Mobilization of Pack-CACTA transposons in Arabidopsis suggests the mechanism of gene shuffling

Pack-TYPE transposons are a unique class of potentially mobile non-autonomous elements that can capture, merge and relocate fragments of chromosomal DNA. It has been postulated that their activity accelerates the evolution of host genes. However, this important presumption is based only on the sequences of currently inactive Pack-TYPE transposons and the acquisition of chromosomal DNA has not been recorded in real time. We have now for the first time witnessed the mobilization of novel Pack-TYPE elements related to the CACTA transposon family over several plant generations. Remarkably, these elements tend to insert into genes as closely spaced direct repeats and they frequently undergo incomplete excisions, resulting in the deletion of one of the end sequences. These properties constitute a mechanism of efficient acquisition of genic DNA residing between neighbouring Pack-TYPE transposons and its subsequent mobilization. Our work documents crucial steps in the formation in vivo of novel Pack-TYPE transposons and thus the mechanism of gene shuffling mediated by this type of mobile element.