Endosymbiosis
What is endosymbiosis?
What evidence supports the endosymbiotic theory of organelle origin?
What is secondary endosymbiosis?
Endosymbiosis, phenomenon in which one organism lives within the body of an unrelated organism, usually in a mutually beneficial relationship, although the relationship could also be parasitic. The term derives from the Greek endo- (meaning āwithinā) and symbiosis (meaning āliving togetherā). The organism living inside its partner organism is called an endosymbiont, and the partner harboring the endosymbiont is called the host. Endosymbiosis has shaped biological evolution, and research suggests that it still plays a role in the natural history of diverse organisms.
An evolutionary tool
Scientists suspect that endosymbiosis played a fundamental role in the evolution of the eukaryotic cell. More specifically, it led to the formation of the organelles mitochondria and chloroplasts, which added to the complexity of the eukaryotic cell and allowed it to thrive in diverse environments. The widely accepted theory is that these organelles evolved from prokaryotic (or proto-eukaryotic) organisms, possibly alphaproteobacteria. These organisms were either in symbiotic association with, engulfed as food by, or parasites of other prokaryotic cells, presumably Asgard archaeaāa group of archaea thought to be the closest prokaryotic relative of eukaryotes. Over millions of years, the internalized prokaryotic cells underwent genome reduction and lost their ability to survive independently of their hosts, in time integrating in the host cells as organelles and conferring on them abilities such as energy production (through mitochondria) and energy transduction (through chloroplasts).
Mitochondria and chloroplasts of eukaryotic cells are unique because they contain their own circular DNA, which is similar to that of bacteria, and replicate independently of the cellās nuclear DNA (but with the help of nuclear-encoded proteins). These organelles divide by splitting in two, as bacteria do, and cannot be formed anew if absent in daughter cells, indicating that the host cell lacks the necessary genetic instructions to produce them. Moreover, the membranes of these organelles resemble those of prokaryotes. Together, these features support the endosymbiotic theory of their origināthese organelles originated as free-living organisms that were incorporated into eukaryotic cells over evolutionary time.
Biologists use the term serial endosymbiosis to describe the sequential endosymbiotic events that gave rise to eukaryotic organelles such as mitochondria and chloroplasts. Owing to their near-universal presence in all eukaryotic cells, most scientists suspect that mitochondria originated before chloroplasts. An exception is the gut microorganism Monocercomonoides, in which mitochondria have been secondarily lost. Advances in imaging and molecular biology have revealed other endosymbiosis-derived structures, such as the recently described nitrogen-fixing nitroplast and the photosynthetic chromatophore in Paulinella, illustrating that endosymbiosis continues to influence cellular complexity.
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An aid to survival
The influence of endosymbiosis extends beyond its role in shaping the evolution of the eukaryotic cell. Several multicellular organisms, particularly invertebrates, have obligate symbiotic prokaryotes living inside their cells and tissues in a mutually beneficial relationship. For example, one estimate suggests that intracellular microbial endosymbionts help about 10ā15 percent of all insects meet their dietary needs. In most cases, these insects feed on unbalanced diets, such as blood, plant sap, and grain, which tend to lack essential nutrients such as amino acids and vitamins that the endosymbiont provides. Host organisms may also depend on endosymbionts for fixing carbon and atmospheric nitrogen to survive under extreme conditions such as in deep-sea hydrothermal vents.
At the other end of the spectrum from mutually beneficial endosymbiosis is parasitic endosymbiosis. In this type of relationship, the parasite spreads within insect populations by altering the hostās reproduction. It skews the host offspring sex ratio so that more infected females are produced. The parasite achieves this by employing several different strategies, such as triggering cytoplasmic incompatibility, parthenogenesis, male killing, and feminization of male offspring.
The reproductive effects exerted by the bacterium Wolbachia have been used to control insect pests and disease vectors such as the dengue-, Zika-, and chikungunya-carrying mosquito Aedes aegypti. When Wolbachia-infected male mosquitoes mate with uninfected females, the embryos fail to develop. This phenomenon, called cytoplasmic incompatibility, also occurs when male and female mosquitoes carrying different strains of the bacterium mate. The bacterium may also reduce egg production, body size, development rate, and the dispersal ability of its insect hosts, although these effects vary depending on the host species and the bacterial strain.
Some strains of Wolbachia are beneficial to their hosts and produce essential nutrients required for survival, as seen in bedbugs and plant hoppers. They may also protect their insect hosts from viral infections.
The integration of a prokaryotic endosymbiont into a eukaryotic host involves specific cellular and genomic changes. In insects, for example, endosymbiont genomes have been reduced to the point that some scientists consider them analogous to cellular organelles. Similar to mitochondria and chloroplasts, these endosymbionts are maternally transmittedāthey are transferred from the host to its offspring via host eggs. In parasitic endosymbiosis, as seen between the gram-negative bacteria Wolbachia and several species of insects, this may affect the host offspring sex ratio. Maternal inheritance is not the only mode of endosymbiont acquisition; in less obligatory associations hosts may also acquire them from the environment.
Types and examples
Depending on the nature of the participants, endosymbiosis is classified as primary or secondary. Primary endosymbiosis gave rise to mitochondria and chloroplasts and involved primitive unicellular prokaryotes (or proto-eukaryotes) and archaea. Secondary endosymbiosis, on the other hand, refers to the internalization of a unicellular organism that already possesses organelles formed through primary endosymbiosis. This process contributed to the emergence of many diverse and complex eukaryotic life-forms on Earth. The presence of three- and four-membraned plastids in euglenoid algae, diatoms, haptophytes, and dinoflagellatesāformed when a primary alga with double-membraned plastids was engulfed by a non-photosynthetic unicellular eukaryoteāprovides evidence for secondary endosymbiosis. In some cases, eukaryotes formed through secondary endosymbiosis retain a highly reduced remnant of the engulfed algal nucleus, called a nucleomorph, which persists between the inner and outer membranes of the plastid, as observed in cryptophyte and chlorarachniophyte algae.
Endosymbiosis can also be classified by the degree of dependence between the host and the endosymbiont. In obligate endosymbiosis, the endosymbiont is so integrated at the cellular and genomic levels that it cannot survive independently, as seen in the nutritional symbionts of insects such as the bacterium Buchnera aphidicola found in aphids. In facultative endosymbiosis, the symbiont can live independently, as in the case of nitrogen-fixing Rhizobium bacteria in the root nodules of leguminous plants. Between these extremes lies a spectrum of associations with varying levels of dependence. One example is the photosynthetic zooxanthellae of corals, which can survive independently of their host, although the host coral usually dies upon losing the endosymbiont, its source of photosynthetic nutrients. This suggests that the mutualistic relationship is obligatory for the host but facultative for the endosymbiont (see also coral bleaching). A rare example of nested endosymbiosis is seen in the citrus mealybug (Planococcus citri). In these insects a betaproteobacterium (the primary endosymbiont) lives inside specialized host cells called bacteriocytes and itself contains a secondary endosymbiont, a gammaproteobacterium. The two bacterial partners are functionally linked, sharing roles in processes such as cellular transport, protein processing, and DNA repair and metabolic exchange. The mealybug depends on this unique tripartite arrangement to obtain essential amino acids for its survival.
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