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.

The study of B chromosomes (Bs) started more than a century ago, while their presence in mammals dates since 1965. As the past two decades have seen huge progress in application of molecular techniques, we decided to throw a glance on new data on Bs in mammals and to review them. We listed 85 mammals with Bs that make 1.94% of karyotypically studied species. Contrary to general view, a typical B chromosome in mammals appears both as sub- or metacentric that is the same size as small chromosomes of standard complement. Both karyotypically stable and unstable species possess Bs. The presence of Bs in certain species influences the cell division, the degree of recombination, the development, a number of quantitative characteristics, the host-parasite interactions and their behaviour. There is at least some data on molecular structure of Bs recorded in nearly a quarter of species. Nevertheless, a more detailed molecular composition of Bs presently known for six mammalian species, confirms the presence of protein coding genes, and the transcriptional activity for some of them. Therefore, the idea that Bs are inert is outdated, but the role of Bs is yet to be determined. The maintenance of Bs is obviously not the same for all species, so the current models must be adapted while bearing in mind that Bs are not inactive as it was once thought.

B chromosomes (Bs) are supernumerary chromosomes relative to the standard karyotype. The maintenance of Bs in the yellow-necked field mice (Apodemus

Species of the Rana kl. esculenta complex from twenty-one localities of the North-Western part of the Balkan Peninsula (Yugoslavia and Macedonia) were analyzed by electophoretic, morphometric and cytogenetic methods. The results of this research revealed that species that belong to the R. kl. esculenta complex from localities in Northern Yugoslavia form a ridibunda/esculenta/lessonae (R-E-L) population system of diploid forms of two parental species and predominantly female hybrids. Pure R. ridibunda populations exist south of the Sava and Danube Rivers. The unequal sex ratio observed in water frog population systems suggests, according to Haldane’s rule, that the heterogametic sex suffers the effects of hybridization. Electrophoretic analysis shows that some introgression of ridibunda alleles into -R. lessonae, as well as lessonae into R. ridibunda has taken place. The occurrence of recombinations, and cytogenetic characteristics of the chromosome complements, does not support pure hybri-dogenesis as a general mode of reproduction in the hybrid form R. kl. esculenta from the examined localities.