Hypothetical model for evolution of SMS-REPs. Our data indicate that in the first step, a progenitor SMS-REP must have arisen in an ancient chromosome. Its structure was almost the same as the proximal SMS-REP at the present time, but included the B region flanking sequences on both sides similar to the distal SMS-REP. The distal SMS-REP resulted from the deletions of two large areas between the A and B, and C and D regions. Secondly, deletion of both flanking sequences of the B region in the progenitor resulted in the proximal SMS-REP. Finally, two terminal deletions and one interstial deletion involving theUPF3A gene accompanied by interchromosomal insertional duplication together with an inversion generated the middle SMS-REP.
