Figure 1

Primary and secondary endosymbiosis. part A: Primary endosymbiosis is proposed to have involved the capture of a cyanobacterium (green elipse) by a eukaryotic heterotroph (red elipse). The cyanobacterium would then have been modified during evolution to give rise to a plastid with two surrounding membranes. part B: The secondary endosymbiotic event that gave rise to the heterokonts is proposed to have involved the engulfment of a red algae with a chloroplast (green elipse inside a red elipse) by a eukaryotic heterotroph (brown elipse). The red alga would have become the heterokont plastid with four surrounding membranes.