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Lecture 22, part 4 of 4
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C. Nullisomy

Nullisomic: individual lacking one chromosome pair (2n-2)

eg. hexaploid wheat 2n=6x=40 nulli 1A.

Nullisomics are usually not found in natural populations. They can be obtained for example by selfing a monosomic to produce disomic, monosomic, and nullisomic progeny. The male gametes lacking a chromosome have a low survival rate or are less competitive so that the percentage of nullisomics from monosomic selfing is low. Only a small percentage of the progeny are monosomic 1 - 10%. In wheat, all 21 possible nullisomics are available in the nullisomic series obtained by Sears. The nullisomic lines differ in morphology for the genes on the missing chromosomes. Nullisomic plants in wheat cultivar Chinese Spring were distinct at seedling and mature plant with the exception of nulli-1B, 7A 7B and 7D. Table 6.58. Nullisomics are usually weak individuals that are difficult to maintain. Exceptions are Nulli-7B and 7D. Nullisomic analysis can be performed to assign dominant genes to specific chromosomes.
Cytogenetic Identification: Nullisomics- n-1 bivalents not always observed; nullisomics in some species show desynapsis and a prevalence of univalents eg. nulli-8 of oat showed 5 II and 30 I while nulli-9, 17 and 19 exhibited 20 II.

Table 6.58 Morphological Features of Nullisomics of Triticum aestivum cv. Chinese Spring
Homoeologous group Morphological features
1. (1A, 1B, 1D)  Three nullisomics of this group are reduced in plant height in varying degree. Spikes are a little less dense than normal, with slightly stiffer glumes; they are both female and male sterile.
2. (2A, 2B, 2D)
All three nullisomics are very dwarfish with greatly reduced tillers; all are male fertile but female sterile. The spikes have thin, papery glumes and are completely awnless.
3. (3A, 3B, 3D) Nullisomics are identified at the seedling stage by their narrow, short, stiff leaves; at maturity, nullisomics are dwarfed with narrow leaves and short spikes.
4. (4A, 4B, 4D) Nullisomics of all three have narrow leaves and slender culms. Mature plants are dwarfed and male sterile.
5. (5A, 5B, 5D) Nullisomics have narrow leaves and slender culms, are late in maturity, spikes are reduced in size and have small glumes and seeds; they are female fertile and male sterile.
6. (6A, 6B, 6D) Nullisomics have narrow leaves, slender culms, and narrow, spreading outer glumes; they are straggly in appearance, and all are female fertile.
7. (7A, 7B, 7D) Nullisomics differ very little from normal at the seedling and maturity stages, are distinguishable only by a slight reduction in vigor and height and by certain spike characters. The seed fertility of 7B and 7D are nearly normal, but is greatly reduced in 7A by pistilloidy.
Adapted from Sears, 1954.

Nulli-tetra complementation

Particular tetrasomics in combination with nullisomics cancel the morphological expression of the nullisomics.
The study of nullisomic-tetrasomic combinations allowed the seven homoeologous groups were established in wheat, each with three homoeolgous chromosomes. Each tetrasomic of the groups 1-7 A, B or D homoeologous chromosomes compensated to some degree for either of the two nullisomics. Sears synthesized the full series of 42 Nulli-Tetra (NT) within each of the 7 homoeologous groups.

N-T 1A-1B   N-T 2A-2B   N-T 3A-3B   N-T 4A-4B   N-T 5A-5B   N-T 6A-6B   N-T 7A-7B
N-T 1A-1D   N-T 2A-2D   N-T 3A-3D   N-T 4A-4D   N-T 5A-5D   N-T 6A-6D   N-T 7A-7D
N-T 1B-1A   N-T 2B-2A   N-T 3B-3A   N-T 4B-4A   N-T 5B-5A   N-T 6B-6A   N-T 7B-7A
N-T 1B-1D   N-T 2B-2D   N-T 3B-3D   N-T 4B-4D   N-T 5B-5D   N-T 6B-6D   N-T 7B-7D
N-T 1D-1A   N-T 2D-2A   N-T 3D-3A   N-T 4D-4A   N-T 5D-5A   N-T 6D-6A   N-T 7D-7A
N-T 1D-1B   N-T 2D-2B   N-T 3D-3B   N-T 4D-4B   N-T 5D-5B   N-T 6D-6B   N-T 7D-7B

N-T 1A-1B (20''+ 1'''') where wheat plant has 4 chromosome 1B, no 1A, the usual 1D and is otherwise diploid for all other chromosome groups.

Nullisomics can be used to assign dominant genes to a chromosome

The chromosomal location of a gene can be determined by crossing a series of nullisomic lines against a line that is homozygous recessive for that trait. In most cases, the progeny will all exhibit the dominant phenotype. The only exception is the one nullisomic line which is missing the chromosome that carries the dominant allele for the trait. In that case, there will be a 0:1 ratio of dominant: recessive progeny.
Gene is not located on the nullisomic chromosome (41 out of 42 crosses)
Gene is located on the nullisomic chromosome (1 out of 42 crosses)

DD  x dd
  D  d
  1 : 0

All progeny exhibit the dominant phenotype.

00  x dd
  D  d
  0 : 1

There is a 0:1 ratio of dominant to recessive phenotype.

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