Polyploidy is whole genome duplication, when all the chromosomes and so, all the genes, of an organism double. In plants, it is very common; easily 3/4 of plants are polyploid.
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| an Oenothera |
Because polyploidy is a genetic effect that takes place inside the cell's nucleus, it is not casually observed. But botanists discovered polyploidy as soon as they had microscopes to look at the contents of the nucleus and when they tried crossing polyploids and got results that didn't make sense based on diploid genetics. (Diploid = 2 copies of the genome, polyploidy = numbers over 2, like 3, 4, and 6 see previous posts in this series link link). Study of polyploidy began in the early 1900s.
However, knowing that Oenothera lamarkiana, now O. glazioviana, the big-flowered evening primrose, is tetraploid (4x), did not reveal whether Oenothera albicaulis, prairie evening primrose, was polyploid, let alone give information about dandelions (Taraxacum) or maples (Acer). Detecting polyploidy takes some effort. Historically one prepared cells at the right step in cell division and counted chromosomes under the microscope. Today, you might compare DNA or genes across a series of plants to see if some have double the amount of DNA or double the markers for those genes. These require all the equipment for DNA counting or sequencing. Historically, botanists writing papers about a particular group of plants, sorting out the number of species present, usually took the time to determine chromosome number for the species they were considering, so gradually knowledge of the frequency of polyploidy increased. Nevertheless, our knowledge of the distribution of polyploidy was patchy across the 20th century. The more we look, the more we find. Virtually all plants have polyploidy in their ancestry and easily 70% are relatively recent polyploids.
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| dandelions, Taraxacum officinale are triploid 3x |
What does whole genome duplication do in nature? Why are so many species polyploid?
Botanists quickly discovered naturally occuring polyploid species that formed from whole genome duplication in hybrids. Wheat is the classic example. Bread wheat, Triticum aestivum, is a species formed by a cross between a gamete from a tetraploid wheat (Triticum turgidum, 4x, the gamete had 2 copies of the basic wheat genome) and a diploid wheat species (T. taushii, 2x gamete with one copy of the genome). The resulting hybrid was sterile or nearly so because it was triploid (3x) and put incomplete groups of chromosomes into its gametes. However a failure of cell divising formed a polyploid seed, with two copies of the genes from the tetraploid parent and two copies of the genes of the diploid parent, making a plant with six total wheat genomes. The hexaploid (6x) is fertile because the duplicated genes paired nicely with each other at cell division (meiosis). This 6x bread wheat, T. aestivum, became, instantly, a new species, because, since its parents had a different number of chromosomes, it was unable to cross with either one. Wheat's story is important to human history, the hexaploid having bigger seeds that fed more people. As botanists worked out relationships in other genera of plants, they often found a tetraploid or hexaploid species that had formed by interspecific hybridization followed by doubling. Examples include upland cotton, Gossypium hirsuta, Moscow (Idaho) salsify Tragopogon miscellus, and cultivated tobacco, Nicotiana tabacum.
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| bread wheat |
It is common to find polyploid species within large plant genera, reflecting repeated cases of species formation by polyploidy. This group of Asian chrysanthemums (Chrysanthemum is an Asian genus) is typical:
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| (Data from Wang et al. see References) |
These are only some of the species of Chrysanthemum, there are about 40. But all across higher plants genus after genus looks like this. At least three polyploidy speciation events are required for even the 10 species of Chrysanthemum I show above: one 2x to 4x polyploidy event, one 4x to 8x, and one 2x x 4x to 6x. There could be more polyploid transitions, though, if, for example, C. indicum and C. japonicum formed independently from different 2x parent species.
.jpg "chrysanthemum in China") |
| chrysanthemum |
Just because a polyploid hybrid is formed, doesn't mean there will be a successful new species. It could form but die without multiplying. The numerous polyploid species seen by botanists are the results of hybridization followed by duplication, followed by survival and expansion.
Generally closely related plants do not share the same habitat, and that applies new polyploid species, they generally occupy different habitats from the parent species. Presumably the novel gene combinations of the new polyploid and a period of natural selection produced plants that could live in habitats not available to the parental species and so the lineage expanded its range. Diploid strawberries from Japan formed a tetraploid hybrid that spread across Asia meeting a different Eurasian diploid to form a northern Asian hexaploid which encountered a North American diploid, Fragaria vesca, in the Arctic where another hybrid formed, doubled and created the octoploid strawberries that were bred to be our cultivated strawberry (See lovelly colored maps of Figure 2 in "Origin and evolution of the octoploid strawberry genome" Scoll down to the map link)
Hybridization followed by polyploidy is a potent form of speciation and adaptation in plants.
Polyploidy caused by interspecific hybridization, the events I have been discussing here, is called allopolyploidy (allo- for "other"). Polyploidy can also occur if the genome of a single plant fails to divide at cell division, and the doubled number gets passed on, autopolyploidy. This is common too, I will explore it in a future post.
Comments and corrections welcome.
References
Edgar, P P., T. J. Poorten, R. VanBuren, and 22 others. 2019. Origin and evolution of the octoploid strawberry genome. Nature Genetics. 51: 541-547. link Accessed 3/23/24.
Ewert, R. F. and S. E. Eichhorn. 2912. Raven Biology of Plants. 8th edition. W.H. Freeman and Company, Publishers. New York, New York.
Heslop-Harrison, Schwarzacher and Liu. 2023. Polyploidy: its consequences and enabling role in plant diversification and evolution. Annals of Botany. 131: 1-9. link
Wang, H., X. Qi, R. Gaol, J. Wang, B. Dong, J. Jiang, S. Chen, Z. Guan, W. Fang, Y. Liao and F. Chen. 2014. Microsatellite polymorphism among Chrysanthemum sp. polyploids: the influence of whole genome duplication. Scientific Reports. 4: 6730. DOI: 10.1038/srep06730. pp. 1-8. Link Accessed 3/22/24.
Wendel, J. F. and R. C. Cronn. 2003. Polyploidy and the evolutionary history of cotton. Advances in Agronomy. 78 Academic Press. New York.139-186. link Accessed 3/22/24.
WheatBP New. Evolution of Wheat link Accessed 3/22/24.
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