Polyploidy

POLYPLOIDY

The organisms having more than two sets of chromosomes by the addition of another chromosome is called polyploidy. It may arise as a result of abnormal mitosis where chromosomes divide by cytoplasm fails to divide during cytokinesis.

Depending upon the number of sets of chromosomes present in polyploidy it is of various types:

The organisms with three sets of chromosomes (2n+n=3n) are called triploids.

The organisms with four sets of chromosomes (2n+2n=4n) are called tetraploids.

The organisms with five sets of chromosomes (2n+3n=5n) are called pentaploid and so on.

The polyploids are more common in plants but rare in animals. About half of 3 lakhs, 25 thousands species of flowering plants are known as polyploids. Approximately, 40% of all flowering plant species and 70-80% of grasses are polyploids. They include agriculturally important plants such as wheat, oats, cotton, potatoes, sugar, cane etc.

Although polyploidy is less common in animals, it is found in some invertebrates.

Types of polyploidy

There are three types of polyploidy, they are:

1) Autopolyploidy

2) Allopolyploidy

3) Auto-allopolyploidy

1) Autopolyploidy-It is a type of polyploidy in which an increase in the number of chromosomes within the same species is caused by abnormal mitosis. For example- if a diploid species has two similar sets of chromosomes (AA) an autotetraploid will have four such genomes (AAAA).

Autopolyploid species are more fertile, hybrid, vigour, produces large flower fruits and seeds and possess other superior characters. Some examples of auto triploids are watermelon, banana, cucumber e.t.c. Similarly, autotetraploid is corn, marigold e.t.c.

Autopolyploids with the odd number of genomes are sexually sterile because the odd chromosome cannot show the process of synapsis.

2) Allopolyploidy-It is a process of multiplication of different sets of chromosomes of F₁ hybrid which is derived by crossing between two homozygous parents. The common examples of allopolyploids are- Raphanobrassicatriticale (first man-made cereal).

Raphanobrassica-A classical example of allopolyploidy

He crossed radish (2n=18) with cabbage (2n=18) and produced F₁ hybrids which were sterile except one of the hybrid plants that produced a few seeds. When these seeds were planted, they grew into new plants which were fertile and viable. On psychological analysis, these fertile plants were found to have tetra polyploids (4n=36).

Raphanobrassicawas not economically important because it has got leaf like that of radish and root like that of cabbage.

NOTE:

Triticum aestivum -Bread wheat (6n=42)

Triticum boeoticum-Ancient wheat grass. (2n=14)

Triticum monococcum-Cultivated ancient wheat (2n=14)

Triticum discococcoides-(4n=18)

Triticum durum-(4n=28)

Triticum vulgare-(6n=42)

Triticum spelta-(6n=42)

Aegilops speltoides-Goat grass/ wild wheat (2n=14)

Aegilops squarrosa-(2n=14)

Origin of wheat-

A classical example of allopolyploidy-

The development of bread wheat (Triticum aestivum) is a good example of allopolyploidy. Before the birth of Christ primitive diploid wheat (Triticum boeoticum) (2n=14) was crossed with diploid goat grass (Aegilops speltoides)(2n=14), produced F₁ sterile hybrids. These F₁hybrids were made into wild allotetraploidTriticum discococcoides (4n=28) by treating with colchicine. TheseTriticum discococcoideslater changed into cultivated wheat (Triticum durum).

LaterTriticum durum(4n=28) was naturally crossed with another wild grassAegilops squarrosa(2n=14) and produced sterile triploids hybrid (3n=21). In the course of evolution, it was converted into allohexaploid wheat varietyTriticum spelta(6n=42). These all are cultivated modern wheat. In this way, a new modern cultivated wheat was developed by allopolyploidy.

3) Auto allopolyploidy-It is an intermediate polyploidy between autopolyploidy and allopolyploidy. It is also known as segmented polyploidy. In this polyploidy, there is a multiplication of different sets of the chromosome but any one set of the chromosome is more than two by doubling (AAAABB) . Eg- It is found in Helianthus tuberosum.

Artificial induction of polyploidy-

A number of chemicals like colchicine are used to induce polyploidy as it inhibits the formation of spindle fibres during mitosis and causes the doubling of chromosomes.

Significance of polyploidy

1) Polyploidy is a basis of heredity and also increases the immunity of organism.

2) Allopolyploids have been used for obtaining fertile hybrids.

3) It also shows the interrelationship among the different groups of organism,

4) It is a source of variation, so it helps in the evolution of life. Eg- Genetically origin of wheat.

5) Modern varieties of plants like paddy, wheat, maize, cotton e.t.c are developed by this technique.

6) Polyploid plants have food adaptability, resistivity and yielding capacity e.t.c. Example wheat, paddy, corn etc.

7) Autopolyploids produce large sized seedless fruits in banana, grapes, watermelon, tomato e.t.c.

Effects of polyploidy

1) The size of the cell increases.

2) The water content is high in the cell due to the presence of large sized vacuole.

3) The growth rate is low.

4) The plant parts are gigantic,

5) The formation of ascorbic acid (vitamin c) and vitamin A in large amount.

Disorders due to mutation in sex chromosomes

Many disorders arise because of mutation in the genes present on the sex chromosomes. These traits are known as the sex-linked disorders and the transmission is called sex-linked inheritance. The recessive gene on the X chromosome affects the males more than the females. Some the sex-linked disorders are described below:

1) Haemophilia- It is also known as bleeder's disease reported in 1803. The affected person is unable to stop bleeding in case of even minor cuts and may lead to death. This is because of the absence of clotting factor VIII.

2) Red-green colour or Daltonism- It is a disease in which the affected person is unable to distinguish between red and green colour, It is caused by X-linked recessive gene.

3)Muscular dystrophy- It is a disease characterised by the deterioration of muscles at an early age. Due to mutation, the body does not produce a protein known as dystrophin which is responsible for releasing calcium from the storage areas of muscle cells during muscular contraction. The affected child becomes invalid at the age of 10 and dies at the age of 20.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.