High-copy sequences reveal distinct evolution of the rye B chromosome
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- B chromosomes (Bs) are supernumerary chromosomes that vary in number among individuals of the same species. Because of their dispensable nature, their non-Mendelian inheritance and their origin from A chromosomes (As), one might assume that Bs followed a different evolutionary pathway from As, this being reflected in differences in their high-copy DNA constitution. We provide detailed insight into the composition and distribution of rye (Secale cereale) B-located high-copy sequences.
- A- and B-specific high-copy sequences were identified in silico. Mobile elements and satellite sequences were verified by fluorescence in situ hybridization (FISH). Replication was analyzed via EdU incorporation.
- Although most repeats are similarly distributed along As and Bs, several transposons are either amplified or depleted on the B. An accumulation of B-enriched satellites was found mostly in the nondisjunction control region of the B, which is transcriptionally active and late-replicating. All B-enriched sequences are not unique to the B but are also present in other Secale species, suggesting the origin of the B from As of the same genus.
- Our findings highlight the differences between As and Bs. Although Bs originated from As, they have since taken a separate evolutionary pathway.