B - CHROMOSOME

The brief note on B-Chromosomes with characteristics and research case studies.

This particular studies has more scope for further experimental evidences.

1. B-CHROMOSOMES
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3. B CHROMOSOMES
 All living organisms consists of normal A-Chromosomal set specific to particular species,
also called as Primary chromosomes.
 Supernumerary chromosomes are the additional or extra chromosomal set present in a
cell, which are dissimilar to normal A-Chromosomal set in the species.
 They are also called as Accessory Chromosomes and lack homologous chromosome part.
 In wild populations, around 100 animal species, 600 plant species especially fungi
contain supernumerary / B-chromosomes.
 Number of B-chromosomes varies.
Example: In maize, 20-30 chromosomes are identified to be responsible for fertility of
pollen grains. In few case, cause increased unpaired chromosomes and infertility.
 B Chromosomes are majorly heterochromatic and largely noncoding.
Example: Supernumerary chromosomes in fungi do not carry genes that are necessary for
basic fungal growth, but may have some functional significance.
 The B Chromosomes are predicted to be evolved from the heterochromatic segments of
normal chromosomes and are non-dividing chromosomes.
 B-Chromosomes are not essential for the life of a species.
Exception: Partly euchromatin region is detected in maize, accountable for pollen fertility.
 In general, supernumeraries would persist in a species unless there was some positive
adaptive advantage (few cases have been identified).
 90% of these B-Chromosomes tend to lack centromere. They are to move towards either of
centrioles/poles during cell division.
Exception: In grasshopper (Myrmeleotettix maculatus), B Chromosomes are found in two
structural types: metacentric and sub-metacentric.
 The occurrence of these chromosomes varies according to the environmental conditions,
increases in warm and dry environments, but absent/rare in cooler localities.
Example: In plants, there is a tendency for B Chromosomes to be present in the germ
line, but are lost from other tissues such as root tips and leaves. There is evidence of
deleterious effects of supernumeraries on pollen fertility and favorable effects or
associations with particular habitats are also known in a number of species.
 The B Chromosomes suppress synapsis (pairing of homologous chromosomes) which
reduces multiple pairing between homologous chromosomes in allopolyploids.
 Thus, increases asymmetry chiasma distribution and has both positive and negative
influence on crossing over and recombination frequencies, thus increasing variation.
 B-Chromosomes might be playing a positive role on normal A-chromosomes in some
other aspects. Vast & detailed research has to be initiated to understand more about it.