Apicomplexa
Encyclopedia
The Apicomplexa are a large group of protist
s, most of which possess a unique organelle
called 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 pseudopod
s are present only in certain gamete
stages. This is a diverse group including organisms such as 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 organelle
s in the sporozoite. The older taxon Sporozoa grouped the Apicomplexa together with the Microsporidia
and Myxosporida. This grouping is no longer regarded as biologically valid and its use is discouraged.
in the gall bladder of a rabbit
. The first member of the phylum
to be named (by Dufour in 1828) was Gregarina ovata in earwig
s. 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.
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.
Roberts and Janovy in 1996 divided the phylum into five taxonomic groups:
The gregarines (subclass Gregarinasina) are generally one-host parasites of invertebrates.
The adeleorins (subclass Coccidiasina, suborder Adeleorina
) are one-host parasites of invertebrates or vertebrates, or two-host parasites that alternately infect haematophagous (blood-feeding) invertebrates and the blood of vertebrates.
The eimeriorins (subclass Coccidiasina, suborder Eimeriorina
), are a diverse group that includes one host species of invertebrates, two-host species of invertebrates, one-host species of vertebrates and two-host species of vertebrates.
The eimeriorins are frequently called the coccidia. Somewhat confusingly this term is often used to include the adeleorins.
Haemospororins (subclass Coccidiasina, suborder Haemosporasina), often known as the malaria parasites, are two-host Apicomplexa that parasitize blood-feeding dipteran fies and the blood of various tetrapod vertebrates.
All species of piroplasms (subclass Piroplasmasina) are two-host parasites infecting ticks and vertebrates.
Class Aconoidasida
Conoid present only in the ookinete of some species
Macrogamete and microgamete develop separately. Syzygy does not occur. Ookinete has a conoid. Sporozoites have three walls. Heteroxenous : alternates between vertebrate host (in which merogony occurs) and invertebrate host (in which sporogony occurs). Usually blood parasites, transmitted by blood-sucking insects
Class Conoidasida
Class Perkinsoridae
s (the conoid), a secretory body (the rhoptry
) and one or more polar rings. Additional slender electron dense secretory bodies (microneme
s) 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:
Replication:
Perkinsus is a parasite of bivalve mollusks and is currently the only known species in this class. It displayes a number of features characteristic of the dinoflagellate
s including laterally inserted heterodynamic flagella. Two other genera have been described in this class - Cryptophagus and Parvilucifera. Rastromonas may also belong to this grouping.
The gregarines are generally parasites of annelid
s, arthropod
s 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 plasmodium. 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 life cycle gamonts are formed. These are released by lysis of the host cells and group together by syzygy. Each gamont forms multiple gamete
s. The gametes fuse with another to form oocysts. The oocysts leave the host to be taken up by a new host.
Coccidians are generally parasites of vertebrate
s. Like gregarines they are commonly parasites of the epithelial cells of the gut but may infect other tissues. The typical coccidial life cycle while similar to that of the gregarines differs in zygote
formation. Some trophozoites enlage and become macrogamete while others divide repeatedly to form microgametes. The microgametes are motile and must reach the macrogamete to fertilize it. The fertilized macrogamete forms a zygote which in its turn forms an oocyst which is normally released from the body.
The Haemosporidians have more complex life cycles 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 oocyte and divides initially by meiosis and then by mitosis 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.
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 while in others it has become the definitive host. In the genera Aggregata, Atoxoplasma, Cystoisospora, Schellackia
and Toxoplasma the original is now definitive while 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 Protococcidiida and Eimeriida. Hypnozoites are found in Karyolysus lacerate and most species of Plasmodium
; transovarial transmission of parasites occurs in life cycles of Karyolysus
and Babesia
.
), 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 vesicle
s called rhoptries and microneme
s, 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 microtubule
s, 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 alveolate
s. 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 dinoflagellate
s. 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. Plastids are generally considered to share a common origin with the chloroplasts of dinoflagellates, and evidence generally 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 animal
s), 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 Adelorina are species that infect invertebrate
s and others that infect vertebrate
s.
The Haemosporina includes the malaria
parasites and their relatives.
