Fig. 1

Plant mitochondrial group II introns evolving. A In its likely evolutionary ancestral state, a group II intron encodes a multifunctional maturase in domain IV of its characteristic six-domain structure (DI through DVI). A fully equipped maturase features reverse transcriptase domains RT0-RT7 (the finger and palm domains), followed by a maturase-specific ‘X’ domain and a DNA endonuclease ‘En’ domain with relevance for intron mobility. Interaction of exon binding sites (EBS 1 and 2) in domain DI with corresponding intron binding sites at the end of the upstream exon are equally important for splicing and retromobility. B Intron-borne maturases frequently degenerate and get lost during plant mitogenome evolution. C Multiple cases of cis-to-trans-splicing transitions are evident for plant mitochondrial group II introns, creating bipartite introns mostly broken in domain DIV or even tripartite group II introns [10]. D Plant mitochondrial group II introns may invade other introns, creating twintrons [14]. E The contribution of group II intron-encoded maturases, their free-standing paralogues in plant mitochondrial or nuclear genomes and those of multiple phylogenetically unrelated splicing factors [e.g. 19, 20, 47, 130] to all the evolutionary processes summarized in the figure is not fully understood at present. F Of particular relevance in this survey is the capability of plant mitochondrial group II introns to occupy new gene locations, creating sequence-related group II paralogues, here grouped into families. G The complete survey of plant mitochondrial group II introns presented includes paralogues that degenerated together with their host genes, or which can be identified as “fossil” introns in intergenic locations. Group II intron illustration have been made manually with ProCreate^®