because they produce a more diverse set up of gametes (Figure 2).

Different species exhibit different levels of tolerance for polyploidy. For example, polyploids form at relatively loftier frequency in flowering plants (ane per 100,000 individuals), suggesting that plants take a remarkably loftier tolerance for polyploidy. This is besides the case for some species of fish and frogs. Nevertheless, higher vertebrates exercise not appear to tolerate polyploidy very well; in fact, it is believed that x% of spontaneous abortions in humans are due to the formation of polyploid zygotes.

Advantages of Polyploidy

© 2005 Nature Publishing Group Comai, L. The advantages and disadvantages of being polyploid. Nature Reviews Genetics 6, 838 (2005). All rights reserved.

Due to the loftier incidence of polyploidy in some taxa, such equally plants, fish, and frogs, there clearly must be some advantages to being polyploid. A common instance in plants is the observation of hybrid vigor, or heterosis, whereby the polyploid offspring of two diploid progenitors is more vigorous and healthy than either of the 2 diploid parents. There are several possible explanations for this observation. One is that the enforced pairing of homologous chromosomes within an allotetraploid prevents recombination between the genomes of the original progenitors, effectively maintaining heterozygosity throughout generations (Figure 3). This heterozygosity prevents the aggregating of recessive mutations in the genomes of later generations, thereby maintaining hybrid vigor. Another of import factor is cistron redundancy. Because the polyploid offspring now have twice as many copies of any particular gene, the offspring are shielded from the deleterious furnishings of recessive mutations. This is specially important during the gametophyte life stage. One might envision that, during the haploid stage of the life bicycle, whatsoever allele that is recessive for a deleterious mutation will not be masked by the presence of a dominant, normally functioning allele, allowing the mutation to cause developmental failure in the pollen or the egg sac. Conversely, a diploid gamete permits the masking of this deleterious allele by the presence of the dominant normal allele, thus protecting the pollen or egg sac from developmental dysfunction. This protective event of polyploidy might be important when small, isolated populations are forced to inbreed.

Some other reward conferred by gene redundancy is the ability to diversify factor function over time. In other words, extra copies of genes that are not required for normal organism function might terminate up being used in new and entirely different ways, leading to new opportunities in evolutionary pick (Adams & Wendel, 2005).

Interestingly, polyploidy can touch on sexuality in ways that provide selective advantages. One way is by disrupting certain self-incompatibility systems, thereby assuasive self-fertilization. This might be the event of the interactions betwixt parental genomes in allopolyploids (Comai et al., 2000). Another way is by favoring the onset of asexual reproduction, which is associated with polyploidy in both plants and animals. This switch in reproductive strategies may ameliorate fitness in static environments.

Disadvantages of Polyploidy

For all the advantages that polyploidy tin can confer to an organism, there are likewise a slap-up number of disadvantages, both observed and hypothesized. One of these disadvantages relates to the relative changes between the size of the genome and the volume of the cell. Cell volume is proportional to the amount of DNA in the jail cell nucleus. For example, doubling a jail cell's genome is expected to double the volume of space occupied past the chromosomes in the nucleus, merely it causes merely a i.6-fold increment in the surface area of the nuclear envelope (Melaragno et al., 1993). This tin can disrupt the balance of factors that normally mediate interactions betwixt the chromosomes and nuclear components, including envelope-spring proteins. The peripheral positioning of telomeric and centromeric heterochromatin may be disturbed also, considering there is less relative surface space on the nuclear envelope to adapt this positioning (Fransz et al., 2002).

Polyploidy tin can also be problematic for the normal completion of mitosis and meiosis. For one, polyploidy increases the occurrence of spindle irregularities, which can pb to the chaotic segregation of chromatids and to the production of aneuploid cells in animals and yeast. Aneuploid cells, which have abnormal numbers of chromosomes, are more readily produced in meioses involving three or more sets of chromosomes than in diploid cells. Autopolyploids take the potential to class multiple arrangements of homologous chromosomes at meiotic metaphase I (Figure ii), which can result in aberrant segregation patterns, such as 3:one or 2:1 plus one laggard. (Laggard chromosomes do non attach properly to the spindle appliance and thus randomly segregate to daughter cells.) These abnormal segregation patterns cannot be resolved into balanced products, and random segregation of multiple chromosome types produces mostly aneuploid gametes (Effigy 3). Chromosome pairing at meiosis I is more than constrained in allopolyploids than in autopolyploids, but the stable maintenance of the ii parental chromosomal complements as well requires the formation of balanced gametes.

Another disadvantage of polyploidy includes potential changes in gene expression. Information technology is more often than not assumed that an increase in the re-create number of all chromosomes would affect all genes equally and should outcome in a compatible increase in factor expression. Possible exceptions would include genes that respond to regulating factors that exercise non change proportionally with ploidy. We at present have experimental prove for such exceptions in several systems. In i interesting example, investigators compared the mRNA levels per genome for 18 genes in 1X, 2X, 3X, and 4X maize. While expression of nearly genes increased with ploidy, some genes demonstrated unexpected deviations from expected expression levels. For example, sucrose synthase showed the expected proportional expression in 2X and 4X tissues, but its expression was three and half dozen times college, respectively, in 1X and 3X tissues. Two other genes showed similar, if less extreme, trends. Altogether, near 10% of these genes demonstrated sensitivity to odd-numbered ploidy (Guo et al., 1996).

