B chromosome

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Description:
In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary or accessory chromosomes). By definition, these chromosomes are not essential for the life of a species, and are lacking in some (usually most) of the individuals. Thus a population would consist of individuals with 0, 1, 2, 3 (etc) supernumeraries.

Most B chromosomes are mainly or entirely heterochromatic, (and so would be largely non-coding) but some, such as the B chromosomes of maize, contain sizeable euchromatic segments. In general it seems unlikely that supernumeraries would persist in a species unless there was some positive adaptive advantage, which in a few cases has been identified. For instance, the British grasshopper Myrmeleotettix maculatus has two structural types of B chromosomes: metacentrics and submetacentrics. The supernumeraries, which have a satellite DNA, occur in warm, dry environments, and are scarce or absent in humid, cooler localities.

In plants there is a tendency for B chromosomes to be present in the germ-line, but to be lost from other tissues such as root tips and leaves. 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.

The evolutionary origin of supernumerary chromosomes is obscure, but presumably they must have been derived from...
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