Apicomplexa

The Apicomplexa are a large group of protists, characterized by the presence of a unique organelle called an apical complex. 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)
• Cryptosporidiosis (Cryptosporidium)
• Coccidian diseases including:
  • Cryptosporidiosis (Cryptosporidium parvum)
  • Cyclosporiasis (Cyclospora cayetanensis)
  • Toxoplasmosis (Toxoplasma gondii)

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Most members have a complex life-cycle, involving both asexual and sexual reproduction. Typically, a host is infected by ingesting cysts, 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..^[1] 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, although some studies suggest they are ultimately derived from green rather than red algae.

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 1901

genera:

• Haemogregarina - taxon created by Danilewsky in 1885
• Cyrilia - taxon created by Lainson in 1981

• family Karyolysidae - taxon created by Wenyon in 1926

genera:

• Karyolysus - taxon created by Labbe in 1894

• family Hepatozoidae - taxon created by Wenyon in 1926

genera:

• 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, ApiDB.org, provides public access to currently available genomic data sets.

References