endosymbios

Endosymbiosis is a biological interaction in which one organism, the endosymbiont, lives inside the cells of another organism, the host, forming a long-term, often mutualistic association. It is a key concept in the evolution of eukaryotic cells and is used to explain how complex cellular structures originated.

The endosymbiotic theory posits that essential organelles such as mitochondria and chloroplasts originated as free-living prokaryotes that became integrated into a host cell. The idea gained prominence through the work of Lynn Margulis in the 20th century, supported by multiple lines of evidence. Mitochondria and plastids contain circular DNA, have ribosomes similar to bacterial 70S ribosomes, possess double membranes, replicate by fission, and share similarities with certain bacteria in their biochemistry and genetics.

Primary endosymbiosis describes the initial uptake of a prokaryote by a host, giving rise to mitochondria (from proteobacteria) and chloroplasts (from cyanobacteria) in plants and algae. Secondary endosymbiosis involves a eukaryotic host engulfing another eukaryotic alga, leading to chloroplasts with additional membranes, a phenomenon observed in several algal lineages. Some groups have also experienced tertiary endosymbiosis. In addition to organelles, many organisms host intracellular endosymbiotic bacteria that contribute to metabolism, such as Buchnera in aphids, illustrating the diversity and ecological importance of endosymbiosis.

Endosymbiosis has profoundly influenced cellular complexity and biological diversity, shaping energy production, biosynthesis, and ecological relationships across life.