SYMBIOGENETIC EVOLUTION OF COMPLEX CELLS AND COMPLEX PLASTIDS

it is generally accepted today that mitochondria and plastids of eukaryotic cells (''eucytes'') have their phylogenetic origins in prokaryotic cells (''protocytes'') that had been taken up into urkaryotic host cells as intracellular symbionts, This concept, strongly supported (among other evidence) by comparisons of rRNA and protein sequence data, has many important consequences for understanding both cellular evolution and cellular compartmentation. According to the Serial Endosymbiont Theory (SET), the eucyte came about by the formation of stable intracellular symbioses of quite different cells. The formation of such symbioses is referred to here as intertaxonic combination (ITC). In addition to mutation and genetic recombination, ITC emerges as a third progressive power in evolution. The situation can be complicated by repeated ITC. This is discussed in detail by taking the evolution of complex plastids as an example. Plastids of this kind, possessing 3 or 4 enveloping membranes instead of 2, are widespread in algae. They appear to be remnants of eukaryotic, and phototrophic, endocytobionts in phagotrophic host cells. The phylogeny of complex plastids could recently be fully reconstructed in the case of cryptomonads, and partly also in the case of Chlorarachnion.