| Apicomplexa | |
|---|---|
 | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | TSAR |
| Clade: | SAR |
| Clade: | Alveolata |
| Phylum: | Apicomplexa Levine, 1970[1][2] |
| Classes & SubclassesPerkins, 2000 | |
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TheApicomplexa (also calledApicomplexia; single:apicomplexan) are organisms of a largephylum of mainlyparasiticalveolates. Most possess a unique form oforganelle structure that comprises a type of non-photosyntheticplastid called anapicoplast—with anapical complex membrane. The organelle's apical shape (e.g., seeCeratium furca) is an adaptation that the apicomplexan applies in penetrating a host cell.
The Apicomplexa are unicellular and spore-forming. Most areobligateendoparasites of animals,[3] exceptNephromyces, asymbiont in marine animals, originally classified as achytrid fungus,[4] and theChromerida, some of which arephotosynthetic partners of corals. Motile structures such asflagella orpseudopods are present only in certaingamete stages.
The Apicomplexa are a diverse group that includes organisms such as thecoccidia,gregarines,piroplasms,haemogregarines, andplasmodia.Diseases caused by Apicomplexa include:
The name Apicomplexa derives from twoLatin words—apex (top) andcomplexus (infolds)—for the set oforganelles in thesporozoite. The Apicomplexa comprise the bulk of what used to be called theSporozoa, a group of parasitic protozoans, in general without flagella, cilia, or pseudopods. Most of the Apicomplexa are motile, however, with agliding mechanism[5] that uses adhesions and small static myosin motors.[6] The other main lines of this obsolete grouping were theAscetosporea (a group ofRhizaria), theMyxozoa (highly derivedcnidariananimals), and theMicrosporidia (derived fromfungi). Sometimes, the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.
The phylum Apicomplexa contains all eukaryotes with a group of structures and organelles collectively termed the apical complex.[7] This complex consists of structural components andsecretory organelles required for invasion ofhostcells during the parasitic stages of theApicomplexan life cycle.[7] Apicomplexa have complex life cycles, involving several stages and typically undergoing bothasexual andsexual replication.[7] All Apicomplexa areobligate parasites for some portion of their life cycle, with some parasitizing two separate hosts for their asexual and sexual stages.[7]
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.[7] Like other eukaryotes, Apicomplexa have anucleus,endoplasmic reticulum andGolgi complex.[7] Apicomplexa generally have a single mitochondrion, as well as anotherendosymbiont-derived organelle called theapicoplast which maintains a separate 35kilobase circular genome (with the exception ofCryptosporidium species andGregarina niphandrodes which lack an apicoplast).[7]
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 arrangedmicrotubules (theconoid), a secretory body (therhoptry) 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 theapical 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 theparasitophorous vacuole and persists for several minutes.[citation needed]
Replication:
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.[9]
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 generaAscogregarina andEimeria.[10]
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 toentosis), which divide to producesporozoites that enter its cells. Eventually, the cells burst, releasingmerozoites, 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.[11]
The apical complex includesvesicles calledrhoptries andmicronemes, 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 ofmicrotubules, called the polar ring, and among the Conoidasida is also a funnel of tubulin proteins called the conoid.[12] 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.[13]
The presence of alveoli and other traits place the Apicomplexa among a group called thealveolates. Several related flagellates, such asPerkinsus andColpodella, have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of thedinoflagellates. They are probably similar to the common ancestor of the two groups.[13]
Another similarity is that many apicomplexan cells contain a singleplastid, called theapicoplast, 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 fromred algae rather thangreen.[14][15]
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.[16]
Perkinsus, while once considered a member of the Apicomplexa, has been moved to a new phylum —Perkinsozoa.[17]
The gregarines are generally parasites ofannelids,arthropods, andmolluscs. They are often found in theguts of their hosts, but may invade the other tissues. In the typical gregarine lifecycle, atrophozoite develops within a host cell into a schizont. This then divides into a number ofmerozoites byschizogony. Themerozoites 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 multiplegametes. The gametes fuse with another to formoocysts. The oocysts leave the host to be taken up by a new host.[18]
