Polyploidy

SIGNIFICANCE: Polyploids have three or more complete sets of chromosomes in their nuclei instead of the two sets found in diploids. Polyploids are especially common in plants, with some examples also existing in animals, and have a prominent role in the evolution of species. Some tissues of diploid organisms are polyploid, while the remaining cells in the organism are diploid.

The Formation of Polyploidy

Most animals are diploid, meaning that they have two sets of chromosomes in their cells; and their gametes (eggs and sperm) are haploid, that is, having one set of chromosomes. Plants, a variety of single-celled eukaryotes, and some insects have individual or parts of an individual’s life cycle when they are haploid. In any case, when there are more than two sets of homologous chromosomes, the cell or organism is considered polyploid. A organism has three sets of homologous chromosomes, a tetraploid has four sets, a dodecaploid has twelve sets, and there are organisms known to have many more than a dozen sets of homologous chromosomes.

How polyploids are formed in nature is still debated. Regardless of what theory is accepted, the first step certainly involves a failure during cell division, in either meiosis or mitosis. For example, if cytokinesis (division of the cytoplasm) fails at the conclusion of meiosis II, the will be diploid. If, by chance, a sperm fertilizes a diploid egg, the resulting zygote will be tetraploid. Although polyploidy might occur this way, biologists have proposed an alternative model involving a triploid intermediate stage.

The triploid intermediate model has been applied primarily to plants, in which polyploidy is better studied. Hybrids between two species are often sterile, but occasionally a diploid from one of the species joins with a normal gamete from the other species, which produces a triploid hybrid. Triploids are also sterile, for the most part, but do produce a small number of gametes, many of which are diploid. This makes the probability that two diploid gametes will join, to form a tetraploid, much higher. This hypothesis is supported by the discovery of triploid hybrid plants that do produce a small number of viable gametes. This type of polyploid, formed as a result of hybridization between two species, is called an allopolyploid. Allopolyploids are typically fertile and represent a new species.

Polyploidy can also occur within a single species, without hybridization, in which case it is called an autopolyploid. Autopolyploids can form in the same way as allopolyploids, but they can also occur as the result of a failure in cell division in a bud. If a cell in the meristematic region (a rapidly dividing group of cells at the tip of a bud) completes but not cytokinesis, it will be a tetraploid cell. All daughter cells from this cell will also be tetraploid, so that any flowers borne on this branch will produce diploid gametes. If the plant is self-compatible, it can then produce tetraploid offspring from these flowers. Autopolyploids are often a little larger and more robust than the diploids that produce them, but they are often so similar they cannot be easily distinguished. An autopolyploid, when formed, represents a new species but is not generally recognized as such unless it looks different enough physically from diploids.

The Genetics of Polyploids

A polyploid has more copies of each gene than a diploid. For example, a tetraploid has four alleles at each locus, which means tetraploids can contain much more individual variability than diploids. This has led some evolutionists to suggest that polyploids should have higher fitness than the diploids from which they came. With more variation, the individual would be preadapted to a much wider range of conditions. Because there are so many extra copies of genes, a certain amount of (loss of genes through mutation or other processes) occurs, with no apparent detriment to the plant.

The pairing behavior of chromosomes in polyploids is also unique. In a diploid, during meiosis, homologous chromosomes associate in pairs. In an autotetraploid there are four homologous chromosomes of each type which associate together in groups of four. In an allotetraploid, the chromosomes from the two species from which they are derived are commonly not completely homologous and do not associate together. Consequently, the pairs of homologous chromosomes from one parent species associate together in pairs, as do the chromosomes from the other parent species. For this reason, sometimes allopolyploids are referred to as amphidiploids, because their pairing behavior looks the same as it does in a diploid. This is also why an is fertile (because meiosis occurs normally), but a hybrid between two diploids commonly is not, because the chromosomes from the two species are unable to pair properly.

Polyploid Plants and Animals

In the plant kingdom, it is estimated by some that 95 percent of pteridophytes (plants, including ferns, that reproduce by spores) and perhaps as many as 80 percent of angiosperms (flowering plants that form seeds inside an ovary) are polyploid, although there is high variability in its occurrence among families of angiosperms. In contrast, polyploidy is uncommon in gymnosperms (plants that have naked seeds that are not within specialized structures). Extensive polyploidy is observed in chrysanthemums, in which chromosome numbers range from 18 to 198. The basic chromosome number (haploid or gamete number of chromosomes) is 9. Polyploids from triploids (with 27 chromosomes) to 22-ploids (198 chromosomes) are observed. The stonecrop Sedum suaveolens, which has the highest chromosome number of any angiosperm, is believed to be about 80-ploid (720 chromosomes). Many important agricultural crops, including wheat, corn, sugarcane, potatoes, coffee, apples, and cotton, are polyploid.

Polyploid animals are less common than polyploid plants but are found among some groups, including crustaceans, earthworms, flatworms, and insects such as weevils, sawflies, and moths. Polyploidy has also been observed in some vertebrates, including tree frogs, lizards, salamanders, and fish. It has been suggested that the genetic redundancy observed in vertebrates may be caused by ancestral polyploidy.

