Types Of Polyploidy

Polyploidy is the condition where an organism possesses more than two sets of chromosomes. It is part of the numerical alterations of chromosomes. There are different types of polyploidy in nature, which range from having two to several sets of chromosomes in organisms.

Types of Polyploidy

Polyploidy may be of two types, depending on the mode of multiplication. They are euploid and aneuploid. 

Euploids

When the chromosome number is an exact multiple of the haploid number, it is called an euploid. Euploidy is of different types- haploids and diploids. 

Haploids 

The Haploids or ‘n’ number of chromosomes in body cells may be normal or abnormal. In lower plants and animals, the haploid condition is normal as the individuals have only one set of chromosomes in their body cells, while the diploid condition is represented only in zygotes.

Haploidy is of interest, and it occurs in diploid individuals. Here, normally, the body cells have two sets of chromosomes, and reduction division takes place to produce haploid gametes. The haploid, however, has no morphological expressions, ie, they cannot directly develop into new individuals. They fused with another gamete to form a diploid individual. Very rarely, haploid individuals are produced during parthenogenesis (parthenos means origin, and genesis means birth. Here, the eggs of an organism develop into individuals without fertilization of the eggs by sperm. 

In some insects, individuals are produced naturally due to parthenogenesis. In plants, parthenogenesis development of eggs results in haploid individuals. Such haploids have been seen in cotton, tomato, Nicotiana, potato, etc. 

Diploids

Diploids are organisms having two sets of chromosomes as seen in humans and other animals. The chromosome number here is 2x.

Polyploids

There are mainly three types of polyploids. 

• Autopolyploid
• Allopolyploid
• Auto-allopolyploid

The following chart is the derivation of these from three diploids with genomes- AA, B₁B₁, and B₂B₂. 

Autopolyploids

An autopolyploid is derived by chromosome multiplication from a single diploid, so that chromosomes come from the same source. For instance, if the diploid genome is AA, an auto triploid would be AAA, an autotetraploid would be AAAA, an auto octoploid would be AAAAAAAA, etc. An autotetraploid would result either by doubling the somatic chromosomes or by the union of two unreduced gametes. Autotriploid can arise by a cross between a tetraploid and a diploid. 

The presence of more than two sets of chromosomes in autopolyploids results in complicated meiosis. During meiosis, homologous chromosomes pair with each other, but when there are more than two homologous chromosomes, such as three, four, etc, pairing will become more complicated. Instead of bivalents, multivalents will be formed. Such pairing with chromosomes of the same parental origin is called auto synthesis. 

The formation of multivalents interferes with the normal separation of chromosomes during meiotic anaphase. In triploids, trivalent will be formed generally, sometimes two may form a bivalent, and one may remain unpaired, forming a univalent. Whether bivalent or trivalent separation is irregular, this results in the formation of gametes with an unbalanced number of chromosomes, some with more than 2n  and some with no chromosomes. 

Consequently, the gametes will be abnormal, and this causes a certain amount of sterility in autopolyploids. In autotetraploids, the gametes will be sterile, and the sterility is due to irregular meiotic circulation. 

Autopolyploids are common in plants but rare in animals. As a general rule, autopolyploids are unsuccessful. Unless they have some form of vegetative propagation. Autotriploids are common in bananas, apples, roses, etc. Autotetraploids are also commonly in occurrence in chillis, Datura, tomatoes, marigolds, grapes, etc. 

Allopolyploids

An allopolyploid is derived from an interspecific diploid hybrid, so that the chromosomes come from different sources. Allopolyploids are more common in nature than autopolyploids, and most natural polyploids are of this kind. 

If AA represents the chromosome complement of one diploid species and the BB chromosome complement of another diploid species, the hybrid produced (F1) will be diploid with the chromosome complement AB. This individual is completely sterile because there is no pairing, and all the chromosomes spread randomly at metaphase I and distribute irregularly at anaphase I. No gametes would survive.

If the chromosomes of this F1 hybrid AB are doubled, the resulting individual is called an allopolyploid (allotetraploid). Here, the AB forms the basic number for the allopolyploids. AAB will be one kind of allotriploid, ABB will be another kind of allopolyploid. ABBB is an allotetraploid, and allotetraploids are AAABB and AABBB. 

The sterility of diploid interspecies hybrids can be overcome by artificially converting them to tetraploids by doubling the chromosomes. The tetraploid will be successful sexually in the fertile allopolyploids. Pairing is strictly autosyndetic. The sterility in the interspecies hybrid is caused by the non-homologous pairing between chromosomes. Because of the lack of homology, allo synthesis will not take place. 

