These parasites carry a leftover algal plastid they can no longer use for sunlight
Apicomplexans are single-celled parasites that break into animal cells using a specialised toolkit at their tip. Most also carry the apicoplast, a relic plastid probably inherited from red algae, which no longer photosynthesises but seems essential for survival. They glide through tissue without flagella, cilia or crawling pseudopods.
The phylum is a large group of spore-forming protozoa, nearly all obligate parasites of animals for at least part of their lives. The exception is Nephromyces, a partner of marine animals once mistaken for a chytrid fungus. Members include coccidia, gregarines, piroplasms and plasmodia, and Cryptosporidium parvum causes the disease cryptosporidiosis. Most of what older textbooks called Sporozoa now sits here.
The name joins Latin words for tip and infolds, describing the apical complex of the infective stage, the sporozoite. This kit includes a ring of microtubules, club-shaped rhoptries and slender micronemes that release enzymes to breach a host cell, and in some lineages a spiralling funnel called the conoid. Dense granules about 0.7 micrometres across empty their contents after the parasite has settled inside its host. A three-layered pellicle braced by flattened sacs called alveoli covers the rest of the cell, pierced only by tiny feeding pores.
Movement is unusual. Instead of beating hairs, apicomplexans glide using adhesion proteins and small stationary myosin motors, which lets them slip through tissues and in and out of cells. Flagella, usually one to three, appear only on gametes. The apicoplast, wrapped in three or four membranes, keeps its own circular genome of about 35 kilobases and appears to help make lipids and haem, though Cryptosporidium and Gregarina niphandrodes have lost it.
Life cycles alternate asexual and sexual phases. Sporozoites invade cells and multiply until the cells burst, releasing merozoites that infect more cells, possibly many times, before gamonts form gametes whose fusion yields new cysts. Many species need two hosts. The alveoli place the phylum among the alveolates, and relatives such as Perkinsus and Colpodella hint at a shared ancestry with dinoflagellates.
Source: Apicomplexa