The Eimeriorina - the largest suborder in this phylum - the life cycle 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 (life cycle 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.
l pathogens, these apicomplexan parasites are eukaryote
s 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. Currently there are no effective vaccine
s 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 the 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 parasite. A NIH-funded database, ApiDB.org, provides public access to currently available genomic data sets. One possible target for drugs is the plastid, and in fact existing drugs such as tetracycline
s which are effective against apicomplexans seem to operate against the plastid.
Most apicomplexans have plastid genomes as well as nuclear ones, although Cryptosporidium
spp. and possibly gregarines are exceptions as they are thought to have lost plastids after the diverging last common ancestor of apicomplexans.
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s, most of which possess a unique organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
called apicoplast
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 algae through secondary endosymbiosis...
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
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
or pseudopod
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
s are present only in certain gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
stages. This is a diverse group including organisms such as coccidia
Coccidia
Coccidia is a subclass of microscopic, spore-forming, single-celled obligate parasites belonging to the apicomplexan class Conoidasida. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa....
, gregarines, piroplasms, haemogregarines, and plasmodia
Plasmodiidae
The Plasmodiidae are a family of apicomplexan parasites, including the type genus Plasmodium, which is responsible for malaria. This genus was created in 1903 by Mesnil.They are one of the four families in the order Haemosporida....
. Diseases caused by apicomplexan organisms include, but are not limited to:
- BabesiosisBabesiosisBabesiosis is a malaria-like parasitic disease caused by infection with Babesia, a genus of protozoal piroplasms. After trypanosomes, Babesia are thought to be the second most common blood parasites of mammals and they can have a major impact on health of domestic animals in areas without severe...
(BabesiaBabesiaBabesia is a protozoan parasite of the blood that causes a hemolytic disease known as Babesiosis. There are over 100 species of Babesia identified; however only a handful have been documented as pathogenic in humans....
) - MalariaMalariaMalaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
(PlasmodiumPlasmodiumPlasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...
) - CoccidiaCoccidiaCoccidia is a subclass of microscopic, spore-forming, single-celled obligate parasites belonging to the apicomplexan class Conoidasida. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa....
n diseases including:- CryptosporidiosisCryptosporidiosisCryptosporidiosis, also known as crypto, is a parasitic disease caused by Cryptosporidium, a protozoan parasite in the phylum Apicomplexa. It affects the intestines of mammals and is typically an acute short-term infection...
(Cryptosporidium parvumCryptosporidium parvumCryptosporidium parvum is one of several protozoal species that cause cryptosporidiosis, a parasitic disease of the mammalian intestinal tract....
) - CyclosporiasisCyclosporiasisCyclosporiasis is an infection with the protozoan Cyclospora cayetanensis, a pathogen transmitted by feces or feces-contaminated fresh produce and water. Outbreaks have been reported due to contaminated raspberries. It is not spread from person to person...
(Cyclospora cayetanensisCyclospora cayetanensisCyclospora cayetanensis is a protozoan that causes disease in humans, and perhaps other primates. It has been linked in the United States from fecally-contaminated imported raspberries and was virtually unknown before about 1990, but has been on the rise since...
) - IsosporiasisIsosporiasisIsosporiasis is a human intestinal disease caused by the parasite Isospora belli. It is found worldwide, especially in tropical and subtropical areas. Infection often occurs in immuno-compromised individuals, notably AIDS patients, and outbreaks have been reported in institutionalized groups in the...
(Isospora belliIsospora belliIsospora belli is a species of internal parasites classified under Coccidia. It is responsible for the condition isosporiasis. Autofluorescence aids detection....
) - ToxoplasmosisToxoplasmosisToxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most genera of warm-blooded animals, including humans, but the primary host is the felid family. Animals are infected by eating infected meat, by ingestion of feces of a cat that has itself...
(Toxoplasma gondiiToxoplasma gondiiToxoplasma gondii is a species of parasitic protozoa in the genus Toxoplasma. The definitive host of T. gondii is the cat, but the parasite can be carried by many warm-blooded animals . Toxoplasmosis, the disease of which T...
)
- Cryptosporidiosis
The name of the taxon Apicomplexa is derived from two Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
words - apex (top) and complexus (infolds) - and refers to a set of organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s in the sporozoite. The older taxon Sporozoa grouped the Apicomplexa together with the Microsporidia
Microsporidia
The microsporidia constitute a phylum of spore-forming unicellular parasites. They were once thought to be protists but are now known to be fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals...
and Myxosporida. This grouping is no longer regarded as biologically valid and its use is discouraged.
