Sporozoa(redirected from Sporozoans)
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Sporozoa(spôr'əzō`ə), phylum of unicellular heterotrophic organisms of the kingdom ProtistaProtista
, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms.
..... Click the link for more information. . Unlike most other protozoansprotozoan
, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues.
..... Click the link for more information. , sporozoans have no cilia or flagella. All species are parasitic and have elaborate life cycles, often requiring more than one host. The best-known sporozoan is Plasmodium falciparum, the causative organism of malariamalaria,
infectious parasitic disease that can be either acute or chronic and is frequently recurrent. Malaria is common in Africa, Central and South America, the Mediterranean countries, Asia, and many of the Pacific islands.
..... Click the link for more information. .
A subphylum of Protozoa, typically with spores. The spores are simple and have no polar filaments. There is a single type of nucleus. There are no cilia or flagella except for flagellated microgametes in some groups. In most Sporozoa there is an alternation of sexual and asexual stages in the life cycle. In the sexual stage, fertilization is by syngamy, that is, the union of male and female gametes. All Sporozoa are parasitic. The subphylum is divided into three classes—Telosporea, Toxoplasmea, and Haplosporea. See Haplosporea, Protozoa
a class of parasitic protozoans including about 2,000 species. The class was identified in 1879 by the German scientist R. Leuckart. Sporozoans are characterized by primary alternation of generations and asexual and sexual reproduction. The principal stages in the life cycle are schizogony (asexual reproduction by multiple segmentation), gamogony (gamete formation and fertilization), and sporogony (formation of spores and sporozoites from the zygote). Schizogony is absent in most gregarines. Sporozoans parasitize the cells, tissues, and cavities of animals and humans. Schizogony leads to an increase in the number of parasites in the body of the host, and sporogony ensures infection of other individuals of the host species. Zygotic reduction is observed in all sporozoans: the first division of the zygote nucleus during sporogony is meiotic, and all the subsequent stages are haploid.
Some sporozoans, for example, most coccidians, have a single host; their distribution is external and by means of oocysts, which are covered with protective sheaths. Other sporozoans, including Plasmodium (the causative agents of malaria), have two hosts: asexual reproduction occurs in one, and the sexual process and sporogony occur in the other. In these sporozoans transmission of the parasite from one host to the other is effected by a bite or by ingestion of the host by another animal. For example, malaria is transmitted to man by means of a mosquito bite, and Haemogregarina is transmitted when a lizard eats an infected tick. In such cases the stages with protective sheaths are absent, and tiny wormlike mononuclear cells—sporozoites—develop in sporocysts and infect the vertebrate host.
Sporozoans include gregarines and Coccidiomorpha; the latter comprise Coccidia and Haemosporidia. Among the haemosporidians are the causative agents of a number of serious human disease (malaria, toxoplasmosis) and diseases of domestic mammals and birds (coccidiosis).
REFERENCEZhizn zhivotnykh, vol. 1. Moscow, 1968. Pages 116–29.
IU. I. POLIANSKII