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Endosymbiosis

Once considered a relative rarity, endosymbiosis, the living together of one organism inside another, has increasingly become recognized as a major factor in the evolution of life forms. The word endosymbiosis comes from Greek words meaning "inside," "with," and "living." Endosymbiosis in biology is a subdivision of the more general concept, symbiosis, which refers to living beings of different species living together for most of the life history of a member of at least one of those species. (In the case of the bacteria it suffices to say "living together of different types" because bacteria often cannot clearly be assigned to species.) Ectosymbiosis is a more familiar notion, an association between organisms of different species where one is attached in some way to the outside of the other. Barnacles adhere to the hairy, wet surfaces of whales where the pattern of barnacle distribution is used by whales to distinguish each other. This is one example of ectosymbiosis.

Endosymbiosis often takes symbiosis proper a step further. As in sexual reproduction, genes from two beings come together giving added abilities to the mutual organism. Unlike in sex, however, the two organisms do not necessarily come apart immediately after their fusion. They may dwell in the same body forever. Indeed, permanent symbiosis has been proven as a means of producing new organisms.

The most stunning and momentous example of endosymbiosis is perhaps that of the photosynthetic parts of algal and plant cells, called plastids, which are now believed to have once been free-living photosynthetic bacteria. Red plastids of red algae are called rhodoplasts. The more familiar green plastids are called chloroplasts. The plastids that give plants and algae their metabolic ability to use light to produce chemical food and energy are the same size, shape, and composition as photosynthetic bacteria. They also divide to reproduce by a process of fission—distinct from the complex mitotic division found in all nonbacterial cells with nuclei, such as plant, algae, and fungal cells. Genetic similarities in long stretches of deoxyribonucleic acid (DNA) show definitively that rhodoplasts are very closely related to cyanobacteria (oxygen-producing, green-tinged bacteria). Therefore, the direct link between cyanobacteria and the plastids of algae and plants is one of ancestry. Free-living cyanobacteria merged with nonphotosynthetic ancestors of the algal cell, including the algae that evolved into plants. Ancestors of plant cells, in other words, acquired their plastids, once free-living cyanobacteria, by endosymbiosis.

Plastids are one of a class of membrane-bounded cell structures called organelles. Others include mitochondria, bodies that react with oxygen to produce energy for the rest of the cell in which they reside. Mitochondria also contain their own DNA and are thought to be the descendants of formerly free-living bacteria. The details of how plastids, mitochondria, and other organelles came to live in permanent endosymbiosis with cells are complicated. The original union leading to the origin of plastids, however, is easy to envision. Some hungry, translucent protists ate delicious photosynthetic cyano-bacteria and failed to digest them. In the light the cyanobacteria could not help but continue its photosynthesis. Hence, the merger, now a green cell, evolved from its cyanobacterial and translucent ancestors. With the passage of time the association became permanent, and resulted in the evolution of algae. Genes between the two types of life were exchanged. Eventually plants evolved from the endosymbiotic union.