Species-specific double-strand break repair and genome evolution in plants

Even closely related eukaryotic species may differ drastically in genome si ze. While insertion of retroelements represents a major source of genome en largement, the mechanism mediating species-specific deletions is fairly obs cure. We analyzed the formation of deletions during double-strand break (DS B) repair in Arabidopsis thaliana and tobacco, two dicotyledonous plant spe cies differing >20-fold in genome size. DSBs were induced by the rare cutti ng restriction endonuclease I-SceI and deletions were identified by loss of function of a negative selectable marker gene containing an I-SceI site. W hereas the partial-use of micro-homologies in junction formation was simila r in both species, in tobacco 40% of the deletions were accompanied by inse rtions. No insertions could be detected in Arabidopsis, where larger deleti ons were more frequent, indicating a putative inverse correlation between g enome size and the average length of deletions. Such a correlation has been postulated before by a theoretical study on the evolution of related insec t genomes and our study now identifies a possible molecular cause for the p henomenon, indicating that species-specific differences in DSB repair might indeed influence genome evolution.