Polyploidy

Not all plant species are diploids. In fact, 75% of all angiosperms are polyploids, which are characterized by having more than two sets of chromosomes in their somatic cells. About 70% of undomesticated grasses and 25% of legumes are polyploids. Among domesticated crop species in general, 75% have been found to be polyploid, with roughly similar percentages found for both annual and perennial species. Major crops, such as wheat, alfalfa, potato, cotton, and sugarcane, are polyploids. There are also plants that do not possess complete sets of chromosomes. Aneuploids have abnormal numbers of chromosomes and vary by the addition or deletion of specific individual chromosomes that otherwise would be present in the normal crop genome. Ploidy reduction produces haploids, which have only a single set of homologous chromosomes instead of the pair found in their diploid counterparts. Haploid plants are very valuable in certain breeding applications. The number of chromosome sets possessed by a crop influences its genetics and thus, the strategies applied for its improvement. Plant breeders can alter chromosome numbers to modify and exploit genetic variability.

1.0Ploidy

Polyploidy can be rather complex. The terminology and symbols used in connection with polyploidy communicate much information—understanding these will help clarify and simplify an otherwise complex topic. • Ploidy refers to the number of chromosome sets in a cell. Prefixes are used to specify the number of chromosome sets in a particular organism. The symbol x is used to indicate the number of chromosomes in a set. Monoploids have one set (1x) and diploids have two sets (2x) of chromosomes, and so forth.

2.0What is Polyploidy 

Polyploidy is a biological condition characterized by cells containing more than the usual pairs of homologous chromosomes. While the majority of eukaryotic species, which possess nuclei in their cells, are diploid—meaning they have two complete sets of chromosomes, each derived from one of two parents—some organisms exhibit polyploidy. This phenomenon is particularly prevalent in plants.

In typical eukaryotes, somatic cells are diploid, generating haploid gametes (such as eggs and sperm) through the process of meiosis. In contrast, a monoploid organism or cell possesses only one set of chromosomes. This term is generally reserved for entities that are conventionally diploid. Notably, male bees and other Hymenoptera are examples of monoploid organisms.

In the realm of plants and multicellular algae, life cycles often involve two alternating multicellular generations. The gametophyte generation is haploid and produces gametes through mitosis, while the sporophyte generation is diploid, generating spores through meiosis.

Polyploidy can arise from abnormal cell division, occurring during mitosis. More commonly, it results from the failure of chromosomes to separate during meiosis or from the fertilization of an egg by multiple sperm. Additionally, polyploidy can be induced in plants and cell cultures through the use of certain chemicals. Colchicine, for instance, is a well-known substance capable of doubling the chromosome count. 

3.0Polyploidy Examples

Wheat exhibits diverse levels of polyploidy, with notable examples including emmer wheat (Triticum turgidum subsp. dicoccum) and durum wheat (Triticum turgidum subsp. durum), both classified as tetraploid with 28 chromosomes. Emmer wheat, an ancient variety, and durum wheat, commonly used for pasta, possess two sets of chromosomes from each parent. On the other hand, bread wheat (Triticum aestivum), the most widely cultivated type, is a hexaploid with 42 chromosomes. Bread wheat originated from natural hybridization events involving different wheat species, including emmer wheat and a wild grass.

4.0Euploidy

There are two general types of ploidy, which include plants that have either one or more complete sets of chromosomes present in their genome (euploids) or those that have partial sets due to the absence of at least one of their individual chromosomes or presence of at least one extra one (aneuploids): 

1. Euploidy refers to the number of chromosome sets in a cell. Prefixes are used to specify the number of chromosome sets in the particular organism. The symbol x is used to indicate the number of chromosomes in a set. Monoploids have one set (1x) and diploids have two sets (2x) of chromosomes, and so forth. 

Haploidy—individual with half (n=x) of the somatic cell chromosome number. 

Diploidy—individual with two sets of the basic, complete genome (2n=2x).  

Polyploidy—individual with more than two basic, complete sets of chromosomes in its somatic cells. Among polyploids, there are two main types: ◦ 

Autoploidy—an individual has more than two complete chromosome sets from a single genome. Also known as autopolyploidy. 

Alloploidy—an individual has two or more genomes contributed from different parental species in their ancestral lineage. Also known as allopolyploidy. Most naturally occurring polyploids are alloploids. Amphidiploids (also called amphiploids) are allotetraploids that contain two sets of genomes from each of two different parents. Amphidiploids are so called because they behave like diploids during meiosis.

