B chromosomes in populations of mammals

B chromosomes (Bs) have been found in 55 out of 4629 living species of mammals. The summarized data show great variability in types of mammalian Bs, including differences in size, shape and molecular composition. This variability extends to the origin, mode of transmission and population dynamics. In general, B chromosomes in mammals do not differ from Bs found in other animal or plant species, but some peculiarities do exist. Most species in which Bs are found are widespread. Some data support the view that Bs may contribute to the successful expansion of some of these species, but it is possible that Bs are just more easily scored in them due to their frequent occurrence. Most of these species are also characterized by cycling fluctuations of abundance and characteristic social organization that produce conditions favorable for Bs to spread. All areas of research on Bs in mammals suffer from lack of data, emphasizing the necessity for intensified research on the molecular structure and ways of maintenance of Bs in populations.