Apicomplexa

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Apicomplexa
Apicomplexa Composite Image.png
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Clade: Diaphoretickes
Clade: TSAR
Clade: SAR
Clade: Alveolata
Phylum: Apicomplexa
Levine, 1970 [1] [2]
Classes & Subclasses Perkins, 2000

The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of (non-photosynthetic) plastid called an apicoplast —with an apical complex membrane. The organelle's apical shape (e.g., Ceratium furca) is an adaptation that the apicomplexan applies in penetrating a host cell.

The Apicomplexa are unicellular and spore-forming. Most are obligate endoparasites of animals, [3] except Nephromyces , a symbiont in marine animals, originally classified as a chytrid fungus, [4] and the Chromerida, some of which are photosynthetic partners of corals. 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:

The name Apicomplexa derives from two Latin words—apex (top) and complexus (infolds)—for the 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, with a gliding mechanism [5] that uses adhesions and small static myosin motors. [6] The other main lines were the Ascetosporea (a group of Rhizaria), the Myxozoa (highly derived cnidarian animals), and the Microsporidia (derived from fungi). Sometimes, the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.

Description

Apicomplexan structure 2023 Apicomplexan.svg
Apicomplexan structure
Some cell types: ookinete, sporozoite, merozoite Ookinete, sporozoite, merozoite.png
Some cell types: ookinete, sporozoite, merozoite

The phylum Apicomplexa contains all eukaryotes with a group of structures and organelles collectively termed the apical complex. [8] This complex consists of structural components and secretory organelles required for invasion of host cells during the parasitic stages of the Apicomplexan life cycle. [8] Apicomplexa have complex life cycles, involving several stages and typically undergoing both asexual and sexual replication. [8] All Apicomplexa are obligate parasites for some portion of their life cycle, with some parasitizing two separate hosts for their asexual and sexual stages. [8]

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. [8] Like other eukaryotes, Apicomplexa have a nucleus, endoplasmic reticulum and Golgi complex. [8] 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). [8]

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.[ 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 genera Ascogregarina and Eimeria . [10]

Life cycle

Apicomplexa life cycle v2.svg
Generic lifecycle of an Apicomplexan: 1-zygote (cyst), 2-sporozoites, 3-merozoites, 4-gametocytes

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.[ citation needed ]

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. [11] 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. [12]

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. [12]

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. [13] [14]

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. [15]

Perkinsus , while once considered a member of the Apicomplexa, has been moved to a new phylum — Perkinsozoa. [16]

Gregarines

Trophozoite of a gregarine Septate gregarine.jpg
Trophozoite of a gregarine

The gregarines are generally parasites of annelids, arthropods, and molluscs. 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. [17]

Coccidians

Dividing Toxoplasma gondii (Coccidia) parasites Toxoplasma gondii.jpg
Dividing Toxoplasma gondii (Coccidia) parasites

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 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. [18]

The main difference between the coccidians and the gregarines is in the gamonts. In the coccidia, 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 coccidians, a single gamont becomes a macrogametocyte, whereas in the gregarines, the gamonts give rise to multiple gametocytes. [19]

Haemosporidia

Trophozoites of the Plasmodium vivax (Haemosporidia) parasite among human red blood cells Immature and mature trophozoites of the Plasmodium vivax parasite PHIL 2720 lores.jpg
Trophozoites of the Plasmodium vivax (Haemosporidia) parasite among human red blood cells

The Haemosporidia 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 lifecycle) 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. [20]

Marosporida

The class Marosporida Mathur, 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 phylogenetic clade containing Aggregata octopianaFrenzel 1885, Merocystis kathae Dakin, 1911 (both Aggregatidae, originally coccidians), Rhytidocystis sp. 1 and Rhytidocystis sp. 2 Janouškovec et al. 2019 (Rhytidocystidae Levine, 1979, originally coccidians, Agamococcidiorida), and Margolisiella islandicaKristmundsson et al. 2011 (closely related to Rhytidocystidae). Marosporida infect marine invertebrates. Members of this clade retain plastid 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. [15]

Ecology and distribution

Two tachyzoites of Toxoplasma gondii, transmission electron microscopy Parasite140105-fig3 Toxoplasmosis in a bar-shouldered dove - TEM of 2 tachyzoites.tif
Two tachyzoites of Toxoplasma gondii , transmission electron microscopy

Many of the apicomplexan parasites are important pathogens of humans and domestic animals. In contrast to bacterial pathogens, these apicomplexan parasites are eukaryotic 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 effective vaccines 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 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 [21] family of websites, which currently provides specialised services for Plasmodium species (PlasmoDB), [22] [23] coccidians (ToxoDB), [24] [25] piroplasms (PiroplasmaDB), [26] and Cryptosporidium species (CryptoDB). [27] [28] 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. [29]

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, KMT5A (Set8), from an animal host to the ancestor of apicomplexans. [30] A second gene—H3K36 methyltransferase (Ashr3 in plants)—may have also been horizontally transferred. [12]

Blood-borne genera

Within the Apicomplexa are three suborders of parasites: [12]

Within the Adelorina are species that infect invertebrates and others that infect vertebrates. 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).

