B chromosome

In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary, accessory, (conditionally-)dispensable, or lineage-specific 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.) B chromosomes.[1] B chromosomes are distinct from marker chromosomes or additional copies of normal chromosomes as they occur in trisomies.

Siberian roe deer metaphase spread with B chromosomes

Origin

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)

Next generation sequencing has shown that the B chromosomes from rye are amalgamations of the rye A chromosomes.[2] Similarly, B chromosomes of the cichlid fish Haplochromis latifasciatus also have been shown to arise from rearrangements of normal A chromosomes.[3]

Function

Most B chromosomes are mainly or entirely heterochromatic (i.e. largely non-coding), but some contain sizeable euchromatic segments (E.g. such as the B chromosomes of maize). In some cases, B chromosomes act as selfish genetic elements. In other cases, B chromosomes provide some positive adaptive advantage. 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.

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.

B chromosomes may play a positive role on normal A chromosomes in some circumstances. In wheat, an allopolyploid, the B chromosomes suppress homologous pairing which reduces multiple pairing between homologous chromosomes.[4] 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, thereby acting as selfish genetic elements. However this effect is counterbalanced for selection against infertility.

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.[5]

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.[5]

The wheat-infecting fungal pathogen Zymoseptoria tritici contains 8 dispensable B-chromosomes - the largest number of dispensable chromosomes observed in fungi.[6]

References
  1. White M.J.D. (1973). The chromosomes (6th ed.). London: Chapman & Hall. pp. 171 et seq. ISBN 0-412-11930-7.
  2. Martis; 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.
  3. Valente; et al. (2014). "Origin and evolution of B chromosomes in the cichlid fish Astatotilapia latifasciata based on integrated genomic analyses". Mol Biol Evol. 31 (8): 2061–2072. doi:10.1093/molbev/msu148. PMID 24770715.
  4. Dvorak, J; Deal, KR; Luo, MC (September 2006). "Discovery and mapping of wheat Ph1 suppressors". Genetics. 174 (1): 17–27. doi:10.1534/genetics.106.058115. PMC 1569802. PMID 16702426.
  5. Covert SF (May 1998). "Supernumerary chromosomes in filamentous fungi". Curr. Genet. 33 (5): 311–9. doi:10.1007/s002940050342. PMID 9618581. Archived from the original on 2001-07-27.
  6. Goodwin SB, M'barek SB, Dhillon B, Wittenberg AH, Crane CF, Hane JK, Foster AJ, Van der Lee TA, Grimwood J, Aerts A, Antoniw J, Bailey A, Bluhm B, Bowler J, Bristow J, van der Burgt A, Canto-Canché B, Churchill AC, Conde-Ferràez L, Cools HJ, Coutinho PM, Csukai M, Dehal P, De Wit P, Donzelli B, van de Geest HC, van Ham RC, Hammond-Kosack KE, Henrissat B, Kilian A, Kobayashi AK, Koopmann E, Kourmpetis Y, Kuzniar A, Lindquist E, Lombard V, Maliepaard C, Martins N, Mehrabi R, Nap JP, Ponomarenko A, Rudd JJ, Salamov A, Schmutz J, Schouten HJ, Shapiro H, Stergiopoulos I, Torriani SF, Tu H, de Vries RP, Waalwijk C, Ware SB, Wiebenga A, Zwiers LH, Oliver RP, Grigoriev IV, Kema GH (2011). "Finished genome of the fungal wheat pathogen Mycosphaerella graminicola reveals dispensome structure, chromosome plasticity, and stealth pathogenesis". PLoS Genet. 7 (6): e1002070. doi:10.1371/journal.pgen.1002070. PMC 3111534. PMID 21695235.

Further reading
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