History
The first apicomplexan protozoan was seen by Antony van Leeuwenhoek who in 1674 saw oocysts of Eimeria stiedaeEimeria stiedae
Eimeria stiedae is a species of Eimeria that causes hepatic coccidiosis in rabbits. It was observed for the first time by Antoni van Leeuwenhoek in 1674.-Life cycle:The rabbits ingest sporulated oocysts...
in the gall bladder of a rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
. The first member of the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
to be named (by Dufour in 1828) was Gregarina ovata in earwig
Earwig
Earwigs make up the insect order Dermaptera, found throughout the Americas, Africa, Eurasia, Australia and New Zealand. With 1,800 species in 12 families, they are one of the smaller insect orders...
s. 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.
Taxonomy
By 1987 a comprehensive survey of the phylum was completed: in all, 4516 species and 339 genera had been named. They consisted of:- the gregarines (subclass Gregarinasida) with 1624 named species and 231 named genera
- the hemogregarines (family Haemogregarinidae) with 399 species and 4 genera
- the eimeriorins (order Eimeriorida) with 1771 species and 43 genera
- the hemospororids (order Haemosporasina with 444 species and 9 genera
- the piroplasmids (order Piroplasmorida) with 173 species and 20 genera
- and a few others (105 species and 32 genera)
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.
Roberts and Janovy in 1996 divided the phylum into five taxonomic groups:
The gregarines (subclass Gregarinasina) are generally one-host parasites of invertebrates.
The adeleorins (subclass Coccidiasina, suborder Adeleorina
Adeleorina
Adeleorina is a suborder of Apicomplexa.Blood parasites belonging to the suborder Adeleorina are collectively known as haemogregarines. Currently their sister group is thought to be the piroplasms....
) are one-host parasites of invertebrates or vertebrates, or two-host parasites that alternately infect haematophagous (blood-feeding) invertebrates and the blood of vertebrates.
The eimeriorins (subclass Coccidiasina, suborder Eimeriorina
Eimeriorina
Eimeriorina is a suborder of apicomplexa....
), are a diverse group that includes one host species of invertebrates, two-host species of invertebrates, one-host species of vertebrates and two-host species of vertebrates.
The eimeriorins are frequently called the coccidia. Somewhat confusingly this term is often used to include the adeleorins.
Haemospororins (subclass Coccidiasina, suborder Haemosporasina), often known as the malaria parasites, are two-host Apicomplexa that parasitize blood-feeding dipteran fies and the blood of various tetrapod vertebrates.
All species of piroplasms (subclass Piroplasmasina) are two-host parasites infecting ticks and vertebrates.
Classification
This scheme is taken from Perkins et al. It is outdated as the Perkinsoridae have since been recognised as a sister group to the dinoflagellates rather that the Apicomplexia. The remainder of the scheme appears to be valid.Class Aconoidasida
Aconoidasida
The Aconoidasida are a class of apicomplexan parasites created by Mehlhorn et al in 1980.Organisms of this phylum bear a tip at one end of their outer membrane. This apical complex includes vesicles called rhoptries and micronemes, which open at the anterior of the cell. These secrete enzymes that...
Conoid present only in the ookinete of some species
- Order Haemospororida
Macrogamete and microgamete develop separately. Syzygy does not occur. Ookinete has a conoid. Sporozoites have three walls. Heteroxenous : alternates between vertebrate host (in which merogony occurs) and invertebrate host (in which sporogony occurs). Usually blood parasites, transmitted by blood-sucking insects
- Order Piroplasmorida
Class Conoidasida
Conoidasida
Conoidasida is a class of protists.The class was defined in 1988 by Levine and contains two subclasses - the coccidia and the gregarines. All members of this class have a complete, hollow, truncated conoid...
- Subclass Gregarinasina
- Order Archigregarinorida
- Order Eugregarinorida
- Suborder AdeleorinaAdeleorinaAdeleorina is a suborder of Apicomplexa.Blood parasites belonging to the suborder Adeleorina are collectively known as haemogregarines. Currently their sister group is thought to be the piroplasms....
- Suborder EimeriorinaEimeriorinaEimeriorina is a suborder of apicomplexa....
