Endosymbionta

Endosymbionta, or endosymbionts, are organisms that live inside the body or cells of another organism (the host) for an extended period, often to mutual benefit. Most known endosymbionts are bacteria, but they can include algae, fungi, and other microbes. They may inhabit specialized host cells or tissues and are typically transmitted from parent to offspring.

Relationships range from mutualistic to parasitic; many endosymbionts provide essential nutrients or metabolic capabilities to their hosts, while hosts offer shelter and resources. Some endosymbionts are essential for host survival (obligate symbionts) whereas others persist only when present (facultative symbionts). Transmission is frequently vertical, ensuring the symbiosis persists across generations, though horizontal acquisition occurs in some systems.

Endosymbiotic theory states that mitochondria and chloroplasts originated from ancient free-living bacteria that entered into endosymbiotic relationships with early eukaryotes. Over time, most of the endosymbiont's genes migrated to the host genome, and the organelles became integral components of the cell.

Well-known examples include mitochondria derived from alpha-proteobacteria and chloroplasts from cyanobacteria. In insects, Buchnera aphidicola provides essential amino acids to aphids, while Wolbachia affects reproduction in many arthropods. Blattabacterium accompanies cockroaches as a nitrogen recycling symbiont, and Sulcia and Nasuia bacteria live in sap-feeding insects to supplement amino acids. Tremblaya mealybug symbionts represent a complex multi-bacterial endosymbiosis.

Endosymbiont genomes are typically reduced and highly adapted to intracellular life, often losing many genes unnecessary in the host environment. This genome reduction and reliance on host cellular machinery reflect long-term co-evolution and intimate integration.

Research on endosymbionts informs evolutionary biology, ecology, and biotechnology. Some endosymbionts are explored for disease control, such as Wolbachia-based strategies to reduce transmission of vector-borne diseases.