Accessory Chromosomes

1. ACCESSORY CHROMOSOMES
Guided by:- Submitted by:-
Dr. K.K.Panigrahi Yashobanta Barik
Asst. prof. ( PBG ) Adm no- 24C/15

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 .

11. THANK YOU