Abstract
The phylum Apicomplexa is a large group of parasitic protists with more than 6,000 described and possibly thousands of undescribed species. All species are obligatory parasites, and potentially every vertebrate and majority of invertebrates host at least one apicomplexan species.
More frequently apicomplexans are specialists with rather high host specificity; nevertheless, generalists with low host specificity exist. Many species are highly pathogenic to their host including human and domestic animals and from medical perspective represent the most important eukaryotic parasites.
Coccidians are omnipresent in vertebrates, e.g., virtually all poultry and rabbits are infected by several host-specific Eimeria spp.; theileriosis is responsible for enormous losses in cattle farming; about 20% of global human population is infected by Toxoplasma gondii; and, finally, Plasmodium falciparum and other Plasmodium species cause globally distributed malaria, which kills millions of people in tropical countries. The phylum Apicomplexa includes morphologically and ecologically diverse protists, such as the gregarines, cryptosporidia, coccidia, haemosporidia, and piroplasms.
The life cycle of majority of Apicomplexa involves sexual and asexual multiplication in the parasitized host and an environmentally resilient cyst forms. Transmission strategies are diverse, from direct transmission to intricate cycles in trophic webs between predators and their prey or involving arthropod vectors.
The phylum is highly successful, thanks to morphological and molecular adaptations. The name is derived from two Latin words, apex (top) and complexus (infolds), and refers to a set of organelles composed from spirally arranged microtubules, polar ring(s), and secretory bodies, such as rhoptries and micronemes.
Apical complex structures mediate entry of the parasite into the host cells, where they usually survive inside a parasitophorous vacuole. Most apicomplexans possess a unique organelle called the apicoplast, which is a highly reduced non-photosynthetic plastid, which retains few functions essential for a parasite survival.
The phylum evolved from a photosynthetic flagellate, and core apicomplexans form a sister group to a free-living marine and freshwater protists (Chromera, Vitrella, and Colpodella).