5.0Autoploidy

Autoploids commonly occur as a result of duplication of the genome(s) of a single species, thus, the genomes possessed by an autoploid are identical. An autotetraploid (4x) has four sets of the same genome, e.g., four sets of the A genome (AAAA). The following are characteristics associated with autoploids:

• Greater ability to colonize new habitats than diploid ancestors—Due to gene buffering, autoploids often show a slower response to selection, but more adaptive potential.
• Dosage effect of gene expression—Additive effect of the alleles increases the number of phenotypes. There is a linear relationship between gene expression and number of gene copies. 
• Larger cells and nuclei compared to their respective diploid counterparts—Because of the greater cell size, autoploids tend to have greater vegetative growth and produce larger structures. This feature has been utilized by breeders to increase yield of crops harvested for their vegetative structures.
• Reduced fertility—Abnormalities in meiosis may interfere with chromosome pairing, resulting in unbalanced chromosomal distribution between daughter cells, and thus, nonviable gametes. Gametes possessing an extra chromosome or missing a chromosome (i.e., aneuploids) are usually nonviable. This feature is utilized by breeders to develop seedless crops such as seedless watermelon.

6.0Alloploidy 

Alloploids arise when the genomes of two or more unrelated species are combined in a single individual.

Several characteristics are associated with alloploidy.

• Sterile unless genomes are doubled and chromosomes pair correctly. Chromosome doubling also occurs spontaneously in nature, mainly through the fusion of unreduced gametes. Colchicine is a chemical that is commonly used to artificially double chromosomes. Colchicine is a toxic natural product that is found in the bulbs of autumn crocus (Colchicum autumnale). It is used as a medicine to treat the human disease gout, but it is also used to induce polyploidy in plants. Originally it was extracted from crocus bulbs, but is now also manufactured synthetically. When plant seeds or meristem tissues (apical tip, shoots, or suckers) are soaked in colchicine, it makes the cell walls permeable and prevents normal chromosome division. 
• Colchicine functions as a disruptor of mitosis by inhibiting microtubule formation, thus preventing the migration of divided chromosomes to the opposite poles and resulting in doubled numbers in a cell. Colchicine is used to make infertile hybrids or haploid plants fertile by restoring doubled chromosomes. The latter plants are known as doubled haploids. An example of the former is development of hexaploid and octaploid triticale.
• Colchicine is also used in the study of karyotypes, which refer to the chromosome constitution of the cell (specifically, the number and morphological appearance of chromosomes in the nucleus). In order to observe chromosomes under a light microscope, cells are treated with the chemical near the middle of mitosis during metaphase—a point in the cell cycle when chromosomes are most dense and therefore most visible. Colchicine treatment arrests these chromosomes by preventing the formation of the spindle microtubules.
• Broadened genetic base. Alloploids (also known as amphiploids) behave like diploids and result in new species.
• Increased allele diversity and heterozygosity—Increase in possible allele combinations can provide expanded opportunities for breeding. 
• Novel phenotypic variation—Genome interactions and changes in gene expression can occur in newly synthesized alloploids. Such gene changes can include transfer of sequences between genomes and gene conversion, loss, or silencing—topics that will be covered in more detail in Molecular Genetics and Biotechnology.

7.0Types of Polyploidy

There are two main types of polyploidy: autoploidy and alloploidy.

Autopolyploidy

Auto- means "self," and autopolyploidy refers to the situation where an individual has multiple sets of chromosomes derived from the same species. Autopolyploids arise through the duplication of the entire chromosome set within a single species.

Autopolyploidy can occur through various mechanisms, such as errors in mitosis or meiosis that lead to the doubling of the chromosome number which can be induced by the chemical or radioactive methods. 

Allopolyploidy

Allo- means "other" or "different," and allopolyploidy involves the combination of chromosome sets from different species. It results from hybridization between two different species, followed by chromosome doubling in the hybrid.

Allopolyploidy often leads to increased genetic diversity and can be an important factor in plant evolution.

Polyploidy in animals

Polyploidy, the condition of having more than two complete sets of chromosomes, is a relatively rare occurrence in animals compared to plants. While commonly found in amphibians, particularly frogs and salamanders, and occasionally in fish species, it remains infrequent in other vertebrates. Polyploidy is extremely rare in mammals and reptiles, with the vast majority of these organisms maintaining a diploid chromosome number.