B-chromosome evolution

B chromosomes are extra chromosomes to the standard complement that occur i n many organisms. They can originate in a number of ways including derivati on from autosomes and sex chromosomes in intra- and interspecies crosses. T heir subsequent molecular evolution resembles that of univalent sex chromos omes, which involves gene silencing, heterochromatinization and the accumul ation of repetitive DNA and transposons. B-chromosome frequencies in popula tions result from a balance between their transmission rates and their effe cts on host fitness. Their long-term evolution is considered to be the outc ome of selection on the host genome to eliminate B chromosomes or suppress their effects and on the B chromosome's ability to escape through the gener ation of new variants. Because B chromosomes interact with the standard chr omosomes, they can play an important role in genome evolution and may be us eful for studying molecular evolutionary processes.