The relationship of a prochlorophyte Prochlorothrix hollandicato green chloroplasts

Abstract

It is generally accepted that chloroplasts arose from one or more endosymbiotic events between an ancestral cyanobacterium and a eukaryote¹. Such an origin fits well in the case of the chloroplasts of rhodophytes that, like cyanobacteria, contain chlorophyll a and phycobilin pigments². 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 pigments³. The 'prochlorophytes'Prochloron didemni (an exosymbiont on didemnid ascidians^4,5) and Prochlorothrix hollandica(a recently discovered, free-living, filamentous form⁶) 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 ft, and lack phycobilins^7,8. 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.