B Chromosomes in Populations of Mammals Revisited

Abstract

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 Therefore, the idea that Bs are inert is outdated, but the role of Bs is yet to be determined.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.

Chromosomal Differentiation in Genetically Isolated Populations of the Marsh-Specialist Crocidura suaveolens (Mammalia: Soricidae)

“…Despite their widespread distribution in wild populations of several animal, plant and fungi species, the evolutionary origin and function of B-chromosomes are largely unknown. These dispensable chromosomes present a particular behavior, thus not following Mendelian segregation laws [34]. Most B-chromosomes are mainly or entirely heterochromatic (i.e., largely non-coding), although in some cases, B-chromosomes can provide some positive adaptive advantage, as suggested by associations with particular habitats [35] or with increases of crossing over and recombination frequencies [36][37][38].…”

Addressing Long-Standing Questions with Advanced Approaches: The 4th B Chromosome Conference

“…Among animals, Bs were reported for several vertebrate taxa including mammals (Vladimir Trifonov, Novosibirsk, Russia, and Camila Moreira, Botucatu, Brazil), frogs (Juan Ferro, Posadas, Argentina), and fish (Cesar Martins, Botucatu, Brazil). Around 85 mammalian species (most of them rodents) are B carriers [Vujošević et al, 2018]. V. Trifonov presented updated information on sequenced B chromosomes for several species including carnivores (red fox and raccoon dog), ruminants (Siberian roe deer and brown brocket deer), and rodents (collared lemming and 2 species of Apodemus ).…”

The occurrence of dot-like micro B chromosomes in Korean field mice Apodemus peninsulae from the shore of the Teletskoye Lake (Altai Mountains)

“…By 2004, Bs have been found in 55 of 4629 living species of mammals (Vujošević and Blagojević 2004). Rubtsov and Borisov (2018) suggest several models of B chromosome origin, while the article by Vujošević et al (2018), provides an updated list of 85 mammalian species with Bs, and gives a detail description of research experiments accomplished on these species. The research article by (Makunin et al 2018) provides novel data on B chromosome content and evolution in the red fox (the first mammalian species with Bs, whose genome has recently been sequenced and assembled , and in the raccoon dog, the carnivore species, where B specific coding genes were discovered almost 13 years ago.…”