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 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 Mendelian 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 Ph locus. 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.
Supernumerary chromosomes in fungi
Chromosome polymorphisms are very common among fungi. Different isolates of the same species often have a different chromosome number, with some of these additional chromosomes being unnecessary for normal growth in culture. The extra chromosomes are known as conditionally dispensable, or supernumerary, because they are dispensable for certain situations, but may confer a selective advantage under different environments.
Supernumerary chromosomes do not carry genes that are necessary for basic fungal growth, but may have some functional significance. For example, it has been discovered that the supernumerary chromosome of the pea pathogen Haematonectria haematococca carries genes that are important to the disease-causing capacity of the fungus. This supernumerary DNA was found to code for a group of enzymes that metabolize toxins, known as phytoalexins, that are secreted by the plant's immune system. It is possible that these supernumerary elements originated in horizontal gene transfer events because sequence analysis often indicates that they have a different evolutionary history from essential chromosomal DNA.
The fungal pathogen Mycosphaerella graminicola contains 8 dispensable B-chromosomes (Goodwin et al. (2011) PLOS Genetics 7:e102070).
||This section includes a list of references, related reading or external links, but the sources of this section remain unclear because it lacks inline citations. (August 2012)|
- Burt, A. and R.L. Trivers (2005). Genes in Conflict: The Biology of Selfish Genetic Elements. Cambridge, MA: Harvard University Press. ISBN 0-674-01713-7.
- Camacho JP, Sharbel TF, Beukeboom LW (February 2000). "B-chromosome evolution". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 355 (1394): 163–78. doi:10.1098/rstb.2000.0556. PMC 1692730. PMID 10724453.
- Camacho, J.P.M. (2004). "B Chromosomes in the Eukaryote Genome". Cytogenetics and Genome Research 106 (2–4). (Special issue)
- Camacho, J.P.M. (2005). "B chromosomes". In T.R. Gregory. The Evolution of the Genome. San Diego: Elsevier. pp. 223–86.
- Jones RN, Rees H (1982). B Chromosomes. New York: Academic Press.
- Perfectti F, Corral JM, Mesa JA, Cabrero J, Bakkali M, López-León MD, Camacho JPM (2004). "Rapid suppression of drive for a parasitic B chromosome". Cytogenetic and Genome Research 106 (2–4): 338–43. doi:10.1159/000079309. PMID 15292613.
- Bakkali M, Camacho JP (2004). "The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa. IV. Transmission of rare B chromosome variants". Cytogenet. Genome Res. 106 (2–4): 332–7. doi:10.1159/000079308. PMID 15292612.
- Bakkali M, Camacho JP (May 2004). "The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa: III. mutation rate of B chromosomes". Heredity 92 (5): 428–33. doi:10.1038/sj.hdy.6800437. PMID 14997182.
- Cabrero J, Bakkali M, Bugrov A, et al. (December 2003). "Multiregional origin of B chromosomes in the grasshopper Eyprepocnemis plorans". Chromosoma 112 (4): 207–11. doi:10.1007/s00412-003-0264-2. PMID 14628147.
- Bakkali M, Cabrero J, Camacho JP (2003). "B-A interchanges are an unlikely pathway for B chromosome integration into the standard genome". Chromosome Res. 11 (2): 115–23. doi:10.1023/A:1022811830616. PMID 12733638.
- Camacho JP, Cabrero J, López-León MD, Bakkali M, Perfectti F (January 2003). "The B chromosomes of the grasshopper Eyprepocnemis plorans and the intragenomic conflict". Genetica 117 (1): 77–84. doi:10.1023/A:1022311320394. PMID 12656575.
- Camacho JP, Bakkali M, Corral JM, et al. (October 2002). "Host recombination is dependent on the degree of parasitism". Proc. Biol. Sci. 269 (1505): 2173–7. doi:10.1098/rspb.2002.2135. PMC 1691133. PMID 12396493.
- Bakkali M, Perfectti F, Camacho JP (January 2002). "The B-chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa: II. Parasitic and neutralized B1 chromosomes". Heredity 88 (1): 14–8. doi:10.1038/sj.hdy.6800002. PMID 11813101.
- Camacho JPM, Corral JM, Mesa JA, Cabrero J, Bakkali M, López-León MD, Perfectti F (2001). "Neutralization of a parasitic B chromosome in the grasshopper Eyprepocnemis plorans". Chromosome Research 9: 106.
- Cabrero J, Lopez-Leon MD, Bakkali M, Camacho JP (October 1999). "Common origin of B chromosome variants in the grasshopper Eyprepocnemis plorans". Heredity 83 (# (Pt 4)): 435–9. doi:10.1038/sj.hdy.6885960. PMID 10583545.
- Bakkali M, Cabrero J, Lopez-Leon MD, Perfectti F, Camacho JP (October 1999). "The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in north africa. I. B variants and frequency". Heredity 83 (# (Pt 4)): 428–34. doi:10.1038/sj.hdy.6885950. PMID 10583544.
- Bakkali M, Cabrero J, López-León MD, Perfectti F, Camacho JPM (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.). London: Chapman & Hall. ISBN 0-412-11930-7.