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Lecture 22, part 4 of 4
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
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
|4. (4A, 4B, 4D)||Nullisomics of all three have narrow leaves
and slender culms. Mature plants are dwarfed and male
|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.|
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.
Gene is not located on the nullisomic chromosome (20 out of 21 crosses)
Gene is located on the nullisomic chromosome (1 out of 21 crosses)
DD x dd
1 : 0
All progeny exhibit the dominant phenotype.
00 x dd
0 : 1
There is a 0:1 ratio of dominant to recessive phenotype.
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Lecture 22, part 4 of 4