Variations in Chromosome Number

 

 Variations in Chromosome Number

Somatic cells of higher plants and animals usually have chromosomes in pairs (2n), that is, two of each kind of chromosome are present in each cell. Mature germ cells, having undergone reduction division, normally have one member of each pair (n).

However, sometimes irregularities occur in normal diploid (2n) chromosome number.  Variation in chromosome number produces two types of individuals or cells : 1. Those whose somatic complements are exact multiples of the basic haploid number characteristic of the species and 2. Those whose somatic complements are irregular multiples of the basic number.

The individuals or cells having chromsomes that are multiples of basic number are called as euploids. They may be haploid, diploid, triploid, tetraploid, etc.

The individuals or cells having irregular chromosome numbers, in which the addition or loss of one or more chromosomes to the complete diploid chromosome complement of an organism are called as aneuploids.

 

Euploidy:

(Gr Eu: Even or true; ploid- unit) Individuals or cells having chromosomes that are multiples of basic number are called as euploids and the phenomenon is called as Euploidy.

The number of chromosomes, in a basic set is called monoploid (x) number. Individuals with two sets of chromosomes are known as diploids (@x), and with more than two sets are called polyploids. The polyploidy types are 3x(triploid), 4x(tetraploid), 5x(Pentaploid), 6x(hexaploid) and so on.

 

The haploid(n) refers to strictly to the number of chromosomes in gametes. In most animals and plants, the haploid number(n) and the monoploid number (x) are the same. However, in case of polyploids, the usage x is done instead of n, e.g., wheat has 42 chromosomes with 6x=42 and x =7. However, gametes contain 21 chromosomes, hence 2x= 42, n=21.

Monoploidy:

The monoploid organsims have one set of chromosomes or one genome(n) in the nuclei of their body cells. The monoploid individuals can arise spontaneously in natural population as rare abnormalities, but in animal kingdom many forms like ants, bees and wasps, the males are normally monoploids (being derived from unfertilized eggs).

Monoploids are characteristically sterile because in their germ cells meiosis is not found normally and chromomosomes lack pairing.

Monoploids plants are often weak and sterile.

Polyploidy:

An organism with more than two sets of chromosomes are called polyploidy. Ploidy levels higher than tetraploid are not commonly found in natural population, but some important crops and ornamental flowering plants are polyploids, e.g., Wheat (hexaploid, 6x), strawberries (octaploid, 8). About one third of all grasss are polyploids.

There are three different types of polyploids, namely i. autopolyploids, ii. Allopolyploids Autopolyploids:

Autopolyploids are those polyploids, which have the same basic set of chromosome multiplied. For instance, if a diploid species has two similar sets of chromosomes (AA), an autotriploid will have (AAA) and an autotetraploid will have (AAAA).

Autopolyploids arise in following ways:

•          By the union of diploid gametes produced in the absence of meiosis or due to abnormal meiosis (autotetraploid)

•          By somatic doubling of the chromosomes in a zygote due to abnormal mitosis

•          By the union of a haploid gamete with the diploid gamete (autotriploid)

Autotriploids are known in Watermelons, sugarbeet, tomato, grapes and banana. They are generally sterile and cannot produce seeds. Therefore, they have great commercial value in producing seedless varieties of fruits.

Autotetraploids(4n) are the most common forms of polyploids. They are usually larger, more succulent and have bigger pollen grains, fruits, etc. Autotetraploids are seen in rye (Secale cereal), corn (Zea mays), red clover (Trifolim pretense).

Cytology of Autopolyploids:

In an autopolyploid, there will be more than two sets of homologous chromosomes. This leads to formation of multivalents instead of bivalents at metaphase of Meiosis I.  An important difference exists between autotriploids and autotetraploids. In autotetraploids normal disjunction is possible giving rise to diploid gametes, while in autotriploids it is not possible.

In an autotriploid, there are three sets of homologous chromosomes. It these sets are normally paired, trivalents would result. Tirvalents cannot disjoin normally and will either disjoin 2:1 chromosomes to two poles or will disjoin 1:1 leaving one chromosome as laggard.

Number of chromosomes in gametes of triploid organisms, therefore, will vary from n to 2n. Most of these gametes are unbalanced leading to high degree of sterility.

In autotetraploids, since there are four sets of chromosomes, quadrivalents are formed,which disjoin in a normal 2:2 manner giving diploid gametes. Rarely, however, a quadrivalent may disjoin 3:1 or may leave a chromosome as a laggard at anaphase I. Therefore, autotetraploids also have a certain degree of sterility, although it will not be as high as autotriploids.

 

Allopolyploidy:

When the polyploidy results due to the doubling of chromosome number in a F₁ hybrid which is derived from two distinct species, then it is called allopolyploidy.

For example, let ‘A’ represent a set of chromosomes in species X and ‘B’ represent another genome in a species Y. The F₁ will then have one A genome and another B genome. The doubling of chromosomes in this F₁ hybrid(AB) will give rise to tetraploid with two A and two B genome. Such a polyploidy is called an allopolyploid or amphidiploid.

 

 

                             Species X            X         Species Y

                                  (AA)                                 (BB)

                                                           

                                                          AB – Diploid F₁ hybrid (sterile)

                                                                        Colchicine

                                                                     AABB – Amphidiploid tetraploid

If the genome are sufficiently dissimilar structurally, no synapsis will occur in the diploid hybrid and high sterility will ensue as the result of the random segregation of unpaired chromosomes. Doubling of the chromosome number to give the tetraploid AABB will, however, provide for regular synapsis and segregation. Genome A will pair with genome A, and genome B will pair with genome B.

