|Classes & Subclasses Perkins, 2000|
The Apicomplexa — also called Apicomplexia — are a large group of parasitic protists, most of which possess a unique organelle, a type of plastid called an apicoplast, and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or pseudopods are present only in certain gamete stages. This is a diverse group including organisms such as the coccidia, gregarines, piroplasms, haemogregarines, and plasmodia. Diseases caused by apicomplexan organisms include, but are not limited to:
The name of the taxon Apicomplexa is derived from two Latin words — apex (top) and complexus (infolds) — and refers to a set of organelles in the sporozoite. The older taxon Sporozoa was created by SChrevel in 1971 and grouped the Apicomplexa together with the Ascetosporea, Microsporidia and Myxosporida. This grouping is no longer regarded as biologically valid and its use is discouraged.
This has rarely been studied at the subclass level. Morrison using molecular date has shown that the haemosporidia are related to the gregarines and that the piroplasms and coccidians are sister groups. Other studies suggest that the haemosporidia and the piroplasma appear to be sister clades and are more closely related to the coccidians than to the gregarines. 
Transposons appear to be rare in this phylum but have been identified in the genera Ascogregarina and Eimeria.
All members of this phylum have an infectious stage - the sporozoite - which possess 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. It is this structure that gives the phylum its name.
A further group of spherical organelles are 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 of about 0.7 micrometers. Secretion of the dense-granule content takes place after parasite invasion and localization within the parasitophorous vacuole and persists for several minutes.
Other morphological findings that are common to all members of this phylum include:
Other features common to this phylum are a lack of cilia, sexual reproduction, use of micropores for feeding, movement by body flexion or gliding and the production of oocysts containing sporozoites as the infective form.
Within this phylum there are three groups - coccidians, gregarines and haemosporidians. The coccidians and gregarines appear to be relatively closely related.
Perkinsus while once considered a member of this phylum has been moved to a new phylum - Perkinsozoa.
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 life cycle 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 life cycle, 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.
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, wheras others divide repeatedly to form microgametes (anisogamy). The microgametes are motile and must reach the macrogamete to fertilize it. The fertilized macrogamete forms a zygote that in its turn forms an oocyst that is normally released from the body. Syzygy, when it occurs, involves markedly anisogamous gametes. The lifecycle is typically haploid, with the only diploid stage occurring in the zygote, which is normally short-lived.
The main difference between the coccidians and the gregarines is in the gamonts. In the coccida these are small, intracellular and without epimerites or mucrons. In the gregarines these are large, extracellular and possess epimerites or mucrons.
A second difference between the coccidia and the gregarines also lies in the gamonts. In the coccidia a single gamonts becomes a macrogametocyte, whereas in the gregarines the gamonts give rise to multiple gametocytes.
The Haemosporidians have more complex lifecycles that alternate between an arthropod and a vertebrate host. The trophozoite parasitises erythrocytes or other tissues in the vertebrate host. Microgametes and macrogametes are always found in the blood. The gametes are taken up by the insect vector during a blood meal. The microgametes migrate within the gut of the insect vector and fuse with the macrogametes. The fertilized macrogamete now becomes an ookinete, which penetrates the body of the vector. The ookinete then transforms into an oocyst and divides initially by meiosis and then by mitosis (haplontic life cycle) to give rise to the sporozoites. The sporozoites escape from the oocyst and migrate within the body of the vector to the salivary glands where they are injected into the new vertebrate host when the insect vector feeds again.
All members of this phylum are parasitic and evolved from a free-living ancestor. It is presumed that this lifestyle evolved at the time of the divergence of dinoﬂagellates and apicomplexans. Further evolution of this phylum has been estimated to have occurred ~ . The oldest extant clade is thought to be the archigregarines.
Many Coccidiomorpha have an intermediate host as well as a primary host, and the evolution of hosts proceeded in different ways and at different times in these groups. For some coccidiomorphs, the original host has become the intermediate host, whereas in others it has become the definitive host. In the genera Aggregata, Atoxoplasma, Cystoisospora, Schellackia and Toxoplasma the original is now definitive, whereas in Akiba, Babesiosoma, Babesia, Haemogregarina, Haemoproteus, Hepatozoon, Karyolysus, Leucocytozoon, Plasmodium, Sarcocystis, and Theileria; the original hosts are now intermediate.
