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Daily RC Article 145

The Complex World of Polyploidy: Nature's Chromosomal Tapestry

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.... Then there is the extremely important phenomenon of polyploidy. Genes are aligned along chromosomes. Every kind of organism has its characteristic number and arrangement of chromosomes. Eggs and sperm (or the appropriate cells in ovules and pollen in case of plants) contain only one set of chromosomes, and is haploid. When they fuse in the act of fertilization, the resulting embryo has two sets of chromosomes and is then diploid. Human beings and chimpanzees are diploid.

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Sometimes, the chromosome number will double (as they normally do in preparation for cell division) but the cell fails to divide. The diploid cell becomes tetraploid with four sets of chromosomes. The newly formed tetraploid organism can breed successfully with other tetraploids of its own kinds, but it cannot breed successfully with either of its parents. So it forms an instant new species. ... The common potatoes grown in Europe are tetraploid derivatives of diploid potatoes that grow wild (and are cultivated) in the Andes. The octoploids form new species – unable to interbreed with the tetraploid parents that formed them.

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Polyploid is the term that describes any organism with more than two sets of chromosomes. Sometimes the complications become too much even for the plants, and they finish up with an odd number of chromosomes (some having been lost among all the cell divisions and matings). Plants with anomalous numbers of chromosomes are said to be ‘aneuploid’. Unlike aneuploid animals who die or tend to be compromised if they live, plants put up with aneuploidy. Sugarcane is aneuploid but that does not stop it being a vigorous major crop.

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There is one further complication. Diploid organisms that are of different species mate to produce fully viable offspring. Usually such crosses fail because the chromosomes of the two parents are incompatible. ... The cells will not produce sound gametes (eggs and sperm, or ovules and pollen) since this requires close cooperation between chromosomes. But if a hybrid organism doubles its chromosomes, it often can produce viable gametes. So we find diploid parents of different species mating to produce diploid, hybrid offspring that are sterile; but the hybrids then double their chromosomes and become tetraploid – and the hybrid tetraploids are then fertile. This happens a lot among plants. Indeed the complications seem endless. A tetraploid plant may mate with a closely related diploid plant to produce a triploid offspring – two sets of chromosomes from the tetraploid parent and one set from the diploid parent. Triploids are sterile – they cannot produce gametes at all – but they may still form viable plants. Thus the cultivated banana is triploid. Because it is sterile, its fruits contain no seeds (as wild banana fruits do). So the domestic banana has to be produced vegetatively, by planting cuttings. Triploid hybrids double their chromosomes to become hexaploid (with six sets of chromosomes). The most famous and important hexaploid organism of all is bread wheat (pasta wheat is diploid). Among trees, Willows (genus Salix) and Acacias provide hundreds of examples of polyploid species.

The article delves into the intricate realm of polyploidy, where organisms possess more than the typical two sets of chromosomes. It highlights how failed cell divisions can lead to tetraploidy, forming instant new species unable to breed with their parent species. Aneuploidy, the presence of an odd number of chromosomes, challenges typical genetic norms but can be accommodated in plants like sugarcane. The piece also explores the potential for hybridization between different species, often resulting in sterile offspring that can regain fertility by doubling their chromosomes. The never-ending complexities of polyploidy are illustrated through various plant examples, including bananas and bread wheat, as well as in trees like willows and acacias, showcasing the remarkable diversity and adaptability within nature's chromosomal tapestry.
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