The below mentioned article provides a note on nullisomy in chromomes.
The cell, tissue or organism in which both the homologues of a chromosome are missing from the somatic chromosome complement is called nullisomic (2n-2), and the condition is called nullisomy. If two pairs of homologous chromosomes are absent, the cell, tissue or organism is known as double nullisomic (2n-2-2). Normally, nullisomics do not survive in diploids; but they survive in polyploid species.
Origin:
Nullisomics are obtained in the selfed progeny of monosomics where “n-1” type male and female gametes fuse to form “2n-2” zygote. Monosomic plants produce two types of gametes, (n) and (n-1). Thus theoretically, their selfed progeny should be: 1 (2n) : 2 (2n-1) : 1 (2n-2), the frequency of nullisomics being 25%. But this is not observed practically due to the following reasons.
1. Male gametes lacking one chromosome (n-1) have low vitality.
2. The n-1 type of pollen has lower competitive ability than normal (n) pollen for fertilization.
3. Univalents of monosomics are lost during meiosis resulting in the production of a higher frequency of (n-1) gametes which are viable on female side. Such egg cells (n-1) produce monosomic offsprings after fertilization with a normal pollen and thus monosomics become more frequent than expected.
In 1954, Sears concluded that (n-1) pollen grains make up only 4% of the functional pollen from monosomics in wheat plants. The frequency of 20-chromosome male gametes which could compete with 21-chromosome male gametes during pollen tube growth ranged from 0 to 10% with the average of 4%.
Monosomic-3B produces upto 10% nullisomics, while several others produce about 1% nullisomics. Although the data may vary for individual chromosomes, the average frequency of nullisomics in the selfed progeny of monosomics is approximately 3% (Table 16.2).
Phenotype:
Nullisomics are weaker as compared to normal disomic plants. They show reduced size, vigour and fertility. All the 21 possible nullisomics in wheat were developed by Sears. There are morphological differences among nullisomics for the different chromosomes of a species.
Uses of Nullisomics:
1. Nullisomics can be used to identify the groups of homoeologous chromosomes. In allopolyploid species like wheat {In ~6x = 42) the effects of nullisomy for particular chromosome may be compensated by tetrasomy for a specific chromosome of the complement.
For example, effects of nullisomy for chromosome 20 (= 2D) in the variety Chinese Spring were compensated by the tetrasomy for chromosome 2 (= 2B) or chromosome 13 (= 2A), while effects of nullisomy for chromosome 17 (= ID) was compensated by chromosome 1 = IB) or 14 (= 1A).
By making all possible combinations of nullisomics and tetrasomics, and selecting those where compensation occurred, Sears in 1954 and Sears and Okamoto in 1957 could classify all the seven groups of 3 homoeologous chromosomes (A, B and D), viz., (i) 1A to 7A, (ii) IB to 7B and (iii) ID to 7D.
2. Nullisomics are used to assign genes to particular chromosomes. The line having a dominant mutant gene to be assigned is crossed to the series of nullisomics for the different chromosomes. The F1 plants will be monosomic, the monosome being contributed by the mutant line. The F1 plants are selfed and the segregation for the concerned gene is studied in F2.
If the concerned gene is located in the chromosome for which the nullisomic used in the given cross is disomic (Fig. 16.8) mutant and nonmutant phenotypes will appear in all the types of F2 plants, viz., disomic, monosomic and nullisomic plants. In case of nullisomics, the frequency of mutant and nonmutant phenotypes will be 3 : 1.
On the other hand, if the mutant gene is located in the chromosome for which the parent is nullisomic (Fig. 16.9), all the nullisomics will show the nonmutant phenotype in the F2, while all the other types of plants, viz., disomics and monosomics will exhibit the mutant phenotype.
3. Production of substitution lines:
The production of substitution lines using nullisomics requires a good fertility and reasonable vigour in the nullisomic for the chromosome to be substituted. In wheat, nullisomic-7B and nullisomic-7D have reasonable fertility, while nullisomics for the other chromosomes are very low in vigour and fertility. The following is a generalised procedure for the production of substitution lines.
(i) The nullisomic line is crossed to the donor variety. The F, progeny will be monosomic, and the monosomic will be contributed by the donor variety.
(ii) The F1 (monosomic) is back crossed to the nullisomic line (used as female). Monosomic plants are selected from the back cross progeny and are back crossed to the nullisomic lines; this is repeated till 5-6 back crosses have been made. In the 6th backcross generation, the genotype of the recurrent parent would be almost completely recovered.
(iii) In the progeny of the 6th backcross, the monosomic plants are identified and selfed, and disomic plants are selected from the progeny; these plants will possess the substituted chromosome from the donor variety in the background of the nullisomic variety.