Apicomplexans

The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. The organelle is an adaptation that the apicomplexan applies in penetration of a host cell.

The Apicomplexa are unicellular and spore-forming. All species are obligate endoparasites of animals, except '' Nephromyces'', a symbiont in marine animals, originally classified as a chytrid fungus. Motile structures such as flagella or pseudopods are present only in certain gamete stages.

The Apicomplexa are a diverse group that includes organisms such as the coccidia, gregarines, piroplasms, haemogregarines, and plasmodia.
Diseases caused by Apicomplexa include:
* Babesiosis (''Babesia'')
* Malaria (''Plasmodium'')
* Cryptosporidiosis (''Cryptosporidium parvum'')
* Cyclosporiasis (''Cyclospora cayetanensis'')
* Cystoisosporiasis (''Cystoisospora belli'' (formerly known as "Isospora Belli"))
* Toxoplasmosis (''Toxoplasma gondii'')

The name of the taxon Apicomplexa derives from two Latin words—''apex'' (top) and ''complexus'' (infolds)—and refers to a set of organelles in the sporozoite. The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group of parasitic protozoans, in general without flagella, cilia, or pseudopods. Most of the Apicomplexa are motile, however, by use of a gliding mechanism
that uses adhesions and small static myosin motors. The other main lines were the Ascetosporea (now in Rhizaria), the Myxozoa (now known to be highly derived cnidarian animals), and the Microsporidia (now known to be derived from fungi). Sometimes, the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.

Description

The phylum Apicomplexa contains all eukaryotes with a group of structures and organelles collectively termed the apical complex. This complex consists of structural components and secretory organelles that are required for invasion of host cells during the parasitic stages of the Apicomplexan life cycle. Apicomplexa have complex life cycles, involving several stages and typically undergoing both asexual and sexual replication. All Apicomplexa are obligate parasites for some portion of their life cycle, with some parasitizing two separate hosts for their asexual and sexual stages.

Besides the conserved apical complex, Apicomplexa are morphologically diverse. Different organisms within Apicomplexa, as well as different life stages for a given apicomplexan, can vary substantially in size, shape, and subcellular structure. Like other eukaryotes, Apicomplexa have a nucleus, endoplasmic reticulum and Golgi complex. Apicomplexa generally have a single mitochondrion, as well as another endosymbiont-derived organelle called the apicoplast which maintains a separate 35 kilobase circular genome (with the exception of '' Cryptosporidium'' species and ''Gregarina niphandrodes'' which lack an apicoplast).

All members of this phylum have an infectious stage—the sporozoite—which possesses three distinct structures in an apical complex. The apical complex consists of 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) surrounded by one or two polar rings may also be present. This structure gives the phylum its name. A further group of spherical organelles is distributed throughout the cell rather than being localized at the apical complex and are known as the dense granules. These typically have a mean diameter around 0.7 μm. Secretion of the dense-granule content takes place after parasite invasion and localization within the parasitophorous vacuole and persists for several minutes.
* Flagella are found only in the motile gamete. These are posteriorly directed and vary in number (usually one to three).
* Basal bodies are present. Although hemosporidians and piroplasmids have normal triplets of microtubules in their basal bodies, coccidians and gregarines have nine singlets.
* The mitochondria have tubular cristae.
* Centrioles, chloroplasts, ejectile organelles, and inclusions are absent.
* The cell is surrounded by a pellicle of three membrane layers (the alveolar structure) penetrated by micropores.

Replication:
* Mitosis is usually closed, with an intranuclear spindle; in some species, it is open at the poles.
* Cell division is usually by schizogony.
* Meiosis occurs in the zygote.

Mobility:

Apicomplexans have a unique gliding capability which enables them to cross through tissues and enter and leave their host cells. This gliding ability is made possible by the use of adhesions and small static myosin motors.

Other features common to this phylum are a lack of cilia, sexual reproduction, use of micropores for feeding, and the production of oocysts containing sporozoites as the infective form.

Transposons appear to be rare in this phylum, but have been identified in the genera ''Ascogregarina'' and ''Eimeria''.

Life cycle

Most members have a complex lifecycle, involving both asexual and sexual reproduction. Typically, a host is infected via an active invasion by the parasites (similar to entosis), which divide to produce sporozoites that enter its cells. Eventually, the cells burst, releasing merozoites, which infect new cells. This may occur several times, until gamonts are produced, forming gametes that fuse to create new cysts. Many variations occur on this basic pattern, however, and many Apicomplexa have more than one host.

The apical complex includes vesicles called rhoptries and micronemes, which open at the anterior of the cell. These secrete enzymes that allow the parasite to enter other cells. The tip is surrounded by a band of microtubules, called the polar ring, and among the Conoidasida is also a funnel of tubulin proteins called the conoid. Over the rest of the cell, except for a diminished mouth called the micropore, the membrane is supported by vesicles called alveoli, forming a semirigid pellicle.

The presence of alveoli and other traits place the Apicomplexa among a group called the alveolates. Several related flagellates, such as '' Perkinsus'' and ''Colpodella'', have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of the dinoflagellates. They are probably similar to the common ancestor of the two groups.

Another similarity is that many apicomplexan cells contain a single plastid, called the apicoplast, surrounded by either three or four membranes. Its functions are thought to include tasks such as lipid and heme biosynthesis, and it appears to be necessary for survival. In general, plastids are considered to have a common origin with the chloroplasts of dinoflagellates, and evidence points to an origin from red algae rather than green.

Subgroups

Within this phylum are four groups — coccidians, gregarines, haemosporidians (or haematozoans, including in addition piroplasms), and marosporidians. The coccidians and haematozoans appear to be relatively closely related.

''Perkinsus '', while once considered a member of the Apicomplexa, has been moved to a new phylum — Perkinsozoa.

Gregarines

The gregarines are generally parasites of annelids, arthropods, and mollusks. They are often found in the guts of their hosts, but may invade the other tissues. In the typical gregarine lifecycle, a trophozoite develops within a host cell into a schizont. This then divides into a number of merozoites by schizogony. The merozoites are released by lysing the host cell, which in turn invade other cells. At some point in the apicomplexan lifecycle, gametocytes are formed. These are released by lysis of the host cells, which group together. Each gametocyte forms multiple gametes. The gametes fuse with another to form oocysts. The oocysts leave the host to be taken up by a new host.

Coccidians

In general, coccidians are parasites of vertebrates. Like gregarines, they are commonly parasites of the epithelial cells of the gut, but may infect other tissues.

The coccidian lifecycle involves merogony, gametogony, and sporogony. While similar to that of the gregarines it differs in zygote formation. Some trophozoites enlarge and become macrogamete, whereas others divide repeatedly to form microgamete