Apicomplexa

Apicomplexa
source
Name	Apicomplexa
Taxon	Apicomplexa
Authority	Levine, 1970
Subdivision ranks	Major groups
Subdivision	Aconoidasida, Conoidasida

Apicomplexa. The Apicomplexa are a large and diverse phylum of exclusively parasitic protists, characterized by a unique apical complex used for invading host cells. They are of immense medical and veterinary importance, causing diseases such as malaria, toxoplasmosis, and coccidiosis in a wide range of animals, including humans. All members are obligate parasites, with complex life cycles often involving multiple hosts and both sexual and asexual reproduction.

Taxonomy and classification

The phylum was formally established by Norman D. Levine in 1970, unifying groups previously classified separately. It is placed within the superphylum Alveolata, sharing a common ancestry with dinoflagellates and ciliates, as supported by molecular phylogenetics. Major classes include the Conoidasida, which possess a complete conical structure, and the Aconoidasida, which lack this feature during the invasive stage. Historically significant taxonomists like Édouard Chatton and William Trager contributed to understanding their relationships. The group encompasses orders such as Eucoccidiorida, which includes the families Sarcocystidae and Eimeriidae, and Haemospororida, containing the genus Plasmodium.

Morphology and ultrastructure

The defining feature is the apical complex, a concentration of organelles including the conoid, rhoptries, and micronemes used for host cell penetration. The cell is surrounded by a pellicle composed of the plasma membrane and underlying alveoli, providing structural rigidity. Most are non-motile in their vegetative stages, though invasive forms like the sporozoite and merozoite exhibit gliding motility. Ultrastructural studies by researchers like Keith Vickerman revealed critical details of their cytoskeleton and invasion apparatus. They generally possess a non-photosynthetic plastid called the apicoplast, a remnant of secondary endosymbiosis, which is essential for metabolism.

Life cycle and reproduction

Life cycles are complex and heteroxenous, often alternating between a definitive host and an intermediate host. Reproduction typically involves both asexual multiplication through processes like schizogony or endodyogeny, and sexual reproduction via gametogony to form zygotes. The zygote develops into an oocyst, which undergoes sporogony to produce infective sporozoites. The life cycle of Plasmodium vivax, studied by Alphonse Laveran and Ronald Ross, involves both Anopheles mosquitoes and human hosts. Environmental transmission can occur via ingestion of oocysts, as seen with Toxoplasma gondii and its spread through cat feces.

Pathogenicity and notable species

Many species are significant pathogens. The genus Plasmodium, with species like Plasmodium falciparum and Plasmodium malariae, is responsible for malaria, a disease historically documented by Hippocrates and a major focus of the World Health Organization. Toxoplasma gondii causes toxoplasmosis, posing risks to fetuses and immunocompromised individuals, with research advanced by Elmer Pfefferkorn. Eimeria tenella and related species cause coccidiosis, a devastating disease in poultry, impacting global agriculture. Cryptosporidium parvum, studied by Ernest Tyzzer, causes severe diarrheal disease and is a concern for water safety.

Evolution and phylogeny

Molecular evidence confirms their evolution from photosynthetic ancestors within the Alveolata, with the apicoplast originating from a red alga via secondary endosymbiosis. Phylogenetic analyses, utilizing genes like SSU rRNA, place the Gregarinasina as often early-branching, with the Coccidiomorpha (including Plasmodium and Toxoplasma) as a derived group. The adaptation to parasitism involved massive gene loss and the acquisition of specialized invasion machinery. Studies of the Mitochondrial genome reveal highly reduced and unusual structures. The evolutionary success of the Haemospororida is linked to their association with Diptera vectors.