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Even closely related eukaryotic species may differ drastically in genome size. While insertion of retroelements represents a major source of genome enlargement, the mechanism mediating species‐ specific deletions is fairly obscure. We analyzed the formation of deletions during double‐strand break (DSB) repair in Arabidopsis thaliana and tobacco, two dicotyledonous plant species differing >20‐fold in genome size. DSBs were induced by the rare cutting restriction endonuclease I‐SceI and deletions were identified by loss of function of a negative selectable marker gene containing an I‐SceI site. Whereas the partial use of micro‐homologies in junction formation was similar in both species, in tobacco 40% of the deletions were accompanied by insertions. No insertions could be detected in Arabidopsis, where larger deletions were more frequent, indicating a putative inverse correlation between genome size and the average length of deletions. Such a correlation has been postulated before by a theoretical study on the evolution of related insect genomes and our study now identifies a possible molecular cause for the phenomenon, indicating that species‐specific differences in DSB repair might indeed influence genome evolution. [ABSTRACT FROM AUTHOR]

To analyze genomic changes resulting from double-strand break (DSB) repair, transgenic tobacco plants were obtained that carried in their genome a restriction site of the rare cutting endonuclease I-SceI within a negative selectable marker gene. After induction of DSB repair via Agrobacterium-mediated transient expression of I-SceI, plant cells were selected that carried a loss-of-function phenotype of the marker. Surprisingly, in addition to deletions, in a number of cases repair was associated with the insertion of unique and repetitive genomic sequences into the break. Thus, DSB repair offers a mechanism for spreading different kinds of sequences into new chromosomal positions. This may have evolutionary consequences particularly for plants, as genomic alterations occurring in meristem cells can be transferred to the next generation. Moreover, transfer DNA (T-DNA), carrying the open reading frame of I-SceI, was found in several cases to be integrated into the transgenic I-SceI site. This indicates that DSB repair also represents a pathway for the integration of T-DNA into the plant genome. [ABSTRACT FROM AUTHOR]