B chromosome

In addition to the normal karyotype, wild populations of many animal and plant species contain supernumerary or B 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. ^[1]

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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 heterochromatic segments of normal chromosomes in the remote past. In general "we may regard supernumeraries as a very special category of genetic polymorphism which, because of manifold types of accumulation mechanisms, does not obey the ordinary Mendelisn laws of inheritance." (White 1973 p173)

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. Bivalent pairing is ensured by a gene on chromosome 5 of the B genome Phlocus. 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 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.

See also

• Intragenomic conflict

References

• Burt, A. and R.L. Trivers (2005). Genes in Conflict: The Biology of Selfish Genetic Elements. Harvard University Press, Cambridge, MA.
• Camacho J.P.M., T.F. Sharbel, and L.W. Beukeboom (2000). B-chromosome evolution. Philosophical Transactions of the Royal Society of London B 355:163-178.
• Camacho, J.P.M. (ed.) (2004). B Chromosomes in the Eukaryote Genome. Special issue of Cytogenetic and Genome Research, vol. 106, nos. 2-4.
• Camacho, J.P.M. (2005). B chromosomes. In The Evolution of the Genome (ed. T.R. Gregory), Elsevier, San Diego, pp. 223-286
• Jones, R.N. and H. Rees (1982). B Chromosomes. Academic Press, New York.
• Perfectti, F., Corral, J. M., Mesa, J. A., Cabrero, J., Bakkali, M., López-León, M. D. and Camacho. J. P. M. (2004). Rapid suppression of drive for a parasitic B chromosome. Cytogenetic and Genome Research, 106: 338-343.
• Bakkali, M. and Camacho J. P. M. (2004). The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in northern Africa. IV. Transmission of rare B chromosome variants. Cytogenetic and Genome Research, 106: 332-337.
• Bakkali, M. and Camacho, J. P. M. (2004). The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in northern Africa. III. Mutation rate of B chromosomes. Heredity, 92: 428-433.
• Cabrero, J., Bakkali, M., Bugrov, A., Warchalowska-Sliwa, E., López-León, M. D., Perfectti, F. and Camacho, J. P. M. (2003). Multiple origin of B chromosomes in the grasshopper Eyprepocnemis plorans. Chromosoma, 112: 207-211.
• Bakkali, M., Cabrero, J. and Camacho, J. P. M. (2003). B-A interchanges are an unlikely pathway for B chromosome integration into the A genome. Chromosome Research, 11: 115-123.
• Camacho, J. P. M., Cabrero, J., López-León, M. D., Bakkali, M. and Perfectti, F. (2003). The B chromosomes of the grasshopper Eyprepocnemis plorans and the intragenomic conflict. Genetica, 117: 77-84.
• Camacho, J. P. M., Bakkali, M., Corral, J. M., Cabrero, J., López-León, M. D., Aranda, I., Martín-Alganza, A. and Perfectti, F. (2002). Host recombination is dependent on the degree of parasitism. Proceedings of the Royal Society of London: Biological Sciences, 269: 2173-2177.
• Bakkali, M., Perfectti, F. and Camacho, J. P. M. (2002). The B-chromosome polymorphism of the grasshopper Eyprepocnemis plorans in northern Africa. II. Parasitic and neutralized B1 chromosome. Heredity, 88: 14-18.
• Camacho, J. P. M., Corral, J. M., Mesa, J. A., Cabrero, J., Bakkali, M., López-León, M. D. and Perfectti, F. (2001). Neutralization of a parasitic B chromosome in the grasshopper Eyprepocnemis plorans. Chromosome Research, 9: 106.
• Cabrero, J., López-León, M. D., Bakkali, M. and Camacho, J. P. M. (1999). Common origin of B chromosome variants in the grasshopper Eyprepocnemis plorans. Heredity, 83: 435-439.
• Bakkali, M., Cabrero, J., López-León, M. D., Perfectti, F. and Camacho, J. P. M. (1999). The B-chromosome polymorphism of the grasshopper Eyprepocnemis plorans in northern Africa. I. B variants and frequency. Heredity, 83: 428-434.
• Bakkali, M., Cabrero, J., López-León, M. D., Perfectti, F. and Camacho, J. P. M. (1998). B-chromosome polymorphism in North-African natural populations of the grasshopper Eyprepocnemis plorans. Cytogenetics and Cell Genetics, 81: 124.
• White M.J.D. 1973. The chromosomes. 6th ed, Chapman & Hall, London.

• B chromosomes in wood mice, genus Apodemus