Apicomplexa

Apicomplexa is one of the biggest and most well-known taxon of parasitic protists. There is about 4,000 known species. Apicomplexans are characterized by a very unique organelle called an apical complex. They form spores, unicellular and are parasites of animals. They move using flagella or pseudopods. One of the most well known diseases caused by one of these organisms is malaria. There is other diseases such as; babesiosis, Cryptosporidiosis, Coccidian diseases, Cyclosporiasis, and Toxoplasmosis.

Classification
Originally most parasitic protists were put in the Sporozoa class. Since its creation the Sporozoa class was found to have protists from numerous unrelated lineages, and has not been used by modern taxonomists for some time. Many of the protists in Sporozoa were found to share some structural components, such as an apical complex of microtubules inside of a cell. The protists possessing these structures were grouped into what we now know as Apicomplexa.

Parasitism
Apicomlexans are a very important part of all the plant and animal life. They infect both invertebrates and vertebrates. Some of the protists in this class can be helpful where as others can cause very severe illnesses. Those that cause malaria are a species in the genus Plasmodium. Around 300 million people are infected with malaria each year, and over 1 million die from it. Coccidiosis and toxoplasmosis can cause illnesses in humans and domesticated animals. Some Apicomlexans have been used experimentally on insects. They try to control the insect population by infecting them with various protists from Apicomplexa.

Anatomy
The apical complex has small vesicles commonly named rhoptries and micronemes. These open at the front of the cell. They release enzymes that allow parasites to enter other cells. The tip of the vesicles are surrounded by a ring of microtubules, called a polar ring. The conoidasida possess a funnel of rods called a conoid. The rest of the cell membrane is held up by alveoli. This forms a pellicle.

Since some Apicomlexans posses alveoli they are placed in a group named alveolates. Other flagellates have structures similar to that of a polar ring and were included in this group. However upon further studies scientists found they are closer to dinoflagellates. Another trait Apicomplexans and alveolates have in common is an apicoplast. The apicoplast is encircled by 3 or 4 other membranes. Scientists think apicoplasts perform functions like lipid synthesis, because it seems to be necessary in order for the apicoplasts to survive.

Life Cycle
The life cycle of many cyst-like protists can be seen in the figure to the left. Apicomplexa have a very intricate life style. It can be summed up into three different processes, sporogony, merogony, and gametogony. Sporogony happens right after a sexual phase. It consists of asexual reproduction that ends in the creation of sporozoites. Sporozoites will mature into organisms that go through another asexual phase. This phase is called merogony. This phase produces merozoites, they are known by other names depending on the species that is performing merogony. There is usually several rounds of merogony, and in some the host organism or cell type switch due to the invasion of the parasite. Gametogony and sporogony can also involve different cell types. Sometimes merozites become gametes through a process known as gametogony.

Just like in other types of sexual reproduction the gametes form a zygote that separates into a form that gives off sporozoites. The sporozoites and merozoites have apical organelles, they are also invasive. Every species of Apicomplexa differentiate in their life cycles, this is a general life cycle.

Related References

 * introduction to the Apicomplexa
 * Apicomplexa wikipedia
 * Apicomplexa tulane.edu