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.

Taxonomy

History

The first Apicomplexa protozoan was seen by Antonie van Leeuwenhoek, who in 1674 saw probably oocysts of Eimeria stiedae in the gall bladder of a rabbit. The first species of the phylum to be described, Gregarina ovata in earwigs intestines, was named by Dufour in 1828. He thought that they were a peculiar group related to the trematodes, at that time included in Vermes. [31] 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. [31]

The older taxon Sporozoa, included in Protozoa, was created by Leuckart in 1879 [32] and adopted by Bütschli in 1880. [33] Through history, it grouped with the current Apicomplexa many unrelated groups. For example, Kudo (1954) included in the Sporozoa species of the Ascetosporea (Rhizaria), Microsporidia (Fungi), Myxozoa (Animalia), and Helicosporidium (Chlorophyta), while Zierdt (1978) included the genus Blastocystis (Stramenopiles). [34] Dermocystidium was also thought to be sporozoan. Not all of these groups had spores, but all were parasitic. [31] However, other parasitic or symbiotic unicellular organisms were included too in protozoan groups outside Sporozoa (Flagellata, Ciliophora and Sarcodina), if they had flagella (e.g., many Kinetoplastida, 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 between bacteria or yeasts.

Sporozoa is no longer regarded as biologically valid and its use is discouraged, [35] although some authors still use it as a synonym for the Apicomplexa. More recently, other groups were excluded from Apicomplexa, e.g., Perkinsus and Colpodella (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: [36] [31]

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. [37]

Jacques Euzéby (1988)

Jacques Euzéby in 1988 [38] 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. [39]

Roberts and Janovy (1996)

Roberts and Janovy in 1996 divided the phylum into the following subclasses and suborders (omitting classes and orders): [40]

These form the following five taxonomic groups:

  1. The gregarines are, in general, one-host parasites of invertebrates.
  2. The adeleorins are one-host parasites of invertebrates or vertebrates, or two-host parasites that alternately infect haematophagous (blood-feeding) invertebrates and the blood of vertebrates.
  3. The eimeriorins 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. This term is often used to include the adeleorins.
  4. Haemospororins, often known as the malaria parasites, are two-host Apicomplexa that parasitize blood-feeding dipteran flies and the blood of various tetrapod vertebrates.
  5. Piroplasms where all the species included are two-host parasites infecting ticks and vertebrates.

Perkins (2000)

Apicomplexa tree..png

Perkins et al. proposed the following scheme. [41] It is outdated as the Perkinsidae have since been recognised as a sister group to the dinoflagellates rather that the Apicomplexia:

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.

The name Protospiromonadida has been proposed for the common ancestor of the Gregarinomorpha and Coccidiomorpha. [42]

Another group of organisms that belong in this taxon are the corallicolids. [43] 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. [44] This genus is found in the renal sac of molgulid ascidian tunicates.

Evolution

Members of this phylum, except for the photosynthetic chromerids, [45] 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. [46] [47] Further evolution of this phylum has been estimated to have occurred about 800  million years ago. [48] The oldest extant clade is thought to be the archigregarines. [46]

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. [49] 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. [15]

Myzozoa
Apicomplexa s.l.

Squirmida ( Digyalum , Filipodium , Platyproteum )

Chrompodellids/Apicomonadea
Apicomplexa s.s.
Gregarines  s.l.

Cryptosporidium

Gregarines s.s.

