Is the Plastid an Endosymbiont?

Hans Ris proposed in 1961 that chloroplasts might be highly derived endosymbiotic microorganisms, originally related to blue-green algae. Evidence from 1966 and earlier is reviewed to test this proposal. Elegant experiments using the unique genetic system offered by Oenothera (the evening primroses) clearly show that plastids carry heritable characters not under nuclear control. Two or even three distinctive kinds of plastids may coexist and retain their identities in a single line of heteroplastidic cells. The distinctive characters of a line of plastids were even maintained in contact with a foreign nuclear genome for more than 10 generations of reproduction of the host plant. Studies in Epilobium, corn, tobacco, and other plants further demonstrate the heritability and mutability of an independent plastid genome. Time-lapse microcinematography, electron microscopy, histochemistry, cell fractionation, tracer and biochemical studies, and DNA hybridization all show that plastids are reproduced only from pre-existing plastids, that they contain DNA differing in many traits from nuclear DNA, that they contain their unique ribosomes, and that even when isolated in vitro or in enucleated cells they still synthesize their own DNA, transcribe at least some RNA, and synthesize some protein. In all of these characters plastids more closely resemble complete blue-green algea than they do other parts of the eukaryote cell.