B chromosome

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B chromosomes in the Siberian deer

B chromosomes are chromosomes that appear in addition to the normal karyotype in some organisms . Synonym they are also called supernumerary (Engl. Supernumerary ) or accessory (Engl. Accessory ) called chromosomes. The regular chromosomes are called A chromosomes in this context . By definition, B chromosomes are only present in some of the specimens of a species or population. In addition, their number is often variable. In some cases they are not found in all tissues.

By irregular behavior during mitosis or meiosis they succeed to become " self-serving " (eng. Selfish ) in the germline to accumulate, so that a non- Mendelian inheritance results, wherein the for chromosomes usual handoff rate is not exceeded by 50%. Which mechanisms are responsible for this has only been clarified in a few cases (see for an overview of such mechanisms). In contrast to marker chromosomes , which have no enrichment mechanisms, B chromosomes have not yet been detected in humans .

B chromosomes are assigned to parasitic or egoistic genetic elements, which also include transposons . In many cases they probably originated from A chromosomes or parts thereof. They were first described in 1907 by Edmund B. Wilson in Hemiptera without their parasitic properties first becoming clear.

The evolution of the B chromosomes presumably largely depends on the interplay of the selection pressure on the host genome in favor of their elimination or shutdown on the one hand and their ability to evade this pressure on the other. Since B chromosomes interact with A chromosomes, where they occur they probably play an important role in genome evolution as a whole. Not all B chromosomes are harmful to the host. Some are neutral in their effect, for some even positive effects are discussed, e.g. B. with chives .

distribution

B chromosomes have so far been described in over 1300 plant species, 500 animal species and some fungi. All larger groups of animals and plants are represented. Unsurprisingly, they were found particularly frequently in well-studied groups. In species with large genomes , Bs are more common than in those with small genomes (e.g. monocot versus dicotyledonous flowering plants, grasshoppers ( Orthoptera ) versus two-winged ( Diptera ) in insects ). In birds , which have comparatively small genomes, B chromosomes have only been found in a single species. The following list gives only a few examples.

Animals: B-chromosomes are widespread in the well-studied grasshoppers (e.g. rarely more than three in Eyprepocnemis plorans ). Other insects with Bs are the wasp Nasonia and the fly Drosophila subsilvestris . Further examples: the flatworm Polycelis nigra (rarely more than three); the New Zealand frog Leiopelma hochstetteri with up to 15 mitotically stable Bs; the fish Poecilia formosa . In 55 (of 4629) mammal species , Bs have so far been found z. B. in wood mice . They were not observed in humans and great apes.

Plants: Up to 34 B-chromosomes have been described in maize plants and up to 20 in chives ( Allium schoenoprasum ). However, in wild plants the maximum number found was three ( Lolium perenne , B. dichromosomatica ), presumably because they are subject to higher selection pressure. With lilies and related plants ( Lilianae ) and grasses ( Poaceae ), two groups that are well studied, B-chromosomes are widespread.

literature

  • Juan Pedro M. Camacho, Timothy F. Sharbel, Leo W. Beukeboom. B-chromosome evolution. 2000. Phil. Trans. R. Soc Lond. B 355: 163-178. doi : 10.1098 / rstb.2000.0556
  • BG Palestis, R. Trivers, A. Burt, RN Jones. The distribution of B chromosomes across species. 2004. Cytogenet. Genome Res. 106: 151-158. doi : 10.1159 / 000079281

annotation

  1. ↑ Klaus Frisch, Non-random segregation of chromosomes - An overview of the literature from 1908 to 1995. 2001. Available online Non-random segregation of chromosomes ( Memento from March 1, 2006 in the Internet Archive ).

Individual evidence

  1. ↑ M. Vujoševic and J. Blagojevic: B chromosomes in populations of mammals. 2004. Cytogenetic and Genome Research 2004; 106: 247-256. doi : 10.1159 / 000079295
  2. ↑ JM Wójcik, AM Wójcik, M. Macholán, J. Piálek, J. Zima. The mammalian model for population studies of B chromosomes: the wood mouse (Apodemus). 2004. Cytogenetic and Genome Research 106: 264-270. doi : 10.1159 / 000079297