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. B chromosomes are distinct from marker chromosomes or additional copies of normal chromosomes as they occur in trisomies. 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. B chromosomes are distinct from marker chromosomes or additional copies of normal chromosomes as they occur in trisomies. 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. Similarly, B chromosomes of the cichlid fish Astatotilapia latifasciata also have been shown to arise from rearrangements of normal A chromosomes. 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 In other cases, B chomosomes 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.