Analysis of re-integrated Ac element positions in the genome of Populus provides a basis for Ac/Ds-transposon activation tagging in trees
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With a view to establish an efficient gene tagging system for forest tree species, we assessed the transposition behaviour of the maize transposable element Ac in poplar. In earlier work, we showed that new integration sites were often located within predicted or known coding sequences. However, somatic transposition behaviour of Ac with regard to conserved chromosome specificity or, more specifically, whether Ac transposition is restricted to the chromosome on which the primary insertion locus (donor) is located or whether it is able to pass chromosomal boundaries, remained unclear. To answer these questions, we took advantage of the publicly available Populus trichocarpa genome sequence (Phytozome v5.0; http://www.phytozome.net) and three 35S::Ac-rolC transgenic hybrid aspen lines to determine the flanking sequences of Ac re-integration sites for tissue sectors from which Ac had been excised. Only about one-third of the analysed re-integrations were positioned within the scaffold containing the primary Ac donor locus, and the majority of re-integrations were found scattered over many unlinked sites on other scaffolds confirming that Ac transposition in poplar does in fact cross chromosome boundaries. The majority of re-integration sites (57.1%) were found within or near coding regions demonstrating that Ac/Ds transposon tagging in poplar holds much promise for the efficient induction of mutants and functional genomics studies in forest tree species.