A contiguous 66-kb barley DNA sequence provides evidence for reversible genome expansion

Organisms with large genomes contain vast amounts of repetitive DNA sequenc es, much of which is composed of retrotransposons. Amplification of retrotr ansposons has been postulated to be a major mechanism increasing genome siz e and leading to "genomic obesity." To gain insights into the relation betw een retrotransposons and genome expansion in a large genome, we have studie d a 66-kb contiguous sequence at the Rarl locus of barley in detail. Three genes were identified in the 66-kb contig, clustered within an interval of 18 kb. Inspection of sequences flanking the gene space unveiled four novel retroelements, designated Nikira,Sukkula, Sabrina, and BAGY-2 and several u nits of the known BARE-1 element. The retroelements identified are responsi ble For at least 15 integration events, predominantly arranged as multiple nested insertions. Strikingly, most of the retroelements exist as solo LTRs (Long Terminal Repeats), indicating that unequal crossing over and/or intr achromosomal recombination between LTRs is a common feature in barley. Our data suggest that intraelement recombination events deleted most of the ori ginal retrotransposon sequences, thereby providing a possible mechanism to counteract retroelement-driven genome expansion.