Figure 6.

Model for the history of group II intron ORFs. Group II intron ORFs probably originated in bacteria where the introns transferred horizontally with high frequency. The ORFs were introduced to chloroplasts and mitochondria perhaps via the ancestral endosymbionts. In euglenoid chloroplasts, the introns lost mobility functions and were transferred vertically, while in the algal lineage the ORFs spread horizontally at a high rate. Introduction of group II intron ORFs to mitochondria resulted in an expansion of the subdomain 4/5 spacer, 7/X spacer and Zn domain, while domain X became highly conserved. The intron ORFs spread horizontally among mitochondria of fungi, liverwort and brown alga, and possibly transferred back to bacteria. In a subset of liverwort introns, the 4/5 and 7/X spacers were expanded further with concomitant loss of subdomains 0, 1 and the Zn domain, giving rise to the matR family of introns in higher plants. Dotted arrows indicate uncertainty for the source or direction of horizontal transfer events. Colored shading of domains indicates development of group-specific motifs or sequence conservation.