The relationship of a prochlorophyte Prochlorothrix hollandica to green chloroplasts

It is generally accepted that chloroplasts arose from one or more endosymbiotic events between an ancestral cyanobacterium and a eukaryote. Such an origin fits well in the case of the chloroplasts of rhodophytes that, like cyanobacteria, contain chlorophyll a and phycobilin pigments. The green chloroplasts from higher plants, green algae, and euglenoids however, contain chlorophyll b as well as chlorophyll a, and lack phycobilins. Consequently, it has been suggested that they arose independently of the rhodophyte chloroplasts, from an ancestral prokaryote containing that complement of pigments. The 'prochlorophytes' Prochloron didemni (an exosymbiont on didemnid ascidians) and Prochlorothrix hollandica (a recently discovered, free-living, filamentous form) have been suggested to be modern counterparts of the ancestor of the green chloroplasts because they are prokaryotes that also contain both chlorophylls a and b, and lack phycobilins. We report here a 16S rRNA-based phylogenetic analysis of P. hollandica. The organism is found to fall within the cyanobacterial line of descent, as do the green chloroplasts, but it is not a specific relative of green chloroplasts. Thus, similar pigment compositions do not necessarily reflect close evolutionary relationships.