Supernumerary B Chromosome

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Supernumerary B-Chromosome

B-Chromosome

B Chromosome are a particular kind of supernumerary / accessory chromosomes, that may or may not be found in an organism as extra chromosome over and above the standard diploid or polyploid chromosome complement. The standard complement consist of chromosomes described as A- chromosomes, including sex chromosomes. First discovered in hemipteran insects Metapodius by Wilson in 1905. Found in the wild populations of many plant, animal and fungi species.

Metapodius confraterna Uhler,

Features

Not essential for the life of a species.They are dispensable; are not found in all individuals of a species and may not be found in all cells of an individual organism A population may consist of individuals with 0, 1, 2, 3 (etc) supernumeraries. They usually have a centromere and possibly coding DNA, but they usually produce no functional protein.

Does not obey the Mendelian laws of inheritance. They are not essential to normal cellular function, and do not follow normal patterns of replication and segregation during cell division. They are not homologous with any of the basic A-chromosomes they are not homologous with any of the basic A chromosomes

Their inheritance is non-Mendelian some times due to non-disjunction during pollen mitosis, thus leading to the production of plants with an increased number of B Chromosmes in the progeny. They do not form synapse with the normal chromosomes They form synapse with themselves.

Genetically inert but may carry functional genetic material.

Most B chromosomes are mainly or entirely heterochromatic, (and so would be largely noncoding) but some, such as the B chromosomes of maize, contain sizeable euchromatic segments.

Smaller than A-Chromosome and have their unique pattern of heterochromatin distribution.

When present in higher no., they suppress vigour and fertility

There is evidence of deleterious effects of supernumeraries on pollen fertility, and favourable effects or associations with particular habitats are also known in a number of species. In maize, their number per cell can vary from 0 to 30 and they adversely affect, development and fertility only when occur, in large amount.

An example of a positive effect of the presence of B-chromosomes has been reported in foxes. Male foxes with supernumerary chromosomes tend to eat better and be more fit overall than foxes lacking the extra chromosomes.

Origin and function unknown

Maybe originated from A-chromosome or chromosome form different species

Found

Appears to be more common in plants (>1000 spp.)than in animals. Found in cereals and forage grasses sps. Eg. Zea, Sorghum, Secale, Avena, Bromus, etc. Especially in cross pollinated sps. B chromosomes are usually restricted only to aerial part and absent in roots (eg. Aegiolops speltoides) and may be eliminated during meiosis due to their division in Anaphase-I followed by elimination at anaphase II.

In animals, the B-chromosomes disappear from the non-reproductive (somatic) tissue and are maintained only in the cell-lines that lead to the reproductive organs. B chromosomes have tendency to accumulate in meiotic cell products resulting in an increase of B number over generations. However this effect is counterbalanced for selection against infertility

Effect of B chromosomes

The most significant effect of B chromosomes is on seed and pollen fertility. Flowering time is generally delayed by Bchromosomes and several characters (plant height, plant weight and tiller number) are adversely affected.

B chromosomes in some species influence the chiasma frequencies among pollen mother cells. It may affect in suppressing or promoting meiotic pairing in a number of intergeneric or interspecific hybrids (eg. Wheat x Aegilops)

The B chromosomes also have the following effects on A chromosomes: increases

asymmetry chiasma distribution increases crossing over and recombination frequencies: increases variation cause increased unpaired chromosomes: infertility

B chromosomes may play a positive role on normal A chromosomes in some circumstances. The B chromosomes suppress homologous pairing which reduces multiple pairing between homologous chromosomes in allopolyploids. Allopolyploids

are polyploids with chromosomes derived from different species

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