- Suborder Adeleorina
- Order Neogregarinorida
- Order Archigregarinorida
- Subclass Coccidiasina
- Order AgamococcidioridaAgamococcidioridaAgamococcidiorida is an order within the subclass Conoidasida of the phylum Apicomplexia. All members of this order are parasitic protozoa.They are found in marine annelids....
- Order EucoccidioridaEucoccidioridaEucoccidiorida is an order of microscopic, spore-forming, single-celled parasites belonging to the apicomplexan class Conoidasida. Protozoans of this order include parasites of humans, and both domesticated and wild animals including birds...
- Order IxorheoridaIxorheoridaIxorheorida is an order within the subclass Conoidasida of the phylum Apicomplexia. All members of this order are parasitic protozoa.The order has one family - Ixorheidae - , this family has one genus - Ixorheis - and this genus has a single species - Ixorheis psychropotae.This species and genus...
- Order ProtococcidioridaProtococcidioridaProtococcidiorida is an order within the subclass Conoidasida of the phylum Apicomplexia. All members of this order are parasitic protozoa.The species in this order are found in marine invertebrates.-Genera:...
- Order Agamococcidiorida
Class Perkinsoridae
- Order Perkinsorida
General morphological features
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 microtubuleMicrotubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s (the conoid), a secretory body (the rhoptry
Rhoptry
A rhoptry is a specialized secretory organelle. They are club-shaped organelles connected by thin necks to the extreme apical pole of the parasite. These organelles, like micronemes, are characteristic of the motile stages of Apicomplexa protozoans. They can vary in number and shape and contain...
) and one or more polar rings. Additional slender electron dense secretory bodies (microneme
Microneme
Micronemes are cellular organs, or organelles, possessed by Apicomplexa protozoans that are restricted to the apical third of the protozoan body. They are surrounded by a typical unit membrane. On electorn microscopy they have an electron-dense matrix due to the high protein content...
s) 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:
- The nucleus is haploid.
- Flagellae are found only in the motile gamete. These are posteriorly directed and vary in number (usually one to three).
- Basal bodiesBasal bodyA basal body is an organelle formed from a centriole, and a short cylindrical array of microtubules. It is found at the base of a eukaryotic undulipodium and serves as a nucleation site for the growth of the axoneme microtubules...
are present. Although hemosporidians and piroplasmids have normal triplets of microtubuleMicrotubuleMicrotubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s in their basal bodies and coccidians and gregarines have 9 singlets. - The mitochondria have tubular cristae.
- A Golgi apparatusGolgi apparatusThe Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
is present. - CentrioleCentrioleA Centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though it is absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
s, chloroplastChloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s, ejectile organelles and inclusions are absent. - Colourless plastidPlastidPlastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s are present in some species. - The cell is surrounded by a pelliclePellicle (biology)The pellicle is a thin layer supporting the cell membrane in various protozoa, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more hydrodynamic. They vary from flexible and elastic to rigid. Although stiff, the pellicle is...
of three membrane layers (the alveolar structure) penetrated by micropores.
Replication:
- MitosisMitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
: usually closed with an intranuclear spindle; in some species it is open at the poles - Cell division: usually by schizogony
- MeiosisMeiosisMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
: Occurs in the zygoteZygoteA zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
General features
Within this phylum there are four groups - Perkinsus, coccidians, gregarines and haemosporidians. The coccidians and gregarines appear to be relatively closely related and Perkinsus appears to be basal within this phylum.Perkinsus is a parasite of bivalve mollusks and is currently the only known species in this class. It displayes a number of features characteristic of the dinoflagellate
Dinoflagellate
The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
s including laterally inserted heterodynamic flagella. Two other genera have been described in this class - Cryptophagus and Parvilucifera. Rastromonas may also belong to this grouping.
The gregarines are generally parasites of annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s, arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s 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 plasmodium. 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 life cycle gamonts are formed. These are released by lysis of the host cells and group together by syzygy. Each gamont forms multiple gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s. The gametes fuse with another to form oocysts. The oocysts leave the host to be taken up by a new host.
