The Apicomplexa (also referred to as Apicomplexia) 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[1] parasites of animals. Motile structures such as flagella or pseudopods are present only in certain gamete stages. This is a diverse group including organisms such as coccidia, gregarines, piroplasms, haemogregarines, and plasmodia. Diseases caused by apicomplexan organisms include, but are not limited to:
While Apicomplexa is not synonymous with the older term "Sporozoa", there is significant overlap between the species included in the two groupings.[2]
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.
General features
Within this phylum there are four groups - Perkensus, coccidians, gregarines and haemosporidians.
Perkensus is a parasite of bivalve mollusks and exhibits characters that are seen in the dinoflagellates including laterally-inserted heterodynamic flagella. It is currently the only known species in this class.
The gregarines are generally parasites of annelids, arthropods and mollusks. They are often found in the guts of their hosts but may invade the other tissues.
In the typical gregarine life cycle a trophozoite develops within a host cell into a plasmodium which divides into merozoites by schizogony.
The merozoites are released by lysing the host cell and then they invade other cells.
Gamonts are formed at some point and emerge from the host cells to group together by syzygy.
Each gamont forms multiple gametes which fuse and form zygocysts which leave the host to be taken up by another.
Coccidians are generally parasites of vertebrates. Like the gregarines they are commonly parasites of the epithelial cells of the gut but may infect other tissues also.
The life cycle is similar to that of the gregarines but differs in the formation of the zygote. Some trpphozoites enlage and become macrogamete while others divide repeatedly to form microgametes. The microgametes are motile and much reach the macrogamete to fertilize it. The fertilzed macrogamete forms a zygote which in turn forms an oocyst which is normally relased from the body.
Taxonomy
The phylum has three classes which have been divided into a number of subclasses and orders
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.
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.[3] 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, and it appears to be necessary for survival. Plastids 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.[4][5]
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 suborders of parasites.
The Haemosporina includes the malaria parasites and thier relatives.
The Eimeriorina - the largest suborder in this phylum - the life cycle involves both sexual and asexual stages. The asexual stages reproduce by schizogony. The male gametocyte produces a large number of gametes and the zygote gives rise to an oocyst which is the infective stage. The majority are monoxenous (infect one host only) but a few are heteroxenous (life cycle involves two or more hosts).
Both the number of families in this later suborder is debated with the number of families being between one and twenty depending on the authority and the number of genera being between nineteen and twenty five. This somewhat unsatisfactory state of affairs awaits resolution with DNA based methods.
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.[6]
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
^Jadwiga Grabda (1991). Marine fish parasitology: an outline. VCH. p. 8. ISBN0895738236.
^Duszynski1, Donald W. (2004-02-21). "The Coccidia of the World"(Online database). Department of Biology, University of New Mexico, and Division of Biology, Kansas State University. ((cite web)): Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: numeric names: authors list (link)
