B chromosome

Siberian roe deer metaphase spread with B chromosomes

B chromosomes are chromosomes which are not needed for the life of a species. They are also called 'supernumerary' or 'accessory chromosomes'.

In addition to normal chromosomes (the karyotype), many species have B chromosomes. Wild populations of many animal, plant, and fungi species have B chromosomes. These chromosomes are not essential for their life, and are lacking in many individuals. So a population might have individuals with 0, 1, 2, 3 (etc.) supernumeraries.^[1]

Most B chromosomes are mainly or entirely heterochromatic, and so do not code for anything. Some, such as the B chromosomes of maize, contain sizeable euchromatic segments, and therefore do code for some products.

It seems unlikely that supernumeraries would survive in a species unless there was some positive advantage. In a few cases this has been identified. For instance, the British grasshopper Myrmeleotettix maculatus has B chromosomes with a satellite DNA. They occur in warm, dry environments, and are scarce or absent in humid, cooler places.

It is thought that B chromosomes are parasitic elements like selfish DNA. Various scenarios have been suggested as to how they arose.^[2]

References

1. White M.J.D. 1973 (1973). The chromosomes. London: 6th ed, Chapman & Hall. pp. 171 et seq. ISBN 0-412-11930-7.
2. Martis M.M. et al 2012; et al. (2012). "Selfish supernumerary chromosome reveals its origin as a mosaic of host genome and organellar sequences". Proc Natl Acad Sci USA. 109 (33): 13343–13346. doi:10.1073/pnas.1204237109. PMC 3421217. PMID 22847450.