Sporozoa

Apicomplexa
Scientific classification
Domain: Eukaryota Kingdom: Chromalveolata Superphylum: Alveolata Phylum: Apicomplexa
Classes & Subclasses
Aconoidasida Haemosporasina Piroplasmasina Conoidasida Coccidiasina Gregarinasina

The Apicomplexa are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or pseudopods are absent except in certain gamete stages. This is a diverse group including organisms such as coccidia, gregarines, piroplasms, haemogregarines, and plasmodia; some diseases caused by apicomplexan organisms include:

• Babesiosis (Babesia)
• Malaria (Plasmodium)
• Coccidian diseases including:
  • Cryptosporidiosis (Cryptosporidium parvum)
  • Cyclosporiasis (Cyclospora cayetanensis)
  • Isosporiasis (Isospora belli)
  • Toxoplasmosis (Toxoplasma gondii)

While "Apicomplexa" is not synonymous with the older term "Sporozoa", there is significant overlap between the species included in the two groupings.^[1]

History

The first apicomplexan protozoan was seen by Antony van Leeuwenhoek who in 1674 saw oocysts of Eimeria stiedae in the gall bladder of a rabbit. The first member of the phylum to be named (by Dufour in 1828) was Gregarina ovata in earwigs. Since then many more have been identified and named. During the quarter century 1826-1850, 41 species and 6 genera of Apicomplexa were named. In the quarter century 1951-1975, 1873 new species and 83 new genera were added.

By 1987 a comprehensive survey of the phylum was completed: in all 4516 species and 339 genera had been named. They consisted of:

• the gregarines (subclass Gregarinasida) with 1624 named species and 231 named genera
• the hemogregarines (family Haemogregarinidae) with 399 species and 4 genera
• the eimeriorins (order Eimeriorida) with 1771 species and 43 genera
• the hemospororids (order Haemospororida with 444 species and 9 genera
• the piroplasmids (order Piroplasmorida) with 173 species and 20 genera
• and a few others (105 species and 32 genera)

Although there has been considerable revision of this phylum it seems likely these numbers are still approximately correct.

Evolution

Many Coccidiomorpha have an intermediate host as well as the primary host, and the evolution of hosts proceeded in different ways and at different times in these groups. In some the original host has become the intermediate host while in others it has become the definitive host. In the genera Aggregata, Atoxoplasma, Cystoisospora, Schellackia and Toxoplasma the original is now definitive while in Akiba, Babesiosoma, Babesia, Haemogregarina, Haemoproteus, Hepatozoon, Karyolysus, Leukocytozoon, Plasmodium, Sarcocystis and Theileria have 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 Protococcidiida and Eimeriida. Hypnozoites are found in Karyolysus lacerate and most species of Plasmodium; transovarial transmission of parasites occurs in life cycles of Karyolysus and Babesia.

Life cycle

Most members have a complex life-cycle, 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. There are many variations on this basic pattern, however, and many Apicomplexa have more than one host.

Generic life cycle of an apicomplexa: 1-zygote (cyst), 2-sporozoites, 3-merozoites, 4-gametocytes.

Apicomplexan structure: 1-polar ring, 2-conoid, 3-micronemes, 4-rhoptries, 5-nucleus, 6-nucleolus, 7-mitochondria, 8-posterior ring, 9-alveoli, 10-golgi apparatus, 11-micropore.

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 there is also a funnel of rods called the conoid..^[2] Over the rest of the cell, except for a diminished mouth called the micropore, the membrane is supported by vesicles called alveoli, forming a semi-rigid pellicle.

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.

Another similarity is that apicomplexan cells contain a single plastid, called the apicoplast, surrounded by either 3 or four membranes. Its functions are thought to include tasks such as lipid synthesis, it appears to be necessary for survival. They are generally considered to share a common origin with the chloroplasts of dinoflagellates, and evidence generally points to an origin from red algae rather than green.^[3][4]

The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group for parasitic protozoans without flagella, pseudopods, or cilia. Most of the Apicomplexa are motile however. The other main lines were the Ascetosporea, the Myxozoa (now known to be derived from animals), and the Microsporidia (now known to be derived from fungi). Sometimes the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.

Blood borne genera

Within the Apicomplexa there are three groups of blood borne parasites. These species lie within in three suborders.

• suborder Adeleorina - 8 genera
• suborder Haemosporina - all genera in this suborder
• suborder Eimeriorina - 2 genera (Lankesterella and Schellackia)

Disease Genomics

As noted above, many of the apicomplexan parasites are important pathogens of human and domestic animals. In contrast to bacterial pathogens, these apicomplexan parasites are eukaryotes and share many metabolic pathways with their animal hosts. This fact makes therapeutic target development extremely difficult – a drug that harms an apicomplexan parasite is also likely to harm its human host. Currently there are no effective vaccines or treatments 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 the 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 parasite. A NIH-funded database, ApiDB.org, provides public access to currently available genomic data sets. 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.^[5]

Most apicomplexans have plastid genomes as well as nuclear ones, although Cryptosporidium spp. and possibly gregarines are exceptions as they are thought to have lost plastids after the diverging last common ancestor of apicomplexans.

References

External links

• "The Taxonomicon & Systema Naturae" (Website database). Taxon: Genus Cryptosporidium. Universal Taxonomic Services, Amsterdam, The Netherlands. 2000. http://www.taxonomy.nl/taxonomicon/.