The allotetraploid, which is obtained from the interspecific hybrid, is fertile. The fertility is due to the formation of bivalents during meiosis, and pairing is auto syndesis type. Auto-syndesis in an allotetraploid makes it fertile. The fertile allotetraploid is also referred to as amphidiploid, as it behaves like a normal diploid during meiosis and forms a bivalent. 

Allotetraploids are rarely sterile due to the formation of a tetravalence during meiosis. Allosyndesis in a diploid species from which the hybrid is formed are closely related. 

Tetraploids are usually aggressive invaders of new territories and are adapted to severe conditions such as drought or cold. A polyploid plant often faces ecological hazards more closely.  They therefore have a wider range of geographical distribution. They have more morphological, genetic, and physiological advancement than normal diploid plants. 

Aneuploids

When the nucleolus contains some numbers more than the exact multiple of the haploid number, it is called aneuploid. Aneuploids are of two types- hyperploid and hypoploid. 

Hyperploidy

In hyperploidy, the chromosome number is greater than 2n. The addition of a third chromosome to one of the bivalents is called trisomy-2n+1, and the addition of two chromosomes to the bivalent is called tetrasomy-2n+2. In a trisomic, any one chromosome is represented thrice, and in a tetrasomic, the extra chromosomes will be of different types, and it is represented as 2n+1+1. 

Trisomy

Trisomics are of three types based on the addition of chromosomes. 

• If the extra number is identical to the other two homologous chromosomes, it is primary trisomy.
• In a secondary trisomy, the third member is an isochromosome (an isochromosome is one in which both arms are identical). It is thought to arise when a centromere divides in the wrong plane. 
• In tertiary trisomy, the extra chromosome is the product of translocation. 

In plants, trisomy has been studied extensively in Datura, maize, tomato, and Nicotiana. In animals, trisomy has been reported in Drosophila and others, including human beings. In man, the trisomic condition leads to many abnormalities both in physical features as well as mental faculties. 

Down syndrome (mongoloidy) was first described in 1866 by Langdon Down. The affected individuals are short in stature, have slanting eyes, broad skulls, and stubby hands. They have low mental faculties and exhibit characteristic abnormalities of plan prints and certain other malformations, especially of the heart, ear, hands, and feet. They rarely attain sexual maturity and fertility. Cytological investigations have revealed that in Down syndrome, chromosome number 21 has an extra copy. 

Tetrasomy

In tetrasomy, any one chromosome is represented four times. Therefore, a tetrasomy is 2n+2. The extra chromosomes are of the same type. 

Aneuploid	Formula	Charcateristic
Nullisomic	2n-2	Euploid with one pair of chromosome less
Monosomic	2n-1	Euploid with one chromosome less
Trisomic	2n+1	Euploid with one extra chromosome
Tetrasomic	2n+2	Euploid with one pair of chromosomes less

Hypoploidy

In hypoploids, the chromosome number is below the diploid number, ie, 2n-1 or fewer chromosomes. It is due to the loss of one or more chromosomes. If a single chromosome is lost from a diploid complement, it is said to be monosomy, and the individuals are called monosomics. 

In nullisomy, an entire pair of homologous chromosomes will be missing, 2n-2, and individuals are called nullisomics. A nullisomic diploid often does not survive. However, a nullisomic polyploid may survive but exhibit reduced vigour and fertility. 

• In monosomic diploid individuals, one of the pairs of homologous chromosomes would undergo a loss of one chromosome and become univalent, while the others are trivalent.
• The diploids cannot withstand such a loss, while polyploids can withstand it because the polyploids have more than two homologous chromosomes for every pair.
• There are instances where diploids with monosomics do not survive (tomato).
• Double and triple monosomic have two or three homologous chromosomes lost from different bivalents. 

Hypoploidy has been noticed in human beings also. A classical example is Turner’s syndrome. Here, there is a loss of one of the X chromosomes, and the female would be XO instead of XX. The female with Turner’s syndrome will have no ovaries and will be sterile. They will also have abnormal faces and webbed necks. 

References

• Agarwal, P. V. |. V. (2004). Cell biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
• https://bio.libretexts.org/Bookshelves/Agriculture_and_Horticulture/Crop_Genetics_(Suza_and_Lamkey)/01%3A_Chapters/1.10%3A_Ploidy-_Polyploidy_Aneuploidy_and_Haploidy

Additional Reading

• Polyploidy Breeding in Plants
• Types of Chromosomal Aberrations