Apicomplexa
Parasitic protozoans with apical complex for host cell invasion.
The Apicomplexa (also called Apicomplexia; single: apicomplexan) are a large phylum of mainly parasitic alveolates, a kind of protozoan. Most possess a unique form of organelle structure comprising a non-photosynthetic plastid called an apicoplast, with an apical complex membrane adapted for penetrating host cells. They are unicellular, spore-forming, and mostly obligate endoparasites of animals, except Nephromyces, a symbiont in marine animals originally classified as a chytrid fungus. Diseases caused by Apicomplexa include malaria, toxoplasmosis, babesiosis, cryptosporidiosis, cyclosporiasis, and cystoisosporiasis.
- field
- Parasitic protozoan phylum
- known_for
- Causing diseases such as malaria and toxoplasmosis; possessing an apical complex for host cell invasion
- characteristics
- Unicellular, spore-forming, obligate endoparasites; have apicoplast; complex life cycles with asexual and sexual reproduction
Lore & Background
The Apicomplexa derive their name from two Latin words—apex (top) and complexus (infolds)—referring to the set of organelles in the sporozoite. They comprise the bulk of what was formerly called Sporozoa, a group of parasitic protozoans generally without flagella, cilia, or pseudopods. Most Apicomplexa can use gliding motility, which relies on adhesions and small static myosin motors. The phylum includes diverse organisms such as coccidia, gregarines, piroplasms, haemogregarines, and plasmodia. All members have an infectious stage—the sporozoite—which possesses three distinct structures in an apical complex: a set of spirally arranged microtubules (the conoid), a secretory body (the rhoptry), and one or more polar rings. Additional slender electron-dense secretory bodies (micronemes) may also be present. The cell is surrounded by a pellicle of three membrane layers (the alveolar structure) penetrated by micropores. Flagella are found only in the motile gamete, and mitochondria have tubular cristae. Most members have a complex lifecycle involving both asexual and sexual reproduction. Typically, a host is infected via active invasion by the parasites, which divide to produce sporozoites that enter cells. Eventually, cells burst, releasing merozoites that infect new cells. This may occur several times until gamonts produce gametes that fuse to create new cysts. Many variations occur, and many Apicomplexa have more than one host.
Reader's Guide
The Apicomplexa are significant as a phylum of obligate parasites that cause major diseases in humans and animals, including malaria (Plasmodium), toxoplasmosis (Toxoplasma gondii), babesiosis (Babesia), cryptosporidiosis (Cryptosporidium parvum), cyclosporiasis (Cyclospora cayetanensis), and cystoisosporiasis (Cystoisospora belli). Their unique apical complex—comprising the conoid, rhoptries, micronemes, and polar rings—is a key adaptation for invading host cells, making them a focus of research into parasitic mechanisms and potential therapeutic targets. The presence of the apicoplast, a non-photosynthetic plastid derived from red algae, is essential for survival and offers a target for drug development. Their complex life cycles, involving both asexual and sexual replication often across multiple hosts, contribute to their persistence and pathogenicity. The phylum also includes symbionts like Nephromyces, and its classification has shifted over time, with Perkinsus now placed in a separate phylum (Perkinsozoa). Understanding Apicomplexa is crucial for controlling the diseases they cause and for studying eukaryotic cell biology and evolution.
Did You Know?
- The name Apicomplexa derives from Latin words meaning 'top' and 'infolds,' referring to the apical complex of organelles in the sporozoite.
- Most Apicomplexa possess a non-photosynthetic plastid called an apicoplast, which is essential for survival and has a separate 35 kilobase circular genome.
- Apicomplexa have a unique gliding motility that uses adhesions and small static myosin motors, enabling them to cross tissues and enter host cells.
- The phylum includes organisms such as coccidia, gregarines, piroplasms, haemogregarines, and plasmodia, causing diseases like malaria and toxoplasmosis.
Frequently Asked Questions
Who is Apicomplexa?
Apicomplexa is a large phylum of single-celled parasitic organisms that belong to the alveolate branch of protozoa. Nearly all members are obligate endoparasites that live inside animal hosts, though the rare exception Nephromyces forms a symbiotic relationship with marine animals.
What are Apicomplexa's signature abilities?
Their defining trait is a specialized apical complex of membrane structures that lets them bore into and invade host cells with remarkable precision. They also harbor a non-photosynthetic plastid called the apicoplast, a cellular feature found nowhere else in the protist world.
What diseases does Apicomplexa cause?
Members of this phylum are behind several major human and veterinary illnesses, including malaria, toxoplasmosis, babesiosis, cryptosporidiosis, and cyclosporiasis. These infections affect hundreds of millions of people and countless livestock animals worldwide.
How does Apicomplexa reproduce?
They cycle through intricate life stages that alternate between asexual multiplication and sexual recombination, often switching between different host organisms. They are also spore-forming, which helps them survive outside a host and transmit between individuals.
Why is Apicomplexa important to the field?
As one of the most medically significant groups of parasitic protozoa, they drive enormous global health burdens and are a central focus of parasitology and drug-development research. Their unique invasion machinery and apicoplast make them a key model for studying how parasites manipulate host biology.
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