B chromosome

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." Similarly, B chromosomes of the cichlid fish Haplochromis latifasciatus also have been shown to arise from rearrangements of normal A chromosomes. Fungal B chromosomes in at least two species appear to have a different origin as they show limited synteny and other similarity measures to A chromosome segments. They have likely originated by horizontal gene transfer. ==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 submetacentric. 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 have a 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. 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 Fusarium solani (syn. 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. == In plants ==

B chromosomes are a significant reflection of genetic diversity between varying plant species. These supernumerary chromosomes are commonly observed in angiosperms, specifically the flowering plants bred through outcrossing. For example, the sister species Aegilops speltoides and Aegilops mutica possess copies of B chromosomes within their aerial tissues, while their roots exhibit an absence of these supernumerary chromosomes. The morphological structure and size of B chromosomes is different from normally-occurring chromosomes in both plants and mammals. Most often in plants, B chromosomes are notably "non-homologous and smaller than the smallest A-chromosome". == References ==