Molecular evidence for the origin of plastids from a cyanobacterium-like ancestor.

The origin of plastids by either a single or multiple endosymbiotic event(s) and the nature of the progenitor(s) of plastids have been the subjects of much controversy. The sequence of the small subunit rRNA (Ssu rRNA) from the plastid of the chlorophyll c-containing alga Cryptomonas phi is presented, allowing for the first time a comparison of this molecule from all of the major land plant and algal lineages. Using a distance matrix method, the phylogenetic relationships among representatives of these lineages have been inferred and the results indicate a common origin of plastids from a cyanobacterium-like ancestor. Within the plastid line of descent, there is a deep dichotomy between the chlorophyte/land plant lineage and the rhodophyte/chromophyte lineage, with the cyanelle of Cyanophora paradoxa forming the deepest branch in the latter group. Interestingly, Euglena gracilis and its colorless relative Astasia longa are more related to the chromophytes than to the chlorophytes, raising once again the question of the origin of the euglenoid plastids.