Marosporida

Aggregatidae ( Aggregata , Merocystis )

Margolisiella

Rhytidocystidae ( Rhytidocystis )

Coccidiomorphea
Coccidia

Hemogregarines

Coccidia with a single host ( Eimeria , Isospora , Cyclospora )

Cyst-forming coccidia ( Toxoplasma , Sarcocystis , Frenkellia )

Hematozoa

Piroplasms ( Babesia , Theileria )

Hemosporidia ( Plasmodium , Leucocytozoon )

Dinoflagellates & Perkinsozoa

Janouškovec et al 2015 presents a somewhat different phylogeny, supporting the work of others showing multiple events of plastids losing photosynthesis. More importantly this work provides the first phylogenetic evidence that there have also been multiple events of plastids becoming genome-free. [50]

See also

Related Research Articles

<i>Plasmodium</i> Genus of parasitic protists that can cause malaria

Plasmodium is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of Plasmodium species involve development in a blood-feeding insect host which then injects parasites into a vertebrate host during a blood meal. Parasites grow within a vertebrate body tissue before entering the bloodstream to infect red blood cells. The ensuing destruction of host red blood cells can result in malaria. During this infection, some parasites are picked up by a blood-feeding insect, continuing the life cycle.

<span class="mw-page-title-main">Coccidia</span> Subclass of protists

Coccidia (Coccidiasina) are a subclass of microscopic, spore-forming, single-celled obligate intracellular parasites belonging to the apicomplexan class Conoidasida. As obligate intracellular parasites, they must live and reproduce within an animal cell. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa.

<i>Eimeria</i> Genus of single-celled organisms

Eimeria is a genus of apicomplexan parasites that includes various species capable of causing the disease coccidiosis in animals such as cattle, poultry and smaller ruminants including sheep and goats. Eimeria species are considered to be monoxenous because the life cycle is completed within a single host, and stenoxenous because they tend to be host specific, although a number of exceptions have been identified. Species of this genus infect a wide variety of hosts. Thirty-one species are known to occur in bats (Chiroptera), two in turtles, and 130 named species infect fish. Two species infect seals. Five species infect llamas and alpacas: E. alpacae, E. ivitaensis, E. lamae, E. macusaniensis, and E. punonensis. A number of species infect rodents, including E. couesii, E. kinsellai, E. palustris, E. ojastii and E. oryzomysi. Others infect poultry, rabbits and cattle. For full species list, see below.

<span class="mw-page-title-main">Gregarinasina</span> Subclass of protists

The gregarines are a group of Apicomplexan alveolates, classified as the Gregarinasina or Gregarinia. The large parasites inhabit the intestines of many invertebrates. They are not found in any vertebrates. Gregarines are closely related to both Toxoplasma and Plasmodium, which cause toxoplasmosis and malaria, respectively. Both protists use protein complexes similar to those that are formed by the gregarines for gliding motility and for invading target cells. This makes the gregarines excellent models for studying gliding motility, with the goal of developing treatment options for both toxoplasmosis and malaria. Thousands of different species of gregarine are expected to be found in insects, and 99% of these gregarine species still need to be described. Each insect species can be the host of multiple gregarine species. One of the most-studied gregarines is Gregarina garnhami. In general, gregarines are regarded as a very successful group of parasites, as their hosts are distributed over the entire planet.

An apicoplast is a derived non-photosynthetic plastid found in most Apicomplexa, including Toxoplasma gondii, and Plasmodium falciparum and other Plasmodium spp., but not in others such as Cryptosporidium. It originated from algae through secondary endosymbiosis; there is debate as to whether this was a green or red alga. The apicoplast is surrounded by four membranes within the outermost part of the endomembrane system. The apicoplast hosts important metabolic pathways like fatty acid synthesis, isoprenoid precursor synthesis and parts of the heme biosynthetic pathway.

<span class="mw-page-title-main">Adeleorina</span> Suborder of microscopic, spore-forming, single-celled parasites in the aplcomplex phylum

Adeleorina is a suborder of parasites in the phylum Apicomplexa.

Chromera velia, also known as a "chromerid", is a unicellular photosynthetic organism in the superphylum Alveolata. It is of interest in the study of apicomplexan parasites, specifically their evolution and accordingly, their unique vulnerabilities to drugs.

<span class="mw-page-title-main">Myzozoa</span> Group of single-celled organisms

Myzozoa is a grouping of specific phyla within Alveolata, that either feed through myzocytosis, or were ancestrally capable of feeding through myzocytosis.

<span class="mw-page-title-main">Apicomplexan life cycle</span> Apicomplexa life cycle

Apicomplexans, a group of intracellular parasites, have life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an apicomplexan organism is typified by a cellular variety with a distinct morphology and biochemistry.

Polychromophilus is a genus of obligate intracellular eukaryotic parasites that infect bats from every continent except Antarctica. They are transmitted by bat flies, which act as an insect vector as well as the parasite’s site of sporogeny. Polychromophilus follows a fairly typical Haemospororidian lifecycle, with gametocytes and gametes restricted to the bloodstream of the host and meronts infecting organs – most notably the lungs and the liver. The type species is Polychromophilus melanipherus, and was described by Dionisi in 1898.

