endosymbioses

Endosymbiosis is a form of symbiosis in which one organism lives inside the body or cells of another. In biology, endosymbiosis is often invoked to explain the origin of key eukaryotic organelles, notably mitochondria and plastids, through long-term associations between a host cell and one or more bacterial partners. These relationships can be obligate, where the host depends on the endosymbiont, or facultative, where both partners benefit but can survive separately.

Primary endosymbiosis refers to an ancestral eukaryotic host that internalized an aerobic bacterium, which evolved into mitochondria, followed by the uptake of a cyanobacterium that gave rise to plastids such as chloroplasts in plants and many algae. Evidence for this includes the double-membrane structure of these organelles, their circular DNA and 70S ribosomes similar to bacteria, and widespread transfer of genes from the endosymbiont to the host nucleus. A modern example of a recent primary endosymbiosis is Paulinella chromatophora, whose chromatophores originate from a cyanobacterium.

Secondary and higher-order endosymbiosis occur when a eukaryotic host engulfs a photosynthetic eukaryote, leading to plastids surrounded by additional membranes. This explains plastids in many algal groups and explains features such as remnants called nucleomorphs in cryptomonads and chlorarachniophytes. Apicomplexans (including Plasmodium) harbor apicoplasts derived from red algal endosymbiosis; other lineages show plastids from green algal ancestors. Some dinoflagellates exhibit tertiary endosymbiosis, involving further rounds of engulfment.

Endosymbiosis is a major driver of eukaryotic evolution, enabling aerobic respiration and photosynthesis and shaping genome organization through endosymbiotic gene transfer. It remains a central topic in evolutionary biology and cell biology.