B chromosome

B- chromosomes are chromosomes that in addition to the normal karyotype occur in some organisms. Synonym they are also referred to as supernumerary (English supernumerary ) or accessory (English accessory ) chromosomes. The regular chromosomes are called in this context A- chromosomes. B- chromosomes are present by definition only in a subset of specimens of a species or a population. Frequently also their number is variable. In some cases, they do not occur in all tissues.

Due to irregular behavior during mitosis or meiosis, they manage to accumulate " selfish " ( engl. selfish ) in the germline, so that results in a non- Mendelian inheritance in which the for chromosomes usual propagation rate is exceeded by 50%. What mechanisms are responsible for this, is clarified only in a few cases (see for an overview of such mechanisms ). In contrast to chromosome markers, which have no enrichment mechanisms, B chromosomes have not yet been demonstrated in humans.

B- chromosomes are associated with the parasitic or selfish genetic elements, including transposons belong. They were in many cases presumably derived from A chromosomes or parts. They were first described in 1907 by Edmund B. Wilson at Hemiptera, without first having their parasitic properties became apparent.

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

Dissemination

B chromosome have been described in over 1,300 species of plants, some fungi, and 500 species. All major animal and plant groups are represented here. Not surprisingly, they were most frequently found in well-studied groups. In species with large genomes Bs are more common than in those with small genomes (eg monocot versus dicot flowering plants, grasshoppers (Orthoptera ) versus (Diptera ) in insects ). In birds that have relatively small genomes, B- chromosomes were detected only in a single species. The following list gives a few examples.

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

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