Similar strategies to increase the likelihood of transmission have evolved in multiple genera. Polyenergid oocysts and tissue cysts are found in representatives of the orders Protococcidiorida and Eimeriida. Hypnozoites are found in Karyolysus lacerate and most species of Plasmodium; transovarial transmission of parasites occurs in lifecycles of Karyolysus and Babesia.
Horizontal gene transfer appears to have occurred early on in this phylum's evolution with the transfer of a histone H4 lysine 20 (H4K20) modifier, Set8, from an animal host to the ancestor of apicomplexans. A second gene - H3K36 methyltransferase (Ashr3 in plants) - may have also be horizontally transferred.
Most members have a complex life-cycle, 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. There are many variations 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 there 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 semi-rigid 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 3 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.
The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group for parasitic protozoans without flagella, pseudopods, or cilia. Most of the Apicomplexa are motile, however. The other main lines were the Ascetosporea, the Myxozoa (now known to be derived from 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.
Within the Apicomplexa there are three suborders of parasites.
Within the Adelorina are species that infect invertebrates and others that infect vertebrates.
The Haemosporina includes the malaria parasites and their relatives.
The Eimeriorina - the largest suborder in this phylum - the lifecycle involves both sexual and asexual stages. The asexual stages reproduce by schizogony. The male gametocyte produces a large number of gametes and the zygote gives rise to an oocyst, which is the infective stage. The majority are monoxenous (infect one host only) but a few are heteroxenous (lifecycle involves two or more hosts).
Both the number of families in this later suborder is debated with the number of families being between one and twenty depending on the authority and the number of genera being between nineteen and twenty five. This somewhat unsatisfactory state of affairs awaits resolution with DNA based methods.
As noted above, many of the apicomplexan parasites are important pathogens of human and domestic animals. In contrast to bacterial pathogens, these apicomplexan parasites are eukaryotic and share many metabolic pathways with their animal hosts. This fact makes therapeutic target development extremely difficult – a drug that harms an apicomplexan parasite is also likely to harm its human host. At the present time, there are no effective vaccines available for most diseases caused by these parasites. Biomedical research on these parasites is challenging because it is often difficult, if not impossible, to maintain live parasite cultures in the laboratory and to genetically manipulate these organisms. In recent years, several of the apicomplexan species have been selected for genome sequencing. The availability of genome sequences provides a new opportunity for scientists to learn more about the evolution and biochemical capacity of these parasites. The predominant source of this genomic information is the EuPathDB family of websites, which currently provide specialised services for Plasmodium species (PlasmoDB), coccidians (ToxoDB), piroplasms (PiroplasmaDB), and Cryptosporidium species (CryptoDB). One possible target for drugs is the plastid, and in fact existing drugs such as tetracyclines, which are effective against apicomplexans seem to operate against the plastid.
Most apicomplexans have an apicoplast, (a non-photosynthetic plastid) and mitochondrial and nuclear genomes, although Cryptosporidium species and gregarines are possible exceptions, as they are thought to have lost their plastids after the diverging last common ancestor of apicomplexans.
The first apicomplexan protozoan was seen by Antonie van Leeuwenhoek, who in 1674 saw oocysts of Eimeria stiedae in the gall bladder of a rabbit. The first member of the phylum to be named (by Dufour in 1828) was Gregarina ovata in earwigs. Since then, many more have been identified and named. During the quarter century 1826-1850, 41 species and 6 genera of Apicomplexa were named. In the quarter-century 1951-1975, 1873 new species and 83 new genera were added.
The field of classifying Apicomplexa is in flux and classification has changed throughout the years since it was formally named in 1970.
By 1987 a comprehensive survey of the phylum was completed: in all, 4516 species and 339 genera had been named. They consisted of:
Although there has been considerable revision of this phylum (the order Haemosporidia now has 17 genera rather than 9), it seems likely these numbers are still approximately correct.
Jacques Euzéby in 1988 created a new class Haemosporidiasina by merging subclass Piroplasmasina and suborder Haemospororina.
The division into Achromatorida and Chromatorida although proposed on morphological grounds may have a biological basis as the ability to store haemozoin appears to have evolved only once.
Roberts and Janovy in 1996 divided the phylum into the following subclasses and suborders (omitting classes and orders):
These form the following five taxonomic groups:
This scheme is taken from Perkins et al. It is outdated as the Perkinsidae have since been recognised as a sister group to the dinoflagellates rather that the Apicomplexia. The remainder of the scheme appears to be valid:
The name Protospiromonadida has been proposed for the common ancestor of the Gregarinomorpha and Coccidiomorpha.
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