Polyploidy in Tissues

Most plants and animals contain particular tissues that are polyploid or polytene, while the rest of the organism is diploid. Polyploidy is observed in multinucleate cells and in cells that have undergone endomitosis, in which the chromosomes condense but the cell does not undergo nuclear or cellular division. For example, in vertebrates, liver cells are binucleate and therefore tetraploid. In addition, in humans, megakaryocytes can have polyploidy levels of up to sixty-four. A is a giant bone-marrow cell with a large, irregularly lobed nucleus that is the precursor to blood platelets. A megakaryocyte does not circulate, but forms platelets by budding. A single megakaryocyte can produce three thousand to four thousand platelets. A platelet is an enucleated, disk-shaped cell in the blood that has a role in blood coagulation. In polytene cells, the replicated copies of the chromosomal DNA remain associated to produce giant chromosomes that have a continuously visible banding pattern. The trophoblast cells of the mammalian placenta are polytene.

Importance of Polyploids to Humans

Most human polyploids die as embryos or fetuses. In a few rare cases, a polyploid infant is born that lives for a few days. In fact, polyploidy is not tolerated in most animal systems. Plants, on the other hand, show none of these problems with polyploidy. Some crop plants are much more productive because they are polyploid. For example, wheat (Triticum aestivum) is an allohexaploid and contains chromosome sets that are derived from three different ancient types. Compared to the species from which it evolved, T. aestivum is far more productive and produces larger grains of wheat. Triticum aestivum was not developed by humans but appears to have arisen by a series of chance events in the past, humans simply recognizing the better qualities of T. aestivum. Another fortuitous example involves three species of mustard that have given rise to black mustard, turnips, cabbage, broccoli, and several other related crops, all of which are allotetraploids.

Polyploids may be induced by the use of drugs such as colchicine, which halts cell division. Because of the advantages of the natural polyploids used in agriculture, many geneticists have experimented with artificially producing polyploids to improve crop yields. One prime example of this approach is Triticale, which represents an allopolyploid produced by hybridizing wheat and rye. Producing artificial polyploids often produces a new variety that has unexpected negative characteristics, so that only a few such polyploids have been successful.

Researchers are exploring the possible role of polyploidy in species survival in stressful conditions. For example, polyploid trees may be better suited to adapting to drought stress or other impacts of climate change.

Key terms

• allopolyploida type of polyploid species that contains genomes from more than one ancestral species
• aneuploida cell or an organism with one or more missing or extra chromosomes; the opposite is “euploid,” a cell with the normal chromosome number
• autopolyploida type of polyploid species that contains more than two sets of chromosomes from the same species
• homologous chromosomeschromosomes that are structurally the same and have the same gene loci, although they may have different alleles (alternative forms of a gene) at many of their shared loci

Bibliography

Adams, Keith L., et al. “Genes Duplicated by Polyploidy Show Unequal Contributions to the Transcriptome and Organ-Specific Reciprocal Silencing.” Proceedings of the National Academy of Sciences 100 (2003): 4649–4654. Print.

Gregory, T. Ryan, ed. The Evolution of the Genome. N.p.: Academic, 2011. Digital file.

Hunter, Kimberley L., et al. “Investigating Polyploidy: Using Marigold Stomates and Fingernail Polish.” American Biology Teacher 64.5 (2002): 364. Print.

Leitch, Illia J., and Michael D. Bennett. “Polyploidy in Angiosperms.” Trends in Plant Science 2.12 (1997): 470–476. Print.

Lewis, Ricki. Human Genetics: Concepts and Applications. 11th ed. [S.I.]: McGraw, 2014. Print.

Linder, H. Peter, and Nigel P. Barker. "Does Polyploidy Facilitate Long-Distance Dispersal?" Annals of Botany 113.7 (2014): 1175–1183. Digital file.

Madlung, A. "Polyploidy and Its Effect on Evolutionary Success: Old Questions Revisited with New Tools." Heredity 110.2 (2013): 99–104. MEDLINE with Full Text. Web. 7 Aug. 2014.

Miller, Orlando J., and Eeva Therman. Human Chromosomes. 4th ed. New York: Springer, 2001. Print.

Raebild, Anders, et al. "Polyploidy--A Tool in Adapting Trees to Future Climate Changes? A Review of Polyploidy in Trees." Forest Ecology and Management, vol. 560, 2024, doi.org/10.1016/j.foreco.2024.121767. Accessed 6 Sept. 2024.

Soltis, Pamela S., and Douglas E. Soltis. Polyploidy and Genome Evolution. Berlin; New York: Springer, 2012. Digital file.

Sumner, Adrian T. Chromosomes: Organization and Function. Hoboken: Wiley, 2008. Digital file.

Van de Peer, Yves, Tia-Lynn Ashman, Pamela S. Soltis, and Douglas E. Soltis. "Polyploidy: An Evolutionary and Ecological Force in Stressful Times." The Plant Cell, vol. 33, no. 1, 2021, pp. 11-26, doi: 10.1093/plcell/koaa015. Accessed 6 Sept. 2024.