Raphanobrassica is a classical example of allopolyploidy. In 1927, G.D Karpechenko, a Russian scientist, reported a cross between Raphanus sativus (2n=18) and Brassica oleraceae (2n=18) to produce F₁ hybrid which was complete sterile, because no pairing occurred in F₁ hybrid as there is no homology between the genomes of Raphanus and Brassica.

However, a number of unreduced (diploid) gametes were formed, and several tetraploid individuals were recovered. On cytological examination these fertile plants were found to have 2n=36 chromosomes, which showed normal pairing into 18 bivalents.

The polyploids occur frequently n plants but rarely in animals, because plants can propagate vegetatively through grafting, cuttings and by rooting. Thus, sterile triploids, pentaploids, etc., are maintained from generation to generation.

Animals usually reproduce sexually through gamete formation. The gamete formation in polyploids is irregular due to abnormal meiosis or failure of chromosome pairing.

The polyploidy is invariably related with gigantism. These plants have been found to contain large sized pollen grains, leaves, stomata, xylem vessels, etc. the polyploid plants show more vigorous vegetative growth.

Polyploidy reduces the fertility of plants in variable degrees.

Ascorbic acid content is higher in tetraploid cabbage and tomatoes than in corresponding diploids.

Polyploidy leads to evolution and is a means of origin of new species, especially in plants.

Aneuploidy:

It is a general, the two members of a pair of homologous chromosomes regularly segregate during meiosis in a normal diploid to give a haploid set of chromosomes in a gamete or a spore.  Whereas, in mitosis two cells of like chromosomal constitution are formed.

Exceptions occur, however, to give cells or organisms deficient or duplicated for a particular chromosome. Aneuploidy (aneu- uneven; ploidy- unit) is the addition or loos of one or more chromosomes to the complete diploid chromosome complement of an organism. Organism whose chromosome number is not an exact multiple of the basic number of the group is called as aneuploidy.

The aneuploidy may be of the following types:

Monosomics:

Diploid organisms which lack one chromosome of a homologous pair are called monosomics. Their genomic formula is 2n-1.

Since, monosomics lack one complete chromosome, they are genetically imbalanced. Therefore, they are either lethal or of reduced vitality. A monosomic individual produces gametes of two types n and n-1. The n-1 gametes do not survive in plants, but, in animals they cause genetic imbalance, which is manifested by high mortality or reduced fertility of resulted organisms.

Polyploids can tolerate the loss of a chromosome with greater ease than a diploid, so monosomics are much more common in polyploidy species. All the 24 monosomics in the tetraploid Nicotiana tobaccum and all 21 monosomics in the wheat – Triticum aestivum have been identified.

Monosomic codition for a particular chromosome may be associated with a characteristic morphology. Therefore, by looking at the morphology of monosomics and that of their progeny, genes can be located on specific chromosomes. For example, in wheat a newly arisen mutation can be located with little difficulty by crossing the mutant plant (as a male) to all of the 21 monosomics, and determing the patterns of segregation that deviate from the expected.

Double monosomics (2n-1-1) or triple monosomics (2n-1-1-1) could also be produced in polyploids like wheat. Double monosomics mean that the chromosome number is 2n-2, like in a nullisomic, but the missing chromosomes are non-homologous.

Nullisomics:

Organsims which lack a single pair of homologous chromosomes are called as Nullisomics. The genomic formula for nullisomics is 2n-2.

These may be occasionally in nature, but seldom survive long enough to be recognized and perpetuate. However, a nullisomic polyploidy (e.g., hexaploid wheat. 6x-2) may survive but exhibit reduced vigour and fertility.

E.R. Sears experimentally produced all the 21 possible nullisomics in wheat, Triticum aestivum. By association certain pheontypes with corresponding chromosome arrangements, nullisomics have been used effectively in locating several different genes in wheat.

  Trisomics:

Individuals having one chromosome extra to the diploid genome are called trisomics. Since, the extra chromosome may belong to any one of the different chromosomes of a haploid complement, the number of possible trisomics is an organism will be equal to its haploid chromosome number. For instance, haploid chromosome number in barley is n=7, consequently, seven trisomics are possible.

Trisomics were obtained for first time in Datura stramonium by A. F Blakelslee and Belling. Since, haploid chromosome number in the Datura is n=12, 12 different monosomics are possible. The morphological differences between the trisomics in Datura  include the shape, size, spine pattern of the capsule, indicating thereby that different genes for capsule types are located on 12 different chromosomes of Datura.

Production of Trisomics:

Trisomics may originate spontaneously due to production of n+1 type of gametes due to rare non-disjunction of a bivalent. However, trisomics are readily obtained by crossing a diploid as male with a triploid as a female or by selfing triploids. In either case, trisomics are obtained in large number and can be identified through phenotypic effects of individual chromsomes.

Cytologic of trisomics:

A trisomics has an extra chromosome which is homologous to one of the chromosomes of the complement. Therefore, it form a trivalent in Meiosis. This trivalent may take a variety of shapes such as an “frying pan’, configuration, or a bivalent and a univalent dough nut shape, etc.