Special types of chromosome: B chromosome

 B-Chromosomes are a particular kind of supernumerary chromosomes, that may or may not be found in an organism as extra chromosomes over and above the standard diploid or polyploid chromosome complement. The standard complement consists of chromosomes described as A chromosomes.

The B-chromosomes are found in the natural populations of many plants and animal species and are recognized on the basis of their following characteristics; (a) they are dispensable; are not found in all individuals of a species and may not be found in all cells of an individual organism; (b) they are not homologous with any of the basic A chromosomes; (c) their inheritance is non-Mendelian, sometimes due to non-disjunction during pollen mitosis (as in some plants); (d) they are usually smaller than A-chromosomes and have their own unique pattern of heterochromatin distribution; (e) in general, they are genetically inert, but may rarely organize nucleoli and carry functional genetic material; (f) when present in high number, they suppress vigour and fertility; (g) their origin and function are unknown.



B-chromosome in plants has been reported in more than 1000 species distributed over bryophytes (including mosses), ferns, gymnosperms, and angiosperms (both monocotyledons and dicotyledons). Mostly, they are found in out-breeders (cross-pollinated species). In the family Gramineae, they are found in 200 species including cereals, and forage grasses These B chromosomes are often absent in well-adapted agronomic strains and are perhaps eliminated due to selection. The B-chromosomes may sometimes be restricted only to aerial parts (absent in roots e.g. Aegilops speltoides, Ae.mutica) and may also be eliminated during meiosis due to their division at anaphase I followed by elimination at anaphase II. B chromosomes may also increase in number due to a drive involving non-disjunction during pollen mitosis, thus leading to the production of plants with two B chromosomes in the progeny OB ♀ x 1B♂.

The most significant effect of B chromosomes is on seed and pollen fertility. Flowering time is generally delayed by B-chromosomes and several characters (plant height, plant weight, and tiller number) are adversely affected. A very important effect of B chromosomes relates to their effect on the pairing of A chromosomes in (i) plants carrying these B-chromosomes and in (ii) species hybrids involving a parent carrying B-chromosomes. In rye, although 0-6 B chromosomes did not influence the chiasmata frequencies, 8 B chromosomes drastically influenced the range of chiasma frequencies among pollen mother cells (PMCs) and among bivalents within PMCs. In maize, the use of various A/B interchange stocks demonstrated that the genetic elements affecting the meiotic pairing of A-chromosomes appeared to be widely distributed throughout the length of B-chromosomes. The effect of B-chromosomes in suppressing or promoting meiotic pairing has been shown in a number of intergeneric or interspecific hybrids (wheat x Aegilops sp.; wheat x rye, Lolium x Festuca; interspecific hybrids within the genus Lolium, etc.).

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