Aneuploid: Types and Applications | Methods | Plant Breeding | Botany

In this article we will discuss about:- 1. Meaning and Types of Aneuploid 2. Applications of Aneuploids in Crop Improvement 3. Limitations.

Meaning and Types of Aneuploid:

The change in chromosome number which involves one or few chromosomes of the genome is called aneuploidy and such individuals are known as aneuploids. In other words, an individual with other than exact multiple of the basic chromosome number is called aneuploid.

Aneuploids are of three types, viz.:

(1) Monosomics,

(2) Nullisomics, and

(3) Polysomics.

These are described below:

(1) Monosomics:

An individual lacking one chromosome from a diploid set (2n – 1) is called monosomic and such condition is known as monosomy. In other words, monosomics contain the normal diploid chromosome set except loss of one chromosome from one pair. Double monsomics lack one chromosome each from two different pairs (2n – 1 – 1).

Monosomics have been reported in Drosophila, Nicotiana, Triticum, cotton and several other crops. Monosomics are viable in polyploid species where loss of a chromosome is balanced by homologous or partially homologous chromosomes from other genomes. In true diploid species, monosomics are generally inviable.

Monosomics may originate in three main ways, viz.:

(i) From diploids,

(ii) From nullisomics, and

(iii) From trisomies

As described below:

(i) From Diploids:

Monosomics may originate spontaneously from diploids. Sometimes non­disjunction during meiosis gives rise to n – 1 gamete. If this gamete is fertilized by a normal (n) gamete, a monosomic zygote (2n – 1) is produced.

(ii) From Nullisomics:

Nullisomics produce n – 1 gametes.

Union of such gamete with normal gamete gives rise to monosomics as shown below:

(iii) From Trisomies:

Trisomics (2n + 1) also give rise to monosomics. Sometimes, non­disjunction of three homologous chromosomes in a trisomic during meiosis gives rise to n – 1 gametes. Union of such gametes wit’- normal one results in the development of monosomic zygote.

Interspecific hybridization and asynaptic strain also lead to the development of monosomics by producing n – 1 type of gametes. Union of n – 1 gamete with normal one gives rise to monosomic individual. Monosomic individuals produce two types of gametes viz., n – 1 and n. In most of the cases, it has been found that n – 1 gametes are more frequent than n type of gametes. The single chromosome remains as laggard and ultimately is lost.

(2) Nullisomics:

An individual lacking one pair of chromosomes from a diploid set (2n – 2) is called nullisomic and such situation is referred to as nullisomy. In other words, nullisomic plants are with one pair of chromosome less than the normal.

Nullisomics are inviable in true diploid species. Among polyploids, hexaploids can tolerate loss of one pair of chromosome more than tetraploids, because they have two other pairs of similar chromosomes in the other genome. Monosomics and nullisomics together are known as hypoploids, which refer to loss of one or two chromosomes from the diploid complements.

(3) Polysomics:

An individual having either single or one pair of extra chromosome in the diploid complement is known as polysomic and such condition is referred to as polysomy.

Polysomics are also known as hyperploids, which refer to addition of one or two chromosomes to a single or two different pairs.

Polysomics are of two types, viz.:

(i) Trisomies, and

(ii) Tetrasomics.

Trisomies are more frequent than tetrasomics.

(i) Trisomics:

Addition of one chromosome to one pair in a diploid set is known as trisomy and such individuals are known as trisomics (2n + 1). Trisomics may be of two types, viz. simple trisomics and double trisomics. When increase in chromosome number is in one pair only (2n + 1), it is known as simple trisomics.

When there is addition of one chromosome in two different pairs, it is called double trisomics (2n + 1 + 1). Trisomics were first reported by Blakeslee in 1910 in Datura. Now they have been reported in tomato, Nicotiana, Secale cereale, Pisum, Oenothera and Drosophila. If a plant has n = 12 chromosomes, it can form 12 different trisomics.

Depending on the nature of extra chromosome, simple trisomics are of three types, viz.:

(a) Primary trisomics,

(b) Secondary trisomics, and

(c) Tertiary trisomics.

Trisomics in which the additional chromosome is normal one are called primary trisomics. Trisomics having additional chromosome as isochromosome are known as secondary trisomics.

Isochromosomes originate by vertical division of centromere. Both the arms of such chromosomes are homogeneous. When additional chromosome in a trisomic is translocated one, it is known as tertiary trisomics.

Origin:

Trisomics may arise in two different ways. In diploid species, sometimes non­disjunction during meiosis leads to the formation of n + 1 and n — 1 type of gametes. Union of n + 1 gamete with normal (n) gamete leads to the development of trisomic individual. A cross between tetrasomic (2n + 2) and normal diploid can also give rise to trisomics.

(ii) Tetrasomics:

Addition of two chromosomes to one pair or two different pairs is known as tetrasomy and such individuals are known as tetrasomics. When there is addition of two chromosomes to one pair (2n + 2), it is called simple tetrasomic and when two chromosomes are added each to two different pairs, it is called double tetrasomic. Trisomies and tetrasomics can be tolerated even by diploid species, whereas monosomies and nullisomics are inviable.

Applications of Aneuploids in Crop Improvement:

Aneuploids are useful in crop improvement in various ways.

Some of the uses of aneuploids in plant breeding are briefly presented below:

1. Locating Genes:

Aneuploids are useful tools for locating genes on a specific chromosome. Monosomics and nullisomics are used for this purpose. Monosomic analysis has been used in wheat, cotton, tobacco, oat and other crops for locating genes on specific chromosome. In case of nullisomics, loss of a pair of chromosome will affect expression of some characters.

The altered characters are considered to be associated with the missing chromosomes. Thus genes on various chromosomes can be located by developing nullisomic series. However, monosomics are better for such analysis than nullisomics, because nullisomics are less vigorous and less fertile than monosomics.

2. Interspecific Gene Transfer:

Monosomics are also used in transferring chromosomes with desirable genes from one species to another.

3. Aneuploids are used for developing alien addition and alien substitution lines in various crops.

4. Primary trisomics are useful in identification of chromosomes involved in translocations.

Limitations of Aneuploids:

1. In diploid species, monosomics are inviable. They can survive only in polyploid species i.e. tetraploid or hexaploid.

2. Nullisomics are inviable in diploid species. They can survive only in highly polyploid species i.e., hexaploids.

3. Trisomics can survive in diploid species.

4. Maintenance of nullisomics and monosomics is difficult.