Apicomplexa

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Apicomplexa
Scientific classification Edit this classification
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

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

pseudopods are present only in certain gamete
stages.

The Apicomplexa are a diverse group that includes organisms such as the

haemogregarines, and plasmodia
. Diseases caused by Apicomplexa include:

The name Apicomplexa derives from two

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[7]
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.

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

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
]

  • Flagella are found only in the motile gamete. These are posteriorly directed and vary in number (usually one to three).
  • Basal bodies are present. Although hemosporidians and piroplasmids have normal triplets of microtubules in their basal bodies, coccidians and gregarines have nine singlets.
  • The
    cristae
    .
  • Centrioles, chloroplasts, ejectile organelles, and inclusions are absent.
  • The cell is surrounded by a
    pellicle
    of three membrane layers (the alveolar structure) penetrated by micropores.

Replication:

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

Mobility:

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

Further information:
Apicomplexa lifecycle stages
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

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

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

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
Main article: Gregarinasina

The gregarines are generally parasites of

oocysts. The oocysts leave the host to be taken up by a new host.[17]

Coccidians

Dividing Toxoplasma gondii (Coccidia) parasites
Main article: Coccidia

In general, coccidians are parasites of

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
Main article: Haemosporida

The Haemosporidia have more complex lifecycles that alternate between an arthropod and a vertebrate host. The trophozoite parasitises

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

Rhytidocystidae Levine, 1979, originally coccidians, Agamococcidiorida), and Margolisiella islandica Kristmundsson 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

Many of the apicomplexan parasites are important pathogens of humans and domestic animals. In contrast to

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

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

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

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

Further information: wikispecies:Apicomplexa

History

The first Apicomplexa protozoan was seen by

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

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]

  • Class Conoidasida
    • Subclass
      p.p.
    • Subclass
      Coccidiasina
      p.p
  • Class Aconoidasida
    • Order
      Haemospororida
      , with 444 species and nine genera
    • Order
      Piroplasmorida
      , with 173 species and 20 genera
  • Other minor groups omitted above, with 105 species and 32 genera

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)

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]

  • Subclass Gregarinasina (the gregarines)
  • Subclass
    Coccidiasina
    • Suborder Adeleorina (the adeleorins)
    • Suborder Eimeriorina (the eimeriorins)
    • Suborder
      Haemospororina
      (the haemospororins)
  • Subclass
    Piroplasmasina
    (the piroplasms)

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)

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:

  • Class Aconoidasida
    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

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

Further information: Alveolate § Phylogeny

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

Vitrella, Piridium
)

Colpodellida (Colpodella
)

Alphamonas, Voromonas
)

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)

Perkinsozoa

Janouškovec et al 2015 presents a somewhat different phylogeny, supporting the work of others showing multiple events of

phylogenetic evidence that there have also been multiple events of plastids becoming genome-free.[50]

See also

References

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External links
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