The discovery of twintrons in land plant mitochondria documents group II intron retromobility in recent evolutionary times.. A functional twintron, “zombie” twintrons and a hypermobile group II intron invading itself in plant mitochondria

The occurrence of group II introns in plant mitochondrial genomes is strikingly different between the six major land plant clades, markedly contrasting their highly conserved counterparts in chloroplast DNA. Their present distribution likely reflects numerous ancient intron gains, duplications and losses during early plant mtDNA evolution before the emergence of seed plants. As a novelty for plant organelles, we here report on five cases of twintrons, introns-within-introns, in the mitogenomes of lycophytes and hornworts. An internal group II intron interrupts an intron-borne maturase of an atp9 intron in Lycopodiaceae, whose splicing precedes splicing of the external intron. An invasive, hypermobile group II intron in cox1, specific to Lycopodiaceae has conquered nine further locations including seven intergenic regions, a previously overlooked sdh3 intron and, most surprisingly, also domain IV of itself. In those cases, splicing of the external introns does not seem to depend on splicing of the internal introns. Similar cases are identified in two other locations in the mtDNAs of hornworts. Although disrupting an intron-encoded protein of a group I intron in one case, we could not detect splicing of the internal group II intron in this mixed twintron combining two different types of mobile ribozymes. We suggest the name “zombie” twintrons (half dead, half alive) for cases where splicing of external introns does not depend on prior splicing of fossilized internal introns.