Fig. 5

Hypothetical framework for the origin and evolution of group II introns and putative models to explain size expansion of Porphyridium mitogenomes. a Shown here is the evolutionary trajectory of group II introns in the tree of life. The proteobacterial primary endosymbiosis is shown with the red line and the cyanobacterial primary endosymbiosis with the green line. Secondary endosymbiosis involving green and red algae are shown by the lighter green lines. The movement of group II introns is shown in different colors (from cyanobacteria: blue; from proteobacteria: purple). Each colored circle containing a letter corresponds to the circles (colored and in alphabetical order) in the phylogenetic tree shown in Fig. 3a (cyanobacteria: blue-circles; proteobacteria: purple-circles). b This image shows three possible models (gene-specific vertical inheritance, horizontal gene transfer, and self-proliferation) to explain the spread of group II introns in Porphyridium mitogenomes