Goussia is a taxonomic genus, first described in 1896 by Labbé, containing parasitic protists which largely target fish and amphibians as their hosts. Members of this genus are homoxenous and often reside in the gastrointestinal tract of the host, however others may be found in organs such as the gallbladder or liver. The genera Goussia, as current phylogenies indicate, is part of the class Conoidasida, which is a subset of the parasitic phylum Apicomplexa; features of this phylum, such as a distinct apical complex containing specialized secretory organelles, an apical polar ring, and a conoid are all present within Goussia, and assist in the mechanical invasion of host tissue. The name Goussia is derived from the French word gousse, meaning pod. This name is based on the bi-valve sporocyst morphology which some Goussians display. Of the original 8 classified Goussians, 6 fit the “pod” morphology. As of this writing, the genera consists of 59 individual species.

<i>Cyclospora</i> Genus of single-celled organisms

Cyclospora is a genus of apicomplexan parasites. It includes the species Cyclospora cayetanensis, the causative agent of cyclosporiasis. Members of Cyclospora are characterized as having oocysts with two sporocysts, each containing two sporozoites.

The genus Schellackia comprises obligate unicellular eukaryotic parasites within the phylum Apicomplexa, and infects numerous species of lizards and amphibians worldwide. Schellackia is transmitted via insect vectors, primarily mites and mosquitoes, which take up the parasite in blood meals. These vectors then subsequently infect reptilian and amphibian which consume the infected insects. The parasites deform erythrocytes of the host into crescents, and can be visualized using a blood smear.

Vitrella brassicaformis (CCMP3155) is a unicellular alga belonging to the eukaryotic supergroup Alveolata. V. brassicaformis and its closest known relative, Chromera velia, are the only two currently described members of the phylum Chromerida, which in turn constitutes part of the taxonomically unranked group Colpodellida. Chromerida is phylogenetically closely related to the phylum Apicomplexa, which includes Plasmodium, the agent of malaria. Notably, both V. brassicaformis and C. velia are photosynthetic, each containing a complex secondary plastid. This characteristic defined the discovery of these so-called 'chromerids,' as their photosynthetic capacity positioned them to shed light upon the evolution of Apicomplexa's non-photosynthetic parasitism. Both genera lack chlorophyll b or c; these absences link the two taxonomically, as algae bearing only chlorophyll a are rare amid the biodiversity of life. Despite their similarities, V. brassicaformis differs significantly from C. velia in morphology, lifecycle, and accessory photosynthetic pigmentation. V. brassicaformis has a green color, with a complex lifecycle involving multiple pathways and a range of sizes and morphologies, while Chromera has a brown color and cycles through a simpler process from generation to generation. The color differences are due to differences in accessory pigments.

The Archigregarinorida are an order of parasitic alveolates in the phylum Apicomplexa. Species in this order infect marine invertebrates — usually annelids, ascidians, hemichordates and sipunculids.

Lankesterella is a genus in the phylum Apicomplexa. Species in this genus infect amphibians, reptiles and birds.

The Eugregarinorida are the most large and diverse order of gregarines — parasitic protists belonging to the phylum Apicomplexa. Eugregarines are found in marine, freshwater and terrestrial habitats. These species possess large trophozoites that are significantly different in morphology and behavior from the sporozoites. This taxon contains most of the known gregarine species.

Filipodium is a genus of parasites in the phylum Apicomplexa. Species in this genus infect marine invertebrates.

Nematopsis (Nee-mah-top-cis) is a genus gregarine Apicomplexan of the family Porosporidae. It is an aquatic parasite of crustaceans with a molluscan intermediate host. Nematopsis has been distinguished from the similar genus Porospora by its resistant and encapsulated oocyst. Little molecular biology has been performed on the members of the Nemaptosis and species are described based on molluscan and crustacean hosts as well as oocyst structure. A total of 38 species have been described and are found all over the world.

<i>Gregarina garnhami</i> Insect-parasitic micro-organism

Gregarina garnhami is a eukaryotic unicellular organism belonging to the Apicomplexa described in 1956 by Canning as a parasite found in several locusts, such as the desert locust, African migratory locust, and Egyptian locust. Especially, the desert locust is the host for this species, as up to 100% of animals can become infected. An estimated thousands of different species of gregarines can be in insects and 99% of these gregarines still need to be described. Each insect is said to host multiple species. A remarkable feature of G. garnhami is its autofluorescence.

References

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