Coccidians are generally parasites of vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s. Like gregarines they are commonly parasites of the epithelial cells of the gut but may infect other tissues. The typical coccidial life cycle while similar to that of the gregarines differs in zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
formation. Some trophozoites enlage and become macrogamete while others divide repeatedly to form microgametes. The microgametes are motile and must reach the macrogamete to fertilize it. The fertilized macrogamete forms a zygote which in its turn forms an oocyst which is normally released from the body.
The Haemosporidians have more complex life cycles 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 oocyte and divides initially by meiosis and then by mitosis 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.
Evolution
Many Coccidiomorpha have an intermediate hostIntermediate host
A secondary host or intermediate host is a host that harbors the parasite only for a short transition period, during which some developmental stage is completed. For trypanosomes, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary 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 while in others it has become the definitive host. In the genera Aggregata, Atoxoplasma, Cystoisospora, Schellackia
Schellackia
Schellackia is a genus of parasitic protozoa within the phylum Apicomplexa. The genus was described in 1919 by Reichenow. Species in this genus infect reptiles and amphibians.The type species is Schellackia bolivari.-Life cycle:...
and Toxoplasma the original is now definitive while in Akiba, Babesiosoma
Babesiosoma
Babesiosoma is a genus of parasitic protozoa in the phylum Apicomplexia.They have two hosts in their life cycle: the vertebrate hosts are fish and the invertebrate vectors are leeches.This genus has been poorly studied and little is known about it....
, Babesia
Babesia
Babesia is a protozoan parasite of the blood that causes a hemolytic disease known as Babesiosis. There are over 100 species of Babesia identified; however only a handful have been documented as pathogenic in humans....
, Haemogregarina
Haemogregarina
Haemogregarina is a genus of haemoprotozoans, parasitic mainly on cold-blooded vertebrates. They are unicellular organisms which are parasitic in the red blood cells.-History:...
, Haemoproteus
Haemoproteus
Haemoproteus is a genus of protozoa that are parasitic in birds, reptiles and amphibians. Its name is derived from Greek: Haima - blood and Proteus - a sea god who had the power of assuming different shapes. The name Haemoproteus was first used in the description of Haemoproteus columbae in the...
, Hepatozoon
Hepatozoon
Hepatozoon is a genus of Apicomplexan protozoa which incorporates over 300 species obligate intraerythrocytic parasites. Species have been described from all groups of tetrapod vertebrates, as well as a wide range of haematophagous arthropods, which serve as both the vectors and definitive hosts of...
, Karyolysus
Karyolysus
Karyolysus is a genus of coccidia. With the exception of K. sonomae whose vertebrate host is the yellow legged frog species in this genus only infect lizards of the genus Lacerta.-History:...
, Leucocytozoon, Plasmodium
Plasmodium
Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...
, Sarcocystis
Sarcocystis
Sarcocystis is a genus of protozoa. Species in this genus infect reptiles, birds and mammals. The name is dervived from Greek: sarx = flesh and kystis = bladder.There are about 130 recognised species in this genus...
and Theileria
Theileria
Theileria is a genus of parasitic protozoan that belongs to the phylum Apicomplexa and is closely related to Plasmodium. Two Theileria species, T. annulata and T. parva, are important cattle parasites. T. annulata causes tropical theileriosis and T. parva causes East Coast fever. Theileria are...
, 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 Protococcidiida and Eimeriida. Hypnozoites are found in Karyolysus lacerate and most species of Plasmodium
Plasmodium
Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...
; transovarial transmission of parasites occurs in life cycles of Karyolysus
Karyolysus
Karyolysus is a genus of coccidia. With the exception of K. sonomae whose vertebrate host is the yellow legged frog species in this genus only infect lizards of the genus Lacerta.-History:...
and Babesia
Babesia
Babesia is a protozoan parasite of the blood that causes a hemolytic disease known as Babesiosis. There are over 100 species of Babesia identified; however only a handful have been documented as pathogenic in humans....
.
Life cycle
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 entosisEntosis
Entosis is a form of cell death that involves the cell dying as a result of becoming engulfed by a neighboring cell. The process was discovered by Overholtzer, et al. as reported in Cell....
