The origin of chloroplasts and the position of eukaryotic algae in the six-kingdom system of life

Symbiosis represents a widely distributed principle of mutual relationships among organisms, belonging to different systematic groups. The cell endosymbiosis is considered to be the most important event, which starts the evolution of all eukaryotic organisms. We recognize about five successful endosymbioses. Some of them resulted in obtaining mitochondria the peroxisomes, perhaps derived from different groups of bacterial ancestors. The special interest represents the origin of the chloroplasts. As it was suggested using the molecular methods, the single-celled cyanobacteria were the ancestors of chloroplasts in glaucophytes, red and green algae. All these groups are classified into kingdom Plantae. Green algae and vascular plants form a monophyletic group. Their chloroplasts are surrounded only by double membrane envelopes, no connection with endoplasmic reticulum or with the nuclear envelope were observed. Endosymbiosis with cyanobacterial chloroplast ancestor is considered to be the primary endosymbiosis. It was of an ancient origin with the age of about 2 billion years. Chloroplasts of other algal divisions were obtained via secondary, sometimes even as tertiary endosymbioses. In dinophytes and euglenophytes, the plastids are the remainder of single celled eukaryotic algae, covered with the triple envelope membranes. Two inner membranes are similar to the chloroplast envelopes in plant chloroplasts, the third outer membrane is of unknown origin. Some experts consider the third plastid membrane as a phagosome (food vacuole) membrane. No connections with the nuclear envelope were observed. In cryptophytes and chromophyte algae (Chromophyta, syn. Heterocontophyta, syn. Ochrista) and Haptophyta (syn. Prymnesiophyta) plastids are covered with four membranes, two additional membranes are derived from the rough endoplasmic reticulum, which is often connected with the outer membrane of nuclear envelope. This kind of endosymbiosis is called the secondary symbiosis, and is much younger than the primary symbioses. Taking in account the six- kingdoms system of life proposed by CAVALIER-SMITH (1998), the secondary endosymbioses are classified into two different kingdoms: Protozoa (Dinophyta, Euglenophyta) and Chromista. The primary endosymbioses form the kingdom Plantae. The classification of Chlorarachniophyta is not clear and recently they are not mentioned as a separate division. Besides Chlorarachnion reptans, some other filarplasmodium forming protozoans containing plastids (Chrysarachnion, Chrysophyceae, Myxochloris, Xanthophyceae) or without plastids (Corallomyxa, Reticulosphaera, GRELL 1991) were recognized