Evolutionary dynamics of introns and their open reading frames in the U7 region of the mitochondrial rnl gene in species of Ceratocystis

Abstract

The mtDNA rnl-U7 region has been examined for the presence of introns in selected species of the genus Ceratocystis. Comparative sequence analysis identified group I and group II introns encoding single and double motif LAGLIDADG open reading frames (ORFs) at the following positions L1671, L1787, and L1923. In addition downstream of the rnl-U7 region group I introns were detected at positions L1971 and L2231, and a group II intron at L2059. A GIY-YIG type ORF was located within one mL1923 LAGLIDADG type ORF and a degenerated GIY-YIG ORF fused to a nad2 gene fragment was found in association with the mL1971 group I intron. The diversity of composite elements that appear to be sporadically distributed among closely related species of Ceratocystis illustrates the potential for homing endonucleases and their associated introns to invade new sites. Phylogenetic analysis showed that single motif LADGLIDADG ORFs related to the mL1923 ORFs have invaded the L1787 group II intron and the L1671 group I intron. Phylogenetic analysis of intron encoded single and double motif LAGLIDADG ORFs also showed that these ORFs transferred four times from group I into group II B1 type introns.