The gregarines are a group of
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. Th ...
n alveolates, classified as the Gregarinasina
or Gregarinia. The large (roughly half a millimeter) parasites inhabit the intestines of many invertebrates. They are not found in any vertebrates. However, gregarines are closely related to both ''
Toxoplasma
''Toxoplasma gondii'' () is an obligate intracellular parasitic protozoan (specifically an apicomplexan) that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but felids, such as d ...
'' and ''
Plasmodium
''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 ver ...
'', which cause
toxoplasmosis
Toxoplasmosis is a parasitic disease caused by '' Toxoplasma gondii'', an apicomplexan. Infections with toxoplasmosis are associated with a variety of neuropsychiatric and behavioral conditions. Occasionally, people may have a few weeks or mont ...
and
malaria
Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...
, respectively. Both protists use protein complexes similar to those that are formed by the gregarines for
gliding motility Gliding motility is a type of translocation used by microorganisms that is independent of propulsive structures such as flagella, pili, and fimbriae. Gliding allows microorganisms to travel along the surface of low aqueous films. The mechanisms of ...
and invading target cells.
This makes them excellent models for studying gliding motility with the goal of developing treatment options for toxoplasmosis and malaria.
Thousands of different species of gregarines are expected to be found in
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...
s, and 99% of these gregarines still need to be described. Each insect can be the host of multiple species. One of the most studied gregarines is ''
Gregarina garnhami
''Gregarina garnhami'' is a Eukaryote, eukaryotic unicellular organism belonging to the Apicomplexa described in 1956 by Canning as a Parasitism, parasite found in several locusts, such as the desert locust, African migratory locust, and Anacridi ...
''.
In general, gregarines are regarded as very successful parasites, as their hosts are spread over the entire planet.
Lifecycle
Gregarines occur in both aquatic and terrestrial environments. Although they are usually transmitted by the
orofaecal route, some are transmitted with the
host's
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 ...
s during
copulation
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetra ...
(''
Monocystis
''Monocystis (Gr., monos, ''single'' + kystisis, ''bladder'')'' is a genus (the type of the family Monocystidae) of acephaline gregarines (subclass Gregarinasina) not having the protoplasm divided into segments by septa and including internal pa ...
'').
In all species, four or more
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 is ...
s (the precise number depends on the species) equipped with an apical complex escape from the
oocysts, a process called excystation, 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 trophozoi ...
s. In some species, the sporozoites and trophozoites are capable of asexual replication – a process called
schizogony or
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.
In all species, two mature trophozoites eventually pair up in a process known as
syzygy and develop into
gamonts. 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 ...
s.
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 multicell ...
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 r ...
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.
![Lankesteria lm](https://upload.wikimedia.org/wikipedia/commons/f/f3/Lankesteria_lm.jpg)
Gregarines have been reported to infect over 3000 invertebrate species.
[Alarcón M E., Huang C-G, Tsai Y-S, Chen W-J, Kumar A (2011) Life cycle and morphology of ''Steinina ctenocephali'' (Ross 1909) comb. nov. (Eugregarinorida: Actinocephalidae), a gregarine of ''Ctenocephalides felis'' (Siphonaptera: Pulicidae) in Taiwan. Zoological Studies 50(6): 763-772]
Taxonomy
The gregarines were recognised as a
taxon
In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
by
Grasse
Grasse (; Provençal oc, Grassa in classical norm or in Mistralian norm ; traditional it, Grassa) is the only subprefecture of the Alpes-Maritimes department in the Provence-Alpes-Côte-d'Azur region on the French Riviera. In 2017, the c ...
in 1953.
The three orders into which they are currently divided were created by Levine ''et al.'' in 1980.
Currently, about 250 genera and 1650 species are known in this taxon. They are divided into three orders based on habitat, host range, and trophozoite morphology.
Most species have
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 (), m ...
lifecycles involving a single invertebrate host. In the lifecycle, the extracellular feeding stage is known as the trophozoite.
Main divisions
Archigregarines are found only in marine habitats. They possess intestinal trophozoites similar in morphology to the infective sporozoites. Phylogenetic analysis suggests this group is paraphyletic and will need division. Generally, four zoites are in each spore in this group.
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. The intestinal
eugregarines are separated into septate – suborder
Septatorina – and
aseptate – suborder
Aseptatorina – depending on whether the trophozoite is superficially divided by a transverse septum. The aseptate species are mostly marine gregarines.