In general, coccidians are parasites ofvertebrates. Like gregarines, they are commonly parasites of theepithelial 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 inzygote formation. Some trophozoites enlarge and becomemacrogamete, whereas others divide repeatedly to formmicrogametes (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.[19]
The main difference between the coccidians and the gregarines is in the gamonts. In the coccidia, these are small, intracellular, and without epimerites ormucrons. 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 coccidians, a single gamont becomes a macrogametocyte, whereas in the gregarines, the gamonts give rise to multiple gametocytes.[20]
The Haemosporidia have more complex lifecycles that alternate between an arthropod and a vertebrate host. The trophozoite parasitiseserythrocytes 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 anookinete, which penetrates the body of the vector. The ookinete then transforms into an oocyst and divides initially by meiosis and then by mitosis (haplontic lifecycle) to give rise to thesporozoites. 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.[21]
The class MarosporidaMathur, Kristmundsson, Gestal, Freeman, and Keeling 2020 is a newly recognized lineage of apicomplexans that is sister to the Coccidia and Hematozoa. It is defined as a phylogeneticclade containingAggregata octopianaFrenzel 1885,Merocystis kathaeDakin, 1911 (both Aggregatidae, originally coccidians),Rhytidocystis sp. 1 andRhytidocystis sp. 2Janouškovec et al. 2019 (RhytidocystidaeLevine, 1979, originally coccidians,Agamococcidiorida), andMargolisiella islandicaKristmundsson et al. 2011 (closely related to Rhytidocystidae). Marosporida infect marine invertebrates. Members of this clade retainplastid genomes and the canonical apicomplexan plastid metabolism. However, marosporidians have the most reduced apicoplast genomes sequenced to date, lack canonical plastidial RNA polymerase and so provide new insights into reductive organelle evolution.[16]
Many of the apicomplexan parasites are important pathogens of humans and domestic animals. In contrast tobacterial pathogens, these apicomplexan parasites areeukaryotic and share many metabolic pathways with their animal hosts. This makes therapeutic target development extremely difficult – a drug that harms an apicomplexan parasite is also likely to harm its human host. At present, no effectivevaccines are 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 forgenome sequencing. The availability of genome sequences provides a new opportunity for scientists to learn more about theevolution and biochemical capacity of these parasites. The predominant source of this genomic information is theEuPathDB[22] family of websites, which currently provides specialised services forPlasmodium species (PlasmoDB),[23][24]coccidians (ToxoDB),[25][26]piroplasms (PiroplasmaDB),[27] andCryptosporidium species (CryptoDB).[28][29] One possible target for drugs is the plastid, and in fact existing drugs such astetracyclines, which are effective against apicomplexans, seem to operate against the plastid.[30]
Many Coccidiomorpha have anintermediate 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 generaAggregata,Atoxoplasma,Cystoisospora,Schellackia, andToxoplasma, the original is now definitive, whereas inAkiba,Babesiosoma,Babesia,Haemogregarina,Haemoproteus,Hepatozoon,Karyolysus,Leucocytozoon,Plasmodium,Sarcocystis, andTheileria, the original hosts are now intermediate.
Similar strategies to increase the likelihood of transmission have evolved in multiple genera. Polyenergidoocysts and tissue cysts are found in representatives of the ordersProtococcidiorida andEimeriida.Hypnozoites are found inKaryolysus lacerate and most species ofPlasmodium; transovarial transmission of parasites occurs in lifecycles ofKaryolysus andBabesia.
Horizontal gene transfer appears to have occurred early on in this phylum's evolution with the transfer of ahistone H4 lysine 20 (H4K20)modifier,KMT5A (Set8), from an animal host to the ancestor of apicomplexans.[31] A second gene—H3K36 methyltransferase (Ashr3 inplants)—may have also been horizontally transferred.[13]
Within the Apicomplexa are three suborders of parasites:[13]
Within the Adelorina are species that infectinvertebrates and others that infectvertebrates. 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 aremonoxenous (infect one host only), but a few areheteroxenous (lifecycle involves two or more hosts).
The number of families in this later suborder is debated, with the number of families being between one and 20 depending on the authority and the number of genera being between 19 and 25.
