The ''Archigregarinorida'' are an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...

alveolates in the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. T ...

. Species in this order infect marine invertebrates — usually

annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

ascidian Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" ...

hemichordate Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include ...

s and

sipunculid The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of unsegmented marine annelid worms. The name ''Sipuncula'' is from the genus name ''Sipunculus'', and comes from the Latin ' ...

Taxonomy

This order was redefined by Levine in 1971. The order currently consists of 76 species in two families — '' Exoschizonidae'' and '' Selenidioididae''. The family ''Exoschizonidae'' contains one

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

— '' Exoschizon'' — which has one

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

. The family '' Selenidioididae'' has six genera: ''

Filipodium ''Filipodium'' is a genus of parasites in the phylum Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthe ...

'' with 3 species, '' Merogregarina'' with one species, '' Meroselenidium'' with one species, ''

Platyproteum The ''Platyproteum'' are a genus of parasitic alveolates in the phylum Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type o ...

'' with one species, '' Selenidioides'' with 11 species and ''

Veloxidium ''Veloxidium'' is a genus of parasitic alveolates in the phylum Apicomplexa. Species in this genus infect marine invertebrates. Taxonomy This species was described in 2012 by Wakeman and Leander.Wakeman KC, Leander BS (2012) Molecular phylogeny ...

'' with one species.

Phylogenetics

DNA studies suggest that the archigregarines are ancestral to the other gregarines.

Phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

suggests that this group is

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

and will need division. The

Neogregarinorida The Neogregarinorida are an order of parasitic alveolates in the phylum Apicomplexa. Species in this order infect insects and are usually found in the fat body, hemolymph, hypodermis, intestine or Malpighian tubules. The most common site of infec ...

appear to be derived from the

Eugregarinorida 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 trophozoite ...

. Assuming this is correct the evolutionary order appears to be: the Archigregarinorida gave rise to the Eugregarinorida who in turn gave rise to the Neogregarinorida. Morrison using molecular data has shown that the

Haemosporidia Haemosporidiasina (Haemosporidia) is a subclass of apicomplexans described by Jacques Euzéby in 1988.Euzéby, J. (1988) Comparative Medical Protozoology, Vol. 3: Apicomplexa, 2: Haemosporidioses, Part 1: Plasmodiids, Haemoproteids, "Piroplasms" ...

appear to nest within the gregarines.

Description

The species in this order are relatively large spindle shaped cells, compared to other apicomplexans and eukaryotes in general (some species are in length). Most gregarines have longitudinal epicytic folds (bundles of

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...

s beneath the cell surface with

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...

like bending behaviour). * Syzygy occurs in all species *

Monoxenous Monoxenous development, or monoxeny, characterizes a parasite whose development is restricted to a single host species. The etymology of the terms monoxeny / monoxenous derives from the two ancient Greek words (), meaning "unique", and (), me ...

— only one

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

in life cycle *

Mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

have tubular

crista A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area fo ...

e and are often distributed near the cell periphery *Apical complex is present in both the

sporozoite 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 i ...

and trophozoite stages *Trophozoites have a large and conspicuous

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

and

nucleolus The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of sig ...

*Inhabit extracellular body cavities of invertebrates such as the

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...

coelom The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...

s and reproductive vesicles *Attachment to host via a

mucron A ''mucron'' is an attachment organelle found in archigregarines - an order of epicellular parasitic Conoidasida.Simdyanov TG, Guillou L, Diakin AY, Mikhailov KV, Schrével J, Aleoshin VV. (2017) A new view on the morphology and phylogeny of eugreg ...

(aseptate gregarines) or an epimerite (septate gregarines) *Possess longitudinal epicytic folds *Have a

myzocytosis Myzocytosis (from Greek: myzein, (') meaning "to suck" and kytos (') meaning "container", hence referring to "cell") is a method of feeding found in some heterotrophic organisms. It is also called "cellular vampirism" as the predatory cell pierces ...

-based mode of feeding

Life cycle

Archigregarines are found only in marine habitats and are transmitted by the orofaecal route.

Merogony 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 i ...

gamogony 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 i ...

and

sporogony 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 ...

are thought to occur in all species in this taxon. In all species four or more

s (the precise number depends on the species) equipped with an apical complex escape from the

oocyst 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 i ...

s, find their way to the appropriate body cavity and penetrate host cells in their immediate environment. The sporozoites emerge within the host cell, begin to feed and develop into larger

trophozoite A trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria-causing ''Plasmodium falciparum'' and those of the '' Giardia'' group. (The complement of the trophozo ...

s. In some species, the sporozoites and trophozoites are capable of asexual replication — a process called

schizogony Fission, in biology, is the division of a single entity into two or more parts and the regeneration of those parts to separate entities resembling the original. The object experiencing fission is usually a cell, but the term may also refer to how ...

merogony 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 i ...

. Most species however appear to lack schizogony in their lifecycles. The intestinal trophozoites are similar in morphology to the infective sporozoites. In all species two mature trophozoites eventually pair up in a process known as (syzygy) and develop into gamonts. The gamonts are aseptate. During syzygy gamont orientation differs between species (side to side, head to tail). A gametocyst wall forms around each pair of gamonts which then begin to divide into hundreds of

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...

Zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...

s are produced by the fusion of two gametes and these in turn become surrounded by an oocyst wall. Within the oocyst

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...

occurs yielding the sporozoites. Hundreds of oocysts accumulate within each gametocyst and these are released via host's

faeces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

or via host death and decay.

References

{{Taxonbar, from=Q4786846 Apicomplexa orders