Apicomplexa

Apicomplexa is a phylum made up almost entirely of parasites. Apicomplexans are distinguished by their unique method of entering host cells. Apicomplexans are a monophyletic group composed entirely of parasitic species. This species is the best known and probably the largest of parasitic protists. There are about 4,000 known species, but this is certainly just an understatement of the actual number. Of the described species seven genera infect humans; these include the following: Plasmodium, Babesia, Cryptosporidium, Isospora, Toxoplasma, Cyclospora and Sarcocystis. Species in the genus Plasmodium cause malaria in humans and other animals; an estimated 300 million people in over 90 countries are infected with malaria, and over 1 million die from it each year. Others apicomplexans cause serious illnesses, such as coccidiosis and toxoplasmosis, in humans and domestic animals. However, apicomplexans that infect insects have been used experimentally to control populations of insect pests. The typical Apicomplexan life cycle is comprised of three stages: sporogony, merogony, and gametogony. The generalized life cycle may vary in its details between genera. Sporogony, an asexual reproductive phase, produces haploid sporozoites, and follows the sexual phase, gamogony. During gamogony male and female gametes are formed, and upon fusion a diploid zygote (called an ookinete or sporoblast) results, and this is where genetic recombination can take place. .Sporogony is followed by a phase of asexual reproduction called merogony, and the cellular products of this phase are called merozoites. Merogony and schizogony are synonyms. Merozoites can invade host cells and undergo multiple rounds of merogony or they may differentiate into gametocytes/gametes (= gamogony.) Parasite amplification, i.e. increase in numbers, takes place during merogony and sporogony. Only the invasive stages--merozoite, sporozoite, and ookinete—contain an apical complex.
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