Apicomplexa

Taxobox
name = Apicomplexa

|Plasmodium
domain = Eukaryota
regnum = Chromalveolata
superphylum = Alveolata
phylum = Apicomplexa
subdivision_ranks = Classes & Subclasses
subdivision = Aconoidasida

*Haemosporasina
*PiroplasmasinaBlastocystea
Conoidasida

*Coccidiasina
*GregarinasinaThe Apicomplexa are a large group of protists, characterized by the presence of a unique organelle called an "apical complex" (see also apicoplast). 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 malarias; some diseases caused by apicomplexan organisms include:

* Babesiosis ("Babesia")
* Malaria ("Plasmodium")
* Coccidian diseases including:
** Cryptosporidiosis ("Cryptosporidium parvum")
** Cyclosporiasis ("Cyclospora cayetanensis")
** Toxoplasmosis ("Toxoplasma gondii")

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.

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..cite web
url=http://biology.unm.edu/biology/coccidia/home.html
title=The Coccidia of the World
last = Duszynski1
first = Donald W.
coauthors = Steve J. Upton and Lee Couch
work =
publisher = Department of Biology, University of New Mexico, and Division of Biology, Kansas State University
date = 2004-02-21
format = Online database ] 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. [cite journal | url = http://www.amjbot.org/cgi/content/full/91/10/1481 | journal = American Journal of Botany | year = 2004 | volume = 91 | pages = 1481–1493 | title = Diversity and evolutionary history of plastids and their hosts | author = Patrick J. Keeling | doi = 10.3732/ajb.91.10.1481 ] [cite journal | doi = 10.1093/nar/gkn483 | year = 2008 | month = Jul | author = Ram, Ev; Naik, R; Ganguli, M; Habib, S | title = DNA organization by the apicoplast-targeted bacterial histone-like protein of Plasmodium falciparum | volume = | issue = | pages = | pmid = 18663012 | journal = Nucleic acids research ]

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")

Blood parasites belonging to the suborder Adeleorina are collectively known as haemogregarines. Currently their sister group is thought to be the piroplasms.

Suborder Adeleorina has ~400 species and has been organised into four large and 4 small genera.

The larger genera are:

*family Haemogregarinidae - taxon created by Neveu-Lemaire in 1901genera:
*"Haemogregarina" - taxon created by Danilewsky in 1885
*"Cyrilia" - taxon created by Lainson in 1981

*family Karyolysidae - taxon created by Wenyon in 1926genera:
*"Karyolysus" - taxon created by Labbe in 1894

*family Hepatozoidae - taxon created by Wenyon in 1926genera:
*"Hepatozoon" - taxon created by Miller in 1908

The smaller genera are :

*"Hemolivia" - taxon created by Petit "et al" in 1990
*"Desseria" - taxon created by Siddall in 1995

*family Dactylosomatidae genera:
*"Dactylosoma"
*"Babesiosoma"

Notes:

Species of the genus "Desseria" infect fish and lack erythrocytic merogony.

The species of the genera "Dactylosoma" and "Babesiosoma" infect fish and reptiles.
Leeches are the only known vectors for these species and their vertebrate hosts are aquatic.

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, [http://ApiDB.org 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. [cite journal | doi = 10.1128/AAC.00394-06 | year = 2006 | month = Sep | author = Dahl, El; Shock, Jl; Shenai, Br; Gut, J; Derisi, Jl; Rosenthal, Pj | title = Tetracyclines specifically target the apicoplast of the malaria parasite Plasmodium falciparum | volume = 50 | issue = 9 | pages = 3124–31 | pmid = 16940111 | pmc = 1563505 | journal = Antimicrobial agents and chemotherapy | url = http://aac.asm.org/cgi/pmidlookup?view=long&pmid=16940111 | format = Free full text ]

Most apicomplexans have plastid genomes as well as nuclear ones, although "Cryptosporidium parvum" is an exception as it has lost its plastid genome.

References

External links

*cite web
coauthors = S.J. Brands (Compiler)
title = The Taxonomicon & Systema Naturae
work = Taxon: Genus Cryptosporidium
publisher = Universal Taxonomic Services, Amsterdam, The Netherlands
date = 2000
url = http://www.taxonomy.nl/taxonomicon/
format = Website database