A polyploid cell has at least one extra set of chromosomes, meaning its cells contain more than two complete sets. Human body cells are diploid, having two sets of chromosomes (2n), one inherited from each parent. In contrast, reproductive cells like sperm and eggs are haploid, containing just one set (n). Polyploidy occurs when a cell has three (triploid, 3n), four (tetraploid, 4n), or more sets of chromosomes. While most animals are diploid, polyploidy is a widespread phenomenon, particularly within the plant kingdom.
How Polyploidy Occurs
Polyploidy originates from errors during cell division. The first pathway involves mistakes during mitosis, the cell division responsible for growth and repair. When mitosis goes awry, it can produce individual polyploid cells within an organism that is otherwise diploid.
A more common route to a fully polyploid organism involves errors during meiosis, the specialized cell division that creates gametes like sperm and eggs. A failure of chromosomes to separate correctly can result in diploid (2n) gametes instead of the usual haploid (n) ones. If one of these abnormal diploid gametes is involved in fertilization, the resulting offspring will be polyploid from its first cell.
This process can lead to two different types of polyploidy. Autopolyploidy occurs when the extra chromosome sets come from a single species, representing a doubling of its own genetic material. In contrast, allopolyploidy results from the hybridization of two different species, combining distinct chromosome sets.
The Role of Polyploidy in Plants
Polyploidy is exceptionally common and often beneficial in the plant kingdom, acting as a driver of evolution and diversification. It is estimated that 15% of speciation events in flowering plants are associated with an increase in ploidy. The presence of extra chromosome sets allows for mutations to accumulate without causing negative effects, fostering greater genetic diversity.
A direct consequence of polyploidy in plants is an increase in cell size, which can contribute to visibly larger organs like fruits, seeds, and flowers. This “gigas” effect is valued in agriculture, where it can translate to increased yields and more robust crops. Many plants we consume daily have been cultivated for their polyploid characteristics.
For instance, the large strawberries sold in supermarkets are octoploid, possessing eight sets of chromosomes (8n). Bread wheat is hexaploid (6n), combining the genomes of three different ancestral grass species, which contributes to its baking qualities. Another familiar example is the triploid (3n) seedless watermelon, whose three chromosome sets disrupt normal seed development.
Polyploidy in the Animal Kingdom
In contrast to plants, polyploidy that affects an entire organism is rare and often lethal in animals, particularly in complex vertebrates like mammals. The addition of whole chromosome sets severely disrupts the intricate networks of gene regulation necessary for animal development. For humans, conditions like triploidy (69 chromosomes) or tetraploidy (92 chromosomes) are fatal, leading to miscarriage early in pregnancy.
Despite its general lethality, polyploidy is not entirely absent in the animal kingdom. It can be found in some invertebrates, such as certain flatworms and leeches, as well as in some fish and amphibians. Salmon and salamanders are well-known examples of animals that can tolerate polyploidy. In some cases, specific tissues within a diploid animal can be polyploid, a condition known as endopolyploidy, which is seen in human liver cells.
Polyploidy should be distinguished from aneuploidy, which is a more common type of chromosomal abnormality in animals. Aneuploidy involves the gain or loss of one or a few individual chromosomes, not an entire set. An example of aneuploidy is Trisomy 21, which causes Down syndrome. Individuals with this condition have three copies of chromosome 21, but their other chromosomes remain in pairs, making the total count 47 instead of 46. This is fundamentally different from the wholesale duplication of every chromosome seen in polyploidy.