serial endosymbiotic theory

(progressive endosymbiotic theory) The proposal that plastids, mitochondria, and possibly also cilia and flagella arose from symbiotic prokaryotic organisms living within a eukaryotic host cell. It is thought that plastids probably originated from organisms similar to present-day blue-green algae while mitochondria arose from aerobic bacteria. Such conclusions are based on various similarities between plastids and mitochondria and free-living prokaryotes. For example, such organelles are self replicating and contain DNA, which in addition to having a different base composition from the nuclear DNA, is circular rather than linear. The ribosomes of mitochondria and chloroplasts are smaller (70S) than those in other parts of the cytoplasm (80S) but similar in size to those of prokaryotes. There is evidence moreover, that the origin of plastids could be polyphyletic, i.e. in different groups of plants different types of symbionts have been incorporated. Thus in the Chlorophyta, Rhodophyta, and higher plants, which all have chloroplasts with a double membrane, it is thought that the plastids are derived from a prokaryotic symbiont. However in other groups of algae the chloroplasts may be surrounded by three or four membranes, implying that these are derived from eukaryotic symbionts. Such evidence has been used in revising classifications at the kingdom and division level.