The Formation of Polyploidy (Genetics & Inherited Conditions)
Most animals are diploid, meaning that they have two homologous 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 triploid 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 daughter cells will be diploid. If, by chance, a diploid 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 gamete from one of the species...
(The entire section is 483 words.)
The Genetics of Polyploids (Genetics & Inherited Conditions)
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 gene silencing (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 allopolyploid is fertile (because meiosis occurs normally), but a hybrid between two diploids...
(The entire section is 239 words.)
Polyploid Plants and Animals (Genetics & Inherited Conditions)
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.
(The entire section is 195 words.)
Polyploidy in Tissues (Genetics & Inherited Conditions)
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 megakaryocyte 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.
(The entire section is 162 words.)
Importance of Polyploids to Humans (Genetics & Inherited Conditions)
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...
(The entire section is 240 words.)
Further Reading (Genetics & Inherited Conditions)
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 (April 15, 2003): 4649-4654. This article shows that with multiple copies of a gene due to polypoidy, some of the copies are silenced.
Gregory, T. Ryan, ed. The Evolution of the Genome. Burlington, Mass.: Elsevier Academic, 2005. Two chapters focus on polyploidy, one dealing with polyploidy in plants, the other with polyploidy in animals.
Hunter, Kimberley L., et al. “Investigating Polyploidy: Using Marigold Stomates and Fingernail Polish.” American Biology Teacher 64, no. 5 (May, 2002): 364. A guide to exploring polyploidy through hands-on learning. Experiment supports National Science Education Standards.
Leitch, Illia J., and Michael D. Bennett. “Polyploidy in Angiosperms.” Trends in Plant Science 2, no. 12 (December, 1997): 470-476. Describes the role of polyploidy in the evolution of higher plants.
Lewis, Ricki. Human Genetics: Concepts and Applications. 9th ed. Dubuque, Iowa: McGraw-Hill, 2009. Gives an overview of polyploidy and aneuploidy in humans. Color ilustrations, and maps.
Miller, Orlando J., and Eeva Therman. Human Chromosomes. 4th ed. New York: Springer, 2001. A textbook about the function and dysfunction of human...
(The entire section is 222 words.)
Web Sites of Interest (Genetics & Inherited Conditions)
Kimball’s Biology Pages. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polyploidy.html. John Kimball, a retired Harvard University biology professor, includes a page about polyploidy in plants and animals and polyploidy in speciation in his online cell biology text.
The Polyploidy Portal. http://www.polyploidy.org/index.php/Main_Page. This site calls itself a “Web entry point to information about polyploidy,” and it contains both basic and advanced information about polyploidy, descriptions of polyploidy-related research projects, educational activities, and a bibliography.
(The entire section is 85 words.)