Apicoplast

An apicoplast is a derived non-photosynthetic plastid found in most Apicomplexa, including malaria parasites such as Plasmodium falciparum, but not in others such as Cryptosporidium. It originated from an alga (there is debate as to whether this was a green or red alga) through secondary endosymbiosis. The apicoplast is surrounded by four membranes within the outermost part of the endomembrane system.

Apicoplasts are a relict, nonphotosynthetic plastid found in most protozoan parasites belonging to the phylum Chromalveolata. Among the most infamous Chromalveolata parasites is Plasmodium falciparum, a causative agent of severe malaria. Because apicoplasts are vital to parasite survival, they provide an enticing target for antimalarial drugs. Specifically, apicoplasts' plant-like properties provide a target for herbicidal drugs. And, with the emergence of malarial strains resistant to current treatments it is paramount that novel therapies, like herbicides, are explored and understood. Furthermore, herbicides may be able to specifically target the parasite's plant-like apicoplast and without any noticeable effect on the mammalian host's cells.

Evidence suggests that the apicoplast is a product of secondary endosymbiosis, and that the apicoplast may be homologous to the secondary plastid of the closely related dinoflagellate algae. An ancient cyanobacterium was first engulfed by a eukaryotic cell but was not digested. The bacterium escaped being digested because it formed a symbiotic relationship with the host eukaryotic cell; both the eukaryote and the bacterium mutually benefited from their novel shared existence. The result of the primary endosymbiosis was a photosynthetic eukaryotic alga. A descendent of this eukaryotic alga was then itself engulfed by a heterotrophic eukaryote with which it formed its own symbiotic relationship and was preserved as a plastid. The apicoplast plastid evolved in its new role to preserve only those functions and genes necessary to beneficially contribute to the host-organelle relationship. The ancestral genome of more than 150 kb was reduced through deletions and rearrangements to its present 35 kb size. During the reorganization of the plastid the apicoplast lost its ability to photosynthesize. These losses of function are hypothesized to have occurred at an early evolutionary stage in order to have allowed sufficient time for the complete degradation of acknowledged photosynthetic relicts and the disappearance of a nucleomorph.

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