The first Apicomplexa protozoan was seen byAntonie van Leeuwenhoek, who in 1674 saw probablyoocysts ofEimeria stiedae in thegall bladder of arabbit. The first species of thephylum to be described,Gregarina ovata, inearwigs' intestines, was named by Dufour in 1828. He thought that they were a peculiar group related to thetrematodes, at that time included inVermes.[32] Since then, many more have been identified and named. During 1826–1850, 41 species and six genera of Apicomplexa were named. In 1951–1975, 1873 new species and 83 new genera were added.[32]
The older taxon Sporozoa, included inProtozoa, was created byLeuckart in 1879[33] and adopted byBütschli in 1880.[34] Through history, it grouped with the current Apicomplexa many unrelated groups. For example, Kudo (1954) included in the Sporozoa species of theAscetosporea (Rhizaria),Microsporidia (Fungi),Myxozoa (Animalia), andHelicosporidium (Chlorophyta), while Zierdt (1978) included the genusBlastocystis (Stramenopiles).[35]Dermocystidium was also thought to be sporozoan. Not all of these groups had spores, but all were parasitic.[32] However, other parasitic or symbiotic unicellular organisms were included too in protozoan groups outside Sporozoa (Flagellata,Ciliophora andSarcodina), if they had flagella (e.g., manyKinetoplastida,Retortamonadida,Diplomonadida,Trichomonadida,Hypermastigida), cilia (e.g.,Balantidium) or pseudopods (e.g.,Entamoeba,Acanthamoeba,Naegleria). If they had cell walls, they also could be included in plant kingdom betweenbacteria oryeasts.
Sporozoa is no longer regarded as biologically valid and its use is discouraged,[36] although some authors still use it as a synonym for the Apicomplexa. More recently, other groups were excluded from Apicomplexa, e.g.,Perkinsus andColpodella (now in Protalveolata).
The field of classifying Apicomplexa is in flux and classification has changed throughout the years since it was formally named in 1970.[1]
By 1987, a comprehensive survey of the phylum was completed: in all, 4516 species and 339 genera had been named. They consisted of:[37][32]
Although considerable revision of this phylum has been done (the order Haemosporidia now has 17 genera rather than 9), these numbers are probably still approximately correct.[38]
Jacques Euzéby in 1988[39] created a new classHaemosporidiasina by merging subclassPiroplasmasina and suborderHaemospororina.
The division into Achromatorida and Chromatorida, although proposed on morphological grounds, may have a biological basis, as the ability to storehaemozoin appears to have evolved only once.[40]
Roberts and Janovy in 1996 divided the phylum into the following subclasses and suborders (omitting classes and orders):[41]
These form the following five taxonomic groups:
Perkins et al. proposed the following scheme.[42] It is outdated as thePerkinsidae have since been recognised as a sister group to the dinoflagellates rather that the Apicomplexia:
The name Protospiromonadida has been proposed for the common ancestor of the Gregarinomorpha and Coccidiomorpha.[43]
Another group of organisms that belong in this taxon are the corallicolids.[44] These are found in coral reef gastric cavities. Their relationship to the others in this phylum has yet to be established.
Another genus has been identified -Nephromyces - which appears to be a sister taxon to the Hematozoa.[45] This genus is found in the renal sac of molgulid ascidiantunicates.
Members of this phylum, except for thephotosynthetic chromerids,[46] are parasitic and evolved from a free-living ancestor. This lifestyle is presumed to have evolved at the time of the divergence of dinoflagellates and apicomplexans.[47][48] Further evolution of this phylum has been estimated to have occurred about800 million years ago.[49] The oldest extant clade is thought to be the archigregarines.[47]
These phylogenetic relations have rarely been studied at the subclass level. The Haemosporidia are related to the gregarines, and the piroplasms and coccidians are sister groups.[50] The Haemosporidia and the Piroplasma appear to be sister clades, and are more closely related to the coccidians than to the gregarines.[10] Marosporida is a sister group to Coccidiomorphea.[16]
| Myzozoa |
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Janouškovec et al. 2015 presents a somewhat different phylogeny, supporting the work of others showing multiple events ofplastids losing photosynthesis. More importantly this work provides the firstphylogenetic evidence that there have also been multiple events of plastids becoming genome-free.[51]
