Apicomplexa
Parasitic protozoans with apical complex for host cell invasion.
The Apicomplexa (singular: apicomplexan) are a large phylum of parasitic protozoans, classified as alveolates. Most have a distinctive organelle called an apicoplast—a non-photosynthetic plastid with an apical complex membrane—which helps them invade host cells. These single-celled organisms produce spores and are almost all obligate endoparasites of animals, with the exception of *Nephromyces*, a marine symbiont once thought to be a chytrid fungus. Flagella or pseudopods appear only in certain gamete stages. The group includes coccidia, gregarines, piroplasms, haemogregarines, and plasmodia. Diseases they cause include babesiosis, malaria, cryptosporidiosis, cyclosporiasis, cystoisosporiasis, and toxoplasmosis.
The name comes from Latin *apex* (top) and *complexus* (infolds), referring to the organelles in the sporozoite. Apicomplexa make up most of what was formerly called Sporozoa—parasitic protozoans generally lacking flagella, cilia, or pseudopods. Most use gliding motility, relying on adhesions and small static myosin motors. The obsolete Sporozoa group also included Ascetosporea, Myxozoa, and Microsporidia. Today, "Sporozoa" is sometimes used as a synonym for Apicomplexa.
All members have an infectious sporozoite stage with an apical complex: spirally arranged microtubules (the conoid), a secretory rhoptry, and one or more polar rings. Additional micronemes and dense granules (about 0.7 μm in diameter) are also present. The cell is surrounded by a three-layered pellicle with micropores. Flagella occur only in motile gametes, are posteriorly directed, and number one to three. Basal bodies exist, but their microtubule structure varies: hemosporidians and piroplasmids have normal triplets, while coccidians and gregarines have nine singlets. Mitochondria have tubular cristae. Centrioles, chloroplasts, ejectile organelles, and inclusions are absent.
Mitosis is usually closed with an intranuclear spindle, though some species have open poles. Cell division occurs by schizogony, and meiosis happens in the zygote. Apicomplexans glide through tissues using adhesions and myosin motors. They lack cilia, reproduce sexually, feed through micropores, and produce oocysts with sporozoites as the infective form. Transposons are rare but found in *Ascogregarina* and *Eimeria*.
Life cycles are complex, involving both asexual and sexual reproduction. Typically, sporozoites actively invade host cells, divide, and burst out as merozoites to infect new cells. This repeats until gamonts form, which produce gametes that fuse into cysts. Many variations exist, and some Apicomplexa use multiple hosts. The apical complex secretes enzymes from rhoptries and micronemes to enter cells. The cell tip has a polar ring and, in Conoidasida, a conoid of tubulin. The rest of the membrane, except for a micropore, is supported by vesicles.
- 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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