From extracellular to intracellular: the establishment of mitochondria and chloroplasts

Abstract

Paracoccus and Rhodopseudomonas are unusual among bacteria in having a majority of the biochemical features of mitochondria; blue-green algae have many of the features of chloroplasts. The theory of serial endo-symbiosis proposes that a primitive eukaryote successively took up bacteria and blue-green algae to yield mitochondria and chloroplasts respectively. Possible characteristics of transitional forms are indicated both by the primitive amoeba, Pelomyxa , which lacks mitochondria but contains a permanent population of endosymbiotic bacteria, and by several anomalous eukaryotic algae, e. g. Cyanophora , which contain cyanelles instead of chloroplasts. Blue-green algae appear to be obvious precursors of red algal chloroplasts but the ancestry of other chloroplasts is less certain, though the epizoic symbiont, Prochloron , may resemble the ancestral green algal chloroplast. We speculate that the chloroplasts of the remaining algae may have had a eukaryotic origin. The evolution of organelles from endosymbiotic precursors would involve their integration with the host cell biochemically, structurally and numerically.