Endosymbionterna

Endosymbionterna are microorganisms that live inside the cells or body tissues of a host organism in a symbiotic relationship. They are most commonly bacteria, but other microorganisms can also be endosymbiotic. The interactions range from mutualistic, where both partners benefit, to parasitic or manipulative, where the endosymbiont benefits at the host’s expense. In many cases, the endosymbiont provides essential services to the host, while the host offers a protected habitat and nutrients.

Endosymbiosis is a key process in the evolution of eukaryotic cells. The most famous examples are organelles such as mitochondria and plastids (including chloroplasts), which are widely regarded as descended from ancient endosymbiotic bacteria. Evidence for this origin includes double membranes around the organelles, circular DNA genomes, ribosomes that resemble bacterial ribosomes, and similarities to free-living relatives.

Notable contemporary endosymbionts include Buchnera aphidicola in aphids, which supplies essential amino acids missing from the insect diet, and Wolbachia species that infect a large fraction of arthropods and can influence reproduction and fitness. In marine and other systems, endosymbiotic algae and other microbes reside inside host cells and contribute photosynthetic or metabolic products that support the host.

Transmission of endosymbionts is often vertical, from parent to offspring, leading to long-term co-evolution and, in many cases, obligate dependence. Some endosymbionts are strictly intracellular and cannot survive outside the host, shaping the biology and ecology of both partners. Research on endosymbionts informs fields from evolutionary biology to agriculture and disease control, highlighting the integral role of intracellular symbioses in living systems.