), 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 vesicle
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
s called rhoptries and microneme
Microneme
Micronemes are cellular organs, or organelles, possessed by Apicomplexa protozoans that are restricted to the apical third of the protozoan body. They are surrounded by a typical unit membrane. On electorn microscopy they have an electron-dense matrix due to the high protein content...
s, 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 microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s, 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 alveolate
Alveolate
The alveolates are a major line of protists.-Phyla:There are four phyla, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:...
s. Several related flagellates, such as Perkinsus
Perkinsus marinus
Perkinsus marinus is a prevalent pathogen of oysters, causing massive mortality in oyster populations. The disease it causes is known as "Dermo" ,and is characterized by proteolytic degradation of oyster tissues...
and Colpodella
Colpodella
Colpodella is a genus of alveolates comprising 5 species, and two further possible species: They share all the synapomorphies of apicomplexans, but are free-living, rather than parasitic. This genus was previously known as Spiromonas...
have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of the dinoflagellate
Dinoflagellate
The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
s. They are probably similar to the common ancestor of the two groups.
Another similarity is that many apicomplexan cells contain a single plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
, called the apicoplast
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 algae through secondary endosymbiosis...
, 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. Plastids are generally considered to share a common origin with the chloroplasts of dinoflagellates, and evidence generally points to an origin from red algae
Red algae
The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds...
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
Ascetosporea
The Ascetosporea are a group of protists that are parasites of animals, especially marine invertebrates. There are two groups, the haplosporids and paramyxids, which are not particularly similar morphologically but consistently group together on molecular trees, which place them near the base of...
, the Myxozoa
Myxozoa
The Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...
(now known to be derived from animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s), and the Microsporidia
Microsporidia
The microsporidia constitute a phylum of spore-forming unicellular parasites. They were once thought to be protists but are now known to be fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals...
(now known to be derived from fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
). Sometimes the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.
Blood-borne genera
Within the Apicomplexa there are three suborders of parasites.- suborder AdeleorinaAdeleorinaAdeleorina is a suborder of Apicomplexa.Blood parasites belonging to the suborder Adeleorina are collectively known as haemogregarines. Currently their sister group is thought to be the piroplasms....
- 8 genera - suborder Haemosporina - all genera in this suborder
- suborder EimeriorinaEimeriorinaEimeriorina is a suborder of apicomplexa....
- 2 genera (LankesterellaLankesterellaLankesterella is a genus of flowering plants from the orchid family, Orchidaceae.- References :*Pridgeon, A.M., Cribb, P.J., Chase, M.A. & Rasmussen, F. eds. . Genera Orchidacearum 1. Oxford Univ. Press....
and SchellackiaSchellackiaSchellackia is a genus of parasitic protozoa within the phylum Apicomplexa. The genus was described in 1919 by Reichenow. Species in this genus infect reptiles and amphibians.The type species is Schellackia bolivari.-Life cycle:...
)
Within the Adelorina are species that infect invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s and others that infect vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s.
The Haemosporina includes the malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
parasites and their relatives.
The Eimeriorina - the largest suborder in this phylum - the life cycle involves both sexual and asexual stages. The asexual stages reproduce by schizogony. The male gametocyte produces a large number of gametes and the zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
gives rise to an oocyst which is the infective stage. The majority are monoxenous (infect one host only) but a few are heteroxenous (life cycle 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.
Disease genomics
As noted above, many of the apicomplexan parasites are important pathogens of human and domestic animals. In contrast to bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l pathogens, these apicomplexan parasites are eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s 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. Currently there are no effective vaccine
Vaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...
s 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 the recent years, several of the apicomplexan species have been selected for genome sequencing. The availability of genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
sequences provides a new opportunity for scientists to learn more about the evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
and biochemical capacity of these parasite. A NIH-funded database, ApiDB.org, provides public access to currently available genomic data sets. One possible target for drugs is the plastid, and in fact existing drugs such as tetracycline
Tetracycline antibiotics
Tetracyclines are a group of broad-spectrum antibiotics whose general usefulness has been reduced with the onset of bacterial resistance. Despite this, they remain the treatment of choice for some specific indications....
s which are effective against apicomplexans seem to operate against the plastid.
Most apicomplexans have plastid genomes as well as nuclear ones, although Cryptosporidium
Cryptosporidium
Cryptosporidium is a protozoan that can cause gastro-intestinal illness with diarrhea in humans.Cryptosporidium is the organism most commonly isolated in HIV positive patients presenting with diarrhea...
spp. and possibly gregarines are exceptions as they are thought to have lost plastids after the diverging last common ancestor of apicomplexans.