BACKGROUND: B chromosomes are supernumerary dispensable parts of the karyotype which appear in some individuals of some populations in some species. Often, they have been considered as 'junk DNA' or genomic parasites without functional genes. SCOPE OF REVIEW: Due to recent advances in sequencing technologies, it became possible to investigate their DNA composition, transcriptional activity and effects on the host transcriptome profile in detail. Here, we review the most recent findings regarding the gene content of B chromosomes and their transcriptional activities and discuss these findings in the context of comparable biological phenomena, like sex chromosomes, aneuploidy and pseudogenes. MAJOR CONCLUSIONS: Recent data suggest that B chromosomes carry transcriptionally active genic sequences which could affect the transcriptome profile of their host genome. GENERAL SIGNIFICANCE: These findings are gradually changing our view that B chromosomes are solely genetically inert selfish elements without any functional genes. This at one side could partly explain the deleterious effects which are associated with their presence. On the other hand it makes B chromosome a nice model for studying regulatory mechanisms of duplicated genes and their evolutionary consequences.

Biology Centre ASCR Institute of Plant Molecular Biology Ceske Budejovice Czech Republic

Helmholtz Center Munich Dept Plant Genome and Systems Biology 85764 Neuherberg Germany

Leibniz Institute of Plant Genetics and Crop Plant Research Gatersleben Corrensstr 3 06466 Stadt Seeland Germany

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