APICOMPLEXA: CHARACTERISTICS, TAXONOMY, SUBGROUPS, MORPHOLOGY - BIOLOGY - 2025

The Apicomplexa is a phylum of unicellular protists comprising about 5000 species, all honeycombed parasitic. Many of these species are of medical and economic importance.

They present a structure called the apical complex, to which the group name alludes. This complex comprises a type of plastid called an apicoplast and a network of microtubules.

Cyst of a hemogregarin (Phyllum Apicomplexa) in the liver of a Gabon (Miniopterus inflatus) Taken and edited from commons.wikimedia.org

The function of the apical complex seems to be to allow the parasite to attach to a host cell and release a substance that causes invagination of the same. This invagination allows the parasite to enter the cell.

Apicomplexa include various groups of organisms such as coccidia, gregarines, piroplasmas, hemogregarins, and plasmodia. They are the cause of numerous diseases in animals and man. These diseases include toxoplasmosis, malaria, cryptosporidiosis, and cyclosporosis.

characteristics

The main characteristic of the group is the presence of the apical complex. This complex is made up of a conoid, or set of microtubules arranged in a spiral; a roptria with secretory function and one or more polar rings.

Additionally, they may have other thin secretory bodies called micronemes. The micronemes are surrounded by one or two polar rings.

Distributed throughout the cell are spherical organelles called dense granules. These have a secretory function and measure about 0.7 μm.

The cell is surrounded by a film and alveolar vesicles penetrated by micropores. They have a haploid nucleus. Mitochondria possess tubular ridges. Plasti are present in only a few species.

The movement is of the sliding type thanks to the use of adhesions and contractile protein molecules (myosin). Some species produce gametes that can be displaced by the presence of flagella or the ability to produce pseudopods.

Another feature is the production of oocysts. Oocysts contain sporozoites which are the infectious form.

Taxonomy

The species that make up this taxon on various occasions have been included in groups as diverse as microsporidia, chlorophytes, among others.

The first species of Apicomplexa, Gregarina ovata, was described by Dufour in 1828. For this description he used isolated specimens of earwig intestines. On that date it was included among the Vermes.

Leuckart, in 1879, erected the taxon Sporozoa, included among the Protozoa, where he located some Apicomplexa. The Sporozoa taxon was later rejected, and most of its members were housed in the Apicomplexa taxon, created in 1970.

Currently some authors consider the taxon as a subphylum within the Myxozoa, but more commonly they are accepted as a phyllum.

Subgroups

Apicomplexes are divided into four subclasses: gregarine and coccidia, located in the Conoidasida class, and hemosporidia and piroplasmas, in the Aconoidasida class.

Gregarinas (Gregarinasina)

They are large parasites (about 0.5 mm) that mainly inhabit the intestines of annelids, arthropods and mollusks, although they can also invade other tissues. Gammon maturation usually occurs in a cellular form and gives rise to multiple gametocytes.

Cocciidia (Coccidiasin)

Individuals in this subclass are obligate intracellular parasites primarily of intestinal epithelial cells, but are also found in the blood, liver, and other organs.

They parasitize both vertebrates and higher invertebrates. The gamontes develop intracellularly and the zygote is generally immobile. Each gamonte becomes a single macrogametocyte.

Cocciidia (Coccidiasin), Coccidia sp. Taken and edited from

Hemosporidia (Haemosporida)

Haemosporidia are intraerythrocytic parasites capable of causing serious diseases in animals and humans. They have complex life cycles that alternate between an arthropod host acting as a vector and a vertebrate host, the definitive host.

Trophozoites parasitize red blood cells or other tissues of the vertebrate host. Among the hemosporidia is Plasmodium, which causes malaria.

Piroplasmas (Piroplasmid)

Piroplasms are vertebrate parasites that use ticks or leeches as vectors. They receive this name because the first species described produced hyperthermia in the bovine hosts they infected.

They have life cycles similar to those of hemosporidia. They are distinguished from these by not forming oocysts or spores. Another difference is that, in the trophozoite phase, they are separated from the erythrocyte by a single membrane. Other blood parasites usually have at least two membranes.

Morphology

All Apicomplexa have the apical complex. Gregarines are divided into two groups by the morphology of the trophozoite or gamonte.

In cephalin gregarines, the body is divided into 3 parts, an epimerite, corresponding to the apical organ for adhesion; a protomerite or anterior section of the cell; and a deuteromerite, which corresponds to the posterior section of the cell.

Acephaline gregarines lack epimerite. In the Accephaniloidea, the trophozoite is unsegmented, while the Cephaniloidea have the body divided into two compartments by an ectoplasmic septum. Gametocytes are rounded.

The shape of the hemosporidium trophozoite can change over time, presenting a ringed shape in the early stages, and later maturing to an amoeboid form. The schizont is large and irregular, while the gametocytes are round or oval.

