Genes on B chromosomes: Old questions revisited with new tools

• The origin and fate of B chromosome-located genic sequences are described.
• B chromosomes contribute and influence the transcriptome of the host.
• B chromosome-located genes are undergoing pseudogenization.

BackgroundB 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 reviewDue 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 conclusionsRecent data suggest that B chromosomes carry transcriptionally active genic sequences which could affect the transcriptome profile of their host genome.General SignificanceThese 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.