Biology and evolution of B chromosomes

Abstract

B chromosomes are dispensable and often selfish elements of the genome which follow their own evolutionary pathway. B chromosomes are a major source of intraspecific variation in nuclear DNA amounts in numerous species and the distribution of Bs among different groups of angiosperms is not random. B chromosome inheritance is irregular and non-Mendelian, and therefore polymorphisms exist with respect to the number of Bs within populations or even within different cell lines of an individual carrying Bs. Drive mechanisms play a major role in the equilibrium of B frequency in populations. The most widely accepted view is that Bs are derived from the A chromosome complement. Some evidence also suggests that Bs can be spontaneously generated in response to the new genomic conditions after interspecific hybridization. The molecular processes that gave rise to Bs during evolution remain unclear. Here, we survey current knowledge on the DNA/chromatin composition, origin, and mitotic and meiotic drive mechanisms of B chromosomes and discuss effects and transcripts associated with Bs.