Urosporidians are aseptate eugregarines that infect the coelomic spaces of marine hosts. Unusually, they tend to lack attachment structures and form gamont pairs that pulsate freely within the
coelomic fluid.
Monocystids are aseptate eugregarines that infect the reproductive vesicles of terrestrial
annelids. These latter species tend to branch closely with
neogregarines and may need to be reclassified. Generally, eight zoites are in each spore in this group.
Neogregarines are found only in terrestrial hosts. These species have reduced trophozoites and tend to infect tissues other than the intestine. Usually, eight zoites are in each spore in this group.
The eugregarines and neogregarines differ in a number of respects. The neogregarines are in general more pathogenic to their hosts. The eugregarines multiply by sporogony and gametogony, while the neogregarines have an additional schizogenic stage – merogony – within their hosts. Merogony may be intracellular or extracellular depending on the species.
DNA studies suggest the
archigregarines are ancestral to the others.
Proposed revisions
Cavalier-Smith has proposed a significant revision of this taxon assuming the
polyphyly of
eugregarines.
He has separated gregarines into three classes. The first of them –
Gregarinomorphea – comprises
Orthogregarinia,
Cryptosporidiidae
''Cryptosporidium'', sometimes informally called crypto, is a genus of apicomplexan parasitic alveolates that can cause a respiratory and gastrointestinal illness (cryptosporidiosis) that primarily involves watery diarrhea (intestinal cryptosp ...
and, additionally,
Rhytidocystidae previously considered as divergent
coccidia
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 ...
ns
or
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. Th ...
''incertae sedis''.
The Orthogregarinia with two new orders
Arthrogregarida and
Vermigregarida was created for the gregarines most closely related to ''
Cryptosporidium''. The second class –
Paragregarea – was created for the archigregarines,
Stenophorida and a new order –
Velocida which itself was created for ''
Urosporoidea'' ''superfam. n.'' and ''
Veloxidium''. The third class was created –
Squirmida – for ''
Filipodium'' and ''
Platyproteum''. Thus, the
eugregarines proved to be split and distributed among these three classes together with some other
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. Th ...
ns.
This point of view was challenged in 2017 by Simdyanov and co-authors, who performed the global integrated analysis of available morphological and molecular phylogenetic data and concluded that
eugregarines are rather a
monophyletic taxon.
Several genera of gregarines are currently not classified: ''
Acuta'', ''
Cephalolobus'', ''
Gregarina'', ''
Levinea'', ''
Menospora'', ''
Nematocystis'', ''
Nematopsis'', ''
Steinina'', and ''
Trichorhynchus''.
Characteristics
*
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 r ...
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 (), m ...
– only one host occurs in lifecycle for almost all species.
*
Mitochondria have tubular
cristae
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 ...
and are often distributed near the cell periphery.
*Apical complex occurs in the sporozoite stage, but is lost in the trophozoite stage in eugregarines and neogregarines.
*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
Nucl ...
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 ...
.
*They inhabit extracellular body cavities of invertebrates such as the intestines, coeloms, and reproductive vesicles.
*Attachment to host occurs by a
mucron (
aseptate gregarines) or an
epimerite (septate gregarines); some gregarines (urosporidians) float freely within extracellular body cavities (coelom).
The parasites 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
microtubules beneath the cell surface with nematode like bending behaviour): crenulations are instead found in the urosporidians.
Molecular biology
The gregarines are able to move and change direction along a surface through gliding motility without the use of
cilia,
flagella, or
lamellipodia
The lamellipodium (plural lamellipodia) (from Latin ''lamella'', related to ', "thin sheet", and the Greek radical ''pod-'', "foot") is a cytoskeletal protein actin projection on the leading edge of the cell. It contains a quasi-two-dimensiona ...
.
This is accomplished through the use of an
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...
and
myosin complex.
The complexes require an actin
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is com ...
to perform their gliding motions.
In the proposed ‘capping’ model, an uncharacterized protein complex moves rearward, moving the parasites forward.
History
The gregarines are among the oldest known parasites, having been described by the physician
Francesco Redi in 1684.
The first formal description was made by Dufour in 1828. He created the genus ''
Gregarina'' and described ''
Gregarina ovata'' from ''
Folficula aricularia''. He considered them to be parasitic worms. Koelliker recognised them as protozoa in 1848.
References
Further reading
*
External links
Tree of Life GregarinaGregarina Movies
{{Taxonbar, from=Q1154061
SAR supergroup subclasses
Conoidasida
Apicomplexa genera