2. INTRODUCTION
In many species, one to many extra chromosomes in addition to the
normal somatic complement are found; these extra chromosome are
called accessory chromosome or supernumerary chromosome or B –
chromosome .
The B – chromosome first reported in maize (Longley,1927 and
Randolph,1928).
Now B-chromosome have been reported in 42 families of angiosperms
including 163 genera and 475 spp.
3. B – chromosome are generally smaller in size than the chromosome of
normal somatic complement but in some species they may be larger
(e.g., in sciara).
One of the most important features of these chromosomes is that their
numbers may considerably among individuals of the same species.
In most case, B chromosomes are largely heterochromatic, while some
species(e.g., maize) they are partially heterochromatic, and in some still
others(e.g., Tradescantia) are entirely euchromatic.
4. CLASSIFICATION
B chromosomes have been classified in two different ways, viz,
1. On the basis of their stability
2. On the basis of their size
On the basis stability , they are two type, viz , Stable and Unstable
1. Stable
The B chromosome of this group are mitotically stable and all
cells of an individual have the same number of B chromosome.
2. Unstable
The B chromosome of this group are mitotically unstable
and given rise to cells with different numbers of B chromosomes with in
the same individual.
5. On the basis of size , B chromosome have been classified in four groups
, viz., Standard type, Small type , Very small type and Large type.
1. Standard type
It is a predominant type . Its length is usually one fourth of
the normal chromosome . The B chrom0somes posses median centromere
and are uniform thickness.
2. Small type
This group consists of all accessory chromosomes with
smaller size but not smaller than the half of the Standard
B chromosome .
6. 3. Very small type
The appear as a small dot like structures and are smaller
than the standard B chromosomes.
4. Large type
In some plants, chromosomes of double size are found which
are known as large accessory chromosomes.
7. BEHAVIOUR AT MITOSIS AND MEIOSIS
The behaviour of B chromosomes during mitosis differs from species to
species. For example, in rye chromosome occupy cental position at
metaphase(Muntzing,1948) while in maize, they occupy peripheral
position at metaphase ( Darlington and Upcott , 1941 )
the meiotic behavior of B chromosomes is studied during pachytene
stage. They have been observed in four different ways as given bellow :
1. they do not pair with A chromosomes .
2. Lower degree of pairing is observed among B
chromosomes. Whenever they pair long B will pair with long B and short
B with short B.
8. 3. When single B chromosome is present , it remain univalent
during pachytene . When two B chromosomes are present , they pair
at pachytene but at metaphase remain univalent . When more numbers
of B chromosomes are present , they result in clumping of chromosomes
at eraly stage of meiosis , but these chromosome act as univalent at
diakiness and metaphase I .
4. Sometimes they show preferential non-disjunction during
meiosis and post meiotic divisions as in case of Lilium and Trillium.
9. EFFECTS
B chromosomes are not essential for the normal growth and
reproduction of plants and thus have no beneficial effect.
However , effects of B chromosomes differ from species to species, In
maize , B chromosomes lead to
1. Reduction in vigour and fertility
2. Production of defective seeds in partially
developed endosperm.
3. Increase in percentage of abortive pollen.
10. In Festuca, 1 or 2 B chromosomes have beneficial effect on vegetative
growth.
B chromosomes were found to have deleterious effect in rye and
positive effect on wheat .