Piroplasmas are generally pear-shaped, however, some species are pleomorphic, and can be ovoid, rounded, amoeboid, comma-shaped, rod-shaped, or elongated rings. The pear shapes are found in pairs receiving the name of bigéminas.

Habitat

The Apicomplexa are obligate endoparasites, which means that they always inhabit the interior of their hosts. Some species are intracellular parasites, others can mature extracellularly.

The number of hosts can vary between one and two. In the case of being two, generally the definitive host is a vertebrate. The intermediate is generally an arthropod.

Reproduction

Apicomplexes reproduce both sexually and asexually. There are modifications in the life cycles and reproduction mechanisms depending on the group of organisms.

-Gregarines

Asexual reproduction

The trophozoite develops into a schizont that divides by schizogony, giving rise to numerous merozoites. Merozoites are released from the host cell by lysis and invade new cells.

This process can be repeated several times. At some point, gametocytes are formed that are released by lysis of host cells.

Sexual reproduction

A gametocyte forms a large number of gametes. Gametes fuse in pairs to form oocysts. The latter leave their host to find a new one.

-Cocciides (Coccidiasin)

Asexual reproduction

Similar to that of gregarines

Sexual reproduction

Some trophozoites increase in size to become individual macrogametes, others divide multiple times to form microgametes. The latter are mobile and seek the macrogamete to fertilize it.

The fertilized macrogamete becomes a short-lived zygote that transforms into an oocyst. The oocyst normally leaves the host.

-Hemosporidia (Haemosporida)

During sexual reproduction the microgametes fuse with the macrogametes. The zygote now becomes an ookinet, which then becomes an oocyst. The latter is initially divided by meiosis and then by mitosis, giving rise to sporozoites.

-Piroplasmas (Piroplasmid)

These organisms have life cycles similar to those of hemosporidia. They differ from them by not forming oocysts or spores.

Different stages of the life cycle of Plasmodium falciparum: Ookinet (mobile zygote), sporozoite (mobile) and merozoite (immobile). Taken and edited from

Diseases

All apicomplexes are parasites, some of them of medical and veterinary importance. Among the diseases they cause are:

Malaria

Also called malaria, it is a disease caused by parasites of the genus Plasmodium. The symptoms are varied with periodic and recurring fevers and chills, sweating and headache.

Nausea, vomiting, coughing, bloody stools, muscle aches, jaundice, and blood clotting defects also occur. As the disease worsens, shock, kidney or liver failure can occur. Additionally, central nervous system disorders, coma and even death can occur.

The vectors of the disease are mosquitoes of the genus Anopheles. Females of this mosquito by feeding on the blood of an infected person can transmit the disease to other healthy people.

One form of direct contagion is through the placenta from the mother to the fetus. Blood transfusions from donors who have had the disease are another form of contagion.

Toxoplasmosis

Caused by the protozoan Toxoplasma gondii, an obligate intracellular parasite. It is transmitted from animals to humans through different routes of contagion.

Several species of felines are the definitive hosts. Toxoplasmosis can cause mild, symptomless infections. Fatal infections are those that mostly affect the fetus, causing the so-called fetal or congenital toxoplasmosis.

The disease can also be complicated when it affects patients with suppressed immune systems, such as people infected with HIV.

Cyclosporidiosis

Opportunistic disease caused by the Cryptosporidium parasite, present in some foods or in contaminated water. The infection is self-limited in immunocompetent individuals, but potentially fatal in immunosuppressed patients.

In the former it presents as watery diarrhea with the presence of mucus, fever, nausea, vomiting, abdominal pain and weight loss. In the latter, symptoms are complicated with loss of up to 10% of body weight, jaundice, and severe malabsorption.

Cyclosporosis

This disease is caused by Cyclospora cayetanensis and transmitted by the fecal-oral route by ingestion of contaminated food or water. It is not transmitted from person to person.

It is a common cause of diarrhea in travelers. Symptoms are severe diarrhea, flatulence, fever, stomach aches, and muscle aches. The main hosts are humans and other primates.

References

1. Apicomplexa. On Wikipedia. Recovered from en.wikipedia.org/wiki/Apicomplexa
2. R. Brusca, GJ Brusca (2003). Invertebrates. 2nd Edition. Sinauer Associates.
3. MT Gleeson (2000). The plastid in Apicomplexa: what use is it? International Journal for Parasitology.
4. ND Levine (1971). Uniform Terminology for the Protozoan Subphylum Apicomplexa. Journal of Eukaryotic Microbiology.
5. ND Levine (1988). Progress in taxonomy of the Apicomplexan protozoa. The Journal of Protozoology.
6. DA Morrison (2009). Evolution of the Apicomplexa: where are we now? Trends in Parasitology.
7. E. Siński, JM Behnke (2004). Apicomplexan parasites: environmental contamination and transmission. Polish Journal of Microbiology.