Global model for group II intron evolution. An ancient reverse transcriptase combined with a structured RNA to form a group II intron retroelement. This ancestral form was present in Eubacteria and had properties of splicing and retromobility. The retroelement form differentiated into eight lineages, of which ML, CL, and B acquired an endonuclease domain. All lineages but class C (IIC) introns were mobile by retrohoming into site-specific target sequences. Introns of three lineages transferred to archaebacteria, while introns of two lineages transferred to mitochondria and chloroplasts. Variant forms of group II introns were produced in each location as noted. Prior to the LECA, group II introns invaded the nucleus where they developed into the spliceosome and non-LTR retroelements. Much later in plants, group II introns transferred to the nucleus, where the IEPs developed into splicing factors that are imported into mitochondria and/or chloroplasts to help splice organellar group II introns. See text for full description. IEP, intron-encoded protein; LTR, long terminal repeat; ORF, open reading frame; RT,reverse transcriptase.