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Apicomplexa
Scientific classification
Domain: Eukaryota
Kingdom: Chromalveolata
Superphylum: Alveolata
Phylum: Apicomplexa
Classes & Subclasses

Aconoidasida

Conoidasida

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[1] 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:

While "Apicomplexa" is not synonymous with the older term "Sporozoa", there is significant overlap between the species included in the two groupings.[2 ]

Contents

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..[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, 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.[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 groups of blood borne parasites. These species lie within in three suborders.

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

  1. ^ Jadwiga Grabda (1991). Marine fish parasitology: an outline. VCH. p. 8. ISBN 0895738236.  
  2. ^ "Introduction to the Apicomplexa". http://www.ucmp.berkeley.edu/protista/apicomplexa.html. Retrieved 2009-05-31.  
  3. ^ Duszynski1, Donald W.; Steve J. Upton and Lee Couch (2004-02-21). "The Coccidia of the World" (Online database). Department of Biology, University of New Mexico, and Division of Biology, Kansas State University. http://biology.unm.edu/biology/coccidia/home.html.  
  4. ^ Patrick J. Keeling (2004). "Diversity and evolutionary history of plastids and their hosts". American Journal of Botany 91: 1481–1493. doi:10.3732/ajb.91.10.1481. http://www.amjbot.org/cgi/content/full/91/10/1481.  
  5. ^ Ram, Ev; Naik, R; Ganguli, M; Habib, S (July 2008). "DNA organization by the apicoplast-targeted bacterial histone-like protein of Plasmodium falciparum". Nucleic acids research. doi:10.1093/nar/gkn483. PMID 18663012.  
  6. ^ Dahl, El; Shock, Jl; Shenai, Br; Gut, J; Derisi, Jl; Rosenthal, Pj (September 2006). "Tetracyclines specifically target the apicoplast of the malaria parasite Plasmodium falciparum" (Free full text). Antimicrobial agents and chemotherapy 50 (9): 3124–31. doi:10.1128/AAC.00394-06. PMID 16940111. PMC 1563505. http://aac.asm.org/cgi/pmidlookup?view=long&pmid=16940111.  

External links

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

SPOROZOA, a large and most important section of the Protozoa, all the members of which are exclusively parasitic in habitat. They are of extremely widespread occurrence; there is hardly one of the chief classes of animals which does not furnish hosts for these parasites, scarcely one of the common tissues or organs of the Metazoan body which may not be liable to infection. Sporozoa differ greatly as regards the effects which they produce upon their hosts. In many, perhaps in most, cases the general health of the infected animal seems to be unimpaired, even though the. parasites may be fairly abundant. Some, however, give rise to dangerous or fatal diseases, while others may cause ravaging epidemics; instances of these are given under the various orders.

Correlated with the mode of life are the two features character istic of all Sporozoa: (a) They absorb only fluid nutriment, osmotically, and so lack any organs for ingesting and digesting solid food; and (b) they reproduce by sporulation, i.e. the formation of minute germs, which are in most instances very numerous and are often enclosed in firm protective envelopes or cases, each case with its contents forming a spore. In addition, the great majority have also another method of reproduction, for increasing the number of the parasites in any individual host; this is distinguished as multiplicative or endogenous reproduction, from the propagative or exogenous method (by means of the resistant spores), which serves for the infection of fresh hosts and secures the dissemination and survival of the species. Further, most if not all forms of Sporozoa 'undergo sexual conjugation at some period or other of the life-cycle.

Beyond this, however, it is impossible to generalize. In response to the exceeding diversity of habitat and of the conditions of life, the parasites exhibit manifold and widely-different types of form, organization and life-history. The recognition of this fact is expressed, at the present day, by the division of the Sporozoa into several well-defined orders, which are grouped in two main divisions, each containing more or less closely related forms. One of these groups consists of the Gregarines, Coccidia and Haer_aosporidia (qq.v.). The other comprises the Myxosporidia, Actinomyxidia, Sarcosporidia and Haplosporidia, the parasites included in the last named order being of comparatively simple structure, and probably near the base of this section. There are, in addition, various other forms (Seroand Exo-sporidia), also primitive in character, but which are as yet too insufficiently known for it to be certain whether they are of distinct ordinal rank, or should be placed with the Haplosporidia. The nomenclature assigned to these two principal divisions of the Sporozoa by different writers has varied according to the particular character on which they have primarily based the arrangement. Of late years, the terms Telosporidia and Neosporidia, proposed by F. Schaudinn (1900), have been most in favour. In the Telosporidia (comprising the Gregarines, Coccidia and Haemosporidia), sporulation does not begin until the close of the vegetative or trophic period, i.e. until growth has ceased; in the Neosporidia (including the remaining orders) growth and sporulation go on coincidently. Recently, however, considerable doubt has been thrown upon the general occurrence of this latter condition in certain Myxosporidia (Microsporidia); and the present writer adopts as preferable, therefore, the terms Ectospora and Endospora (qq.v.), invented by E. Metschnikoff and made use of by F. Mesnil (1899), which indicate a universal distinction between the two groups in their manner of sporulation. This distinction is probably the most fundamental one, and itself supports a conclusion which is, on other grounds, becoming more and more likely, namely, that these two divisions are not related phylogenetically; but have, on the contrary, a radically different origin. In other words, under the heading Sporozoa, as at present used, are included two entirely independent series of Protozoan parasites; the general resemblances which these exhibit are due to convergence brought about by their specialized mode of life.

The most recent and comprehensive account of the group is that by E. A. Minchin (in Lankester's Treatise on Zoology, pt. i., London, 1903), to which the present writer is much indebted; another useful treatise is that of F. Doflein, Die Protozoen als Parasiten u. Krankheitserreger (G. Fischer, Jena, 1901). Earlier accounts are those of M. Liihe, Ergebnisse der neuren Sporozoenforschung (Jena, 1900); Wasielewski, Sporozoenkunde (Jena, 1896); Y. Delage and E. Hhrouard in Traite de zoologie concreie, pt. i., Paris, 1896); E. R. Lankester, art. "Protozoa" in Ency. Brit. 9th' ed. (1886), and O. Biitschli in Bronn's Klassen u. Ordnungen des Thierreichs, I. i. (1882). There is a systematic enumeration of the group by A. Labbh in Das Thierreich, 5. (Berlin, 1899); and the classification and phylogeny are considered by E. Mesnil (Soc. Biol., vol. jub. p. 258, Paris, 1899), and by H. Crawley in Amer. Nat. (1905), xxxix. 607.

(H. M. Wo.)


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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

Contents

Translingual

Etymology

Proper noun

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Sporozoa

  1. a taxonomic infraphylum, within subphylum Apicomplexa - the sporozoans
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See also


Wikispecies

Up to date as of January 23, 2010

From Wikispecies

Taxonavigation

Main Page
Cladus: Eukaryota
Regnum: Protista
Subregnum: Biciliata
Infraregnum: Alveolata
Phylum: Myzozoa
Subphylum: Apicomplexa
Infraphylum: Sporozoa
Classes: Aconoidasida - Coccidea - Gregarinea - Hematozoa - Piroplasmea

References

  • Thomas Cavalier-Smith and E. E. Chao, Protalveolate phylogeny and systematics and the origins of Sporozoa and dinoflagellates (phylum Myzozoa nom. nov.), Europ. J. Protistol. 40, 185-212 (2004).
  • Thomas Cavalier-Smith, Protist phylogeny and the high-level classification of Protozoa, Europ. J. Protistol. 39, 338-348 (2003).

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