FIGURE 1. Secondary symbiogenetic origin and membrane topology of cryptomonads.

After the primary endosymbiotic incorporation of a cyanobacterium to form the first chloroplast (green), many of its genes were transferred into the host nucleus. After this ancestral plant diversified to form green plants, red algae and glaucophytes, more complex algae were formed by independent secondary symbioses involving green or red algae, after which many or all plastid protein genes were transferred from the algal to the secondary host's nucleus. Shown here is the symbiosis of a red alga to form cryptomonads, where (as in all chromists) the food vacuole membrane fused with the RER. This fusion did not occur in alveolates, chlorarachneans or euglenoids. Former cyanobacterial genes now inserted in nucleomorph or nuclear chromosomes are shown in green, and former red algal genes now in the host nucleus in red. In cryptomonads, the chloroplast and nucleomorph (former red algal nucleus) are topologically in the periplastid space (starch- and ribosome-containing residual cytoplasm of the former red algal cell, yellow) in the periplastid membrane (former red algal plasma membrane), which is located in the lumen of the host's rough endoplasmic reticulum (RER). Chloroplast proteins are coded by three genomes (chloroplast, nucleomorph and nucleus) and mitochondrial proteins by two genomes (mitochondrion and nucleus). Nucleomorph and periplastid proteins are coded by two genomes (nucleus and nucleomorph). Coloured dots indicate protein translocation pathways in the periplastid complex: nuclear- or nucleomorph-encoded proteins targeted to the chloroplast are green; nuclear-encoded proteins imported into the periplastid space, and both nuclear- and nucleomorph-encoded proteins imported into the nucleomorph, are red. Mitochondrial proteins (brown) are encoded by both nuclear and mitochondrial genomes.