Fig. 9

A schematic view of B chromosome evolution. During the first step, a proto-B is derived from multi-A sequences as a result of genomic rearrangements. The proto-B gains the sequences from A genome for its survival and successful transmission. In the second step, the proto-B accumulates further sequences with a series of TE insertions, ampliconic sequences, gene like fragments and the formation of unique sequences that are specific to the B. Finally, a mature B evolves, providing extra genomic material which may contains genes for diverse functions