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The replicative retrotransposon life cycle offers the potential for explosive increases in copy number and consequent inflation of genome size. The BARE-1 retrotransposon family of barley is conserved, disperse, and transcriptionally active. To assess the role of BARE-1 in genome evolution, we determined the copy number of its integrase, its reverse transcriptase, and its long terminal repeat (LTR) domains throughout the genus Hordeum. On average, BARE-1 contributes 13.7 x 10(3) full-length copies, amounting to 2.9% of the genome. The number increases with genome size. Two LTRs are associated with each internal domain in intact retrotransposons, but surprisingly, BARE-1 LTRs were considerably more prevalent than would be expected from the numbers of intact elements. The excess in LTRs increases as both genome size and BARE-1 genomic fraction decrease. Intrachromosomal homologous recombination between LTRs could explain the excess, removing BARE-1 elements and leaving behind solo LTRs, thereby reducing the complement of functional retrotransposons in the genome and providing at least a partial "return ticket from genomic obesity."
Vicient et al. (Wed,) studied this question.