De novo evolution of satellite DNA on the rye B chromosome.

The most distinctive region of the rye B chromosome is a subtelomeric domain that contains an exceptional concentration of B-chromosome-specific sequences. At metaphase this domain appears to be the physical counterpart of the subtelomeric heterochromatic regions present on standard rye chromosomes, but its conformation at interphase is less condensed. In this report we show that the two sequence families that have been previously found to make up the bulk of the domain have been assembled from fragments of a variety of sequence elements, giving rise to their ostensibly foreign origin. A single mechanism, probably based on synthesis-dependent strand annealing (SDSA), is responsible for their assembly. We provide evidence for sequential evolution of one family on the B chromosome itself. The extent of these rearrangements and the complexity of the higher-order organization of the B-chromosome-specific families indicate that instability is a property of the domain itself, rather than of any single sequence. Indirect evidence suggests that particular fragments may have been selected to confer different properties on the domain and that rearrangements are frequently selected for their effect on DNA structure. The current organization appears to represent a transient stage in the evolution of a conventional heterochromatic region from complex sequences.