Epigenetic instability can pose however another claiming for polyploids. Epigenetics refers to changes in phenotype and gene expression that are not caused by changes in DNA sequence. Co-ordinate to the genomic shock hypothesis, disturbances in the genome, such as polyploidization, may lead to widespread changes in epigenetic regulation. Although there are few instances of documented epigenetic instability in autopolyploids, there are a couple of intriguing examples worth mentioning. In 1 example, transgene silencing occurred more often in Arabidopsis thaliana tetraploids than in A. thaliana diploids, suggesting an event of ploidy on chromosome remodeling (Mittelsten Scheid et al., 1996). Nevertheless, several factors cannot exist ruled out in the observation of this miracle, including duplication of the strong 35S promoter from cauliflower mosaic virus in the transgene. In another case, the activation of a DNA transposon of the Spm/CACTA family was observed in autopolyploids. Unfortunately, the generality of this change could not be determined because multiple independent autopolyploids were not examined.

Conversely, all-encompassing testify for epigenetic remodeling is available in allopolyploids. Structural genomic changes, such equally Dna methylation, and expression changes are reported to back-trail the transition to alloploidy in several plant systems, including Arabidopsis and wheat (Shaked et al., 2001). The most detailed information is available for the model system Arabidopsis. For instance, in a cross of A. thaliana and A. arenosa, epigenetically regulated genes were identified by comparing transcripts from the autotetraploid parents to transcripts from the neoallopolyploid progeny. A. thaliana genes afflicted past epigenetic regulation were defined as those that responded to the transition from autopolyploidy to allopolyploidy. Altogether, betwixt 2% and 2.5% of A. thaliana genes were estimated to have undergone regulatory changes during the transition to allopolyploidy. A more detailed microarray study that examined the regulation of 26,000 genes in Arabidopsis neoallopolyploids detected a transcriptome divergence betwixt the progenitors of more than than fifteen%, due to genes that were highly expressed in A. thaliana and non in A. arenosa or vice versa. Significantly, expression of approximately five% of the genes diverged from the mid-parent value in two independently derived allotetraploids, consequent with nonadditive factor regulation later on hybridization (Wang et al., 2006). Taken together, these results suggest that the instability syndrome of neoallopolyploids may be attributed primarily to regulatory divergence betwixt the parental species, leading to genomic incompatibilities in the allopolyploid offspring.

Aneuploidy might likewise be a gene in epigenetic remodeling in neoallopolyploids, either past altering the dosage of factors that are encoded by chromosomes that have greater or fewer than the expected number of copies leading to changes in imprinted loci, or by exposing unpaired chromatin regions to epigenetic remodeling mechanisms. In the latter case, this susceptibility of meiotically unpaired Deoxyribonucleic acid to silencing was beginning reported for the fungus Neurospora crassa, but it appears to be a full general miracle. Therefore, some of the epigenetic instability that is observed in allopolyploids might effect from aneuploidy.

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 invasive allopolyploid Spartina angelica. Yet, it is not clear whether the success of this species can exist 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 considering their genomes confer hybrid vigor and buffer against the effects of inbreeding. However, fertility barriers between species often demand to be overcome in order to form successful allopolyploids, and these barriers may have an epigenetic footing.

Summary

Recent studies accept provided interesting insights into the regulatory and genomic consequences of polyploidy. Together with the emerging bear witness of ancestral duplication through polyploidization in model plant, mucus, and animate being species, knowledge of these consequences has stimulated thinking about the relationship between early polyploidization events, the success of the polyploidy, and the long-term fate of new species.

References and Recommended Reading

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This commodity was adjusted from Comai, L., The advantages and disadvantages of beingness polyploid. Nature Reviews Genetics half-dozen, 838-845 (2005) (link to article)

Adams, K. L., & Wendel, J. F. Polyploidy and genome evolution in plants. Current Stance in Institute Biology 8, 135-141 (2005)

Comai, L. et al. Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids. Constitute Cell 12, 1551-1568 (2000)

Fransz, P., et al. Interphase chromosomes in Arabidopsis are organized besides defined chromocenters from which euchromatin loops emanate. Proceedings of the National University of Sciences 99, 14584-14589 (2002)

Guo, M., Davis, D., & Birchler, J. A. Dosage effects on gene expression in a maize ploidy series. Genetics 142, 1349-1355 (1996)

Melaragno, J. E., Mehrotra, B., & Coleman, A. Westward. Relationship between endopolyploidy and cell size in epidermal tissue of Arabidopsis. Establish Cell 5, 1661-1668 (1993)

Mittelsten Scheid, O., et al. A change of ploidy can modify epigenetic silencing. Proceedings of the National Academy of Sciences 93, 7114-7119 (1996)

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 Cell 13, 1749-1759 (2001)

Van de Peer, Y., & Meyer, A. Chapter 6: Large-scale gene and aboriginal genome duplications. In The Development of the Genome, ed. T. R. Gregory, 330-363 (San Diego, Elsevier, 2005)

Wang, J., et al. Genome-broad nonadditive cistron regulation in Arabidopsis allotetraploids. Genetics 172, 507-517 (2006)

Source: https://www.nature.com/scitable/topicpage/polyploidy-1552814/

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