Apicomplexan

Apicomplexan is a protozoa. As the apicomplexan is the spore-producing phylum, apicomplexan is termed sporozoan. All apicomplexans are parasitic organisms, which are lacking in contractile vacuoles and locomotor processes. Important Apicomplexa characteristics are, it lives within the cells or body cavities of other organisms. One apicomplexan can live within the other apicomplexans. Some Sporozoa live as pathogens. Genes of Plasmodium cause malaria. Coccidiosis is caused by Eimeria and Isospora. Sporozoa intakes their food from the host or host’s cytoplasm or fluids of the body.  The respiration and excretion of apicomplexa occur by simple diffusion through the cell membrane. Apicomplexan protozoa can undergo both sexual and asexual reproduction. During sexual reproduction, it will precede spores. While undergoing binary or multiple fission reactions during asexual reproduction, this is termed schizogony. 

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Reproduction in Apicomplexa 

Apicomplexa can undergo both sexual and asexual reproduction. During asexual reproduction, Apicomplexan replicates via binary fission or multiple fission. This method of reproduction is also termed schizogony. Apicomplexan undergoes various stages during its reproduction. They are named gametogony, sporogony, and merogony. 

Gametogony

Sporozoans undergo a gametogony state when they are going to reproduce through sexual reproduction. During this stage, gamonts are developed to produce gametes. These gametes get fused with the nucleus to form a cyst. Then the cyst starts to develop into a young one. 

Merogony

Apicomplexan undergoes the stage merogony during asexual reproduction.  After Sporozoa entering into the host cell, a trophozoite starts to increase its size and repeatedly replicates its nucleus and other organelles. During this process, the organism is termed meront or schizont. Further in the next stage cytokinesis subdivides the multinucleated meronts into numerous identical daughter cells. This stage is called merozoites. When the host cell gets ruptured, the identical daughter cells start to release into the blood. We can observe this kind of reproduction in  Theileria, Plasmodium, Babesia, and Toxoplasma gondii.

Sporogony 

Apicomplexan undergo sporogony both during asexual and sexual reproduction. During this process, the zygote is formed and this is termed karyogamy. The zygote formed undergoes meiosis and multiple fission. Sporozoites are formed as an end product. 

Other forms of Apicomplexan replications are endopolygeny and endodyogeny. 

During asexual reproduction, endodyogeny occurs. This is an unusual process, in which two daughter cells will develop inside a mother cell and get separated from the mother cell. When several organisms developed at once by internal budding, this is known as endopolygeny. 

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Major Parasites Under Apicomplexan Protozoa 

Gregarines

Gregarines are parasites belonging to the family Gregarinasina, which are found in annelids, arthropods, and mollusks. They widely target the guts of the host cells, they can also invade other tissues. Gregarines usually undergo asexual reproduction. Here, the trophozoite in the host cell develops into schizont and is further divided into merozoites. The divided merozoites released from the host start to invade other cells. To some extent, the gametocytes are formed and each gametocyte develops into multiple gametes. These gametes fuse with the nucleus of others to form oocysts. The grown oocysts start targeting the new host cells. 

Coccidians

Coccidians are a kind of parasite. Which usually targets vertebrates. Like gregarines, they choose epithelial cells of the gut of vertebrates as a host. But it may also infect other tissues for the host. The coccidian will undergo stages like merogony, gametogony, and sporogony during its reproduction. During sexual reproduction, the zygotes are formed and trophozoites get enlarged to form macrogametes. Likewise, others are divided to form microgametes. The formation of microgametes is known as anisogamy. The microgametes developed are motile and can travel and reach the macrogamete to fertilize it.  Here, the fertilized macrogamete developed into a zygote and forms oocyst. The completely developed oocyst release to the body and starts targeting the new cells. 

The major difference between gregarines and coccodians is in gamonts. Coccodia does not have epimerites or microns and has small intracellular. Also, in coccidia, a single gamont develops into a macrogametocyte. But gregarines have epimerites or mucrons with large extracellular. Here, the single gamont gives rise to multiple gametocytes. 

Haemosporidia 

Haemosporidia belongs to the class Heamosporid. The life cycle of Haemosporidia is complex. Because it gets alternate between vertebrate host and an arthropod host. Here, the trophozoite targets erythrocytes or other tissues in vertebrates. Both the micro and macrogametes are found in the blood. The vector insects will take these gametes from the blood while sucking the blood. The microgametes migrate within the guts and fuse with macrogametes to fertilize it. The macrogamete later develops into an ookinete, which enters the body of the vector. Then the ookinete transforms into an oocyst and undergoes meiosis and mitosis and gives rise to sporozoites. The sporozoites escaped from oocyst migrate within the body of the vector and reach the new vertebrate host during the next feed of vector insect. 

Marosporida

The class Marosporida is a newly recognized lineage of apicomplexans. It is a sister class of Coccidia and Hematozoa. This class contains species like Rhytidocystis sp. 1 and Rhytidocystis sp. 2, Aggregata octopiana, and Margolisiella islandica.  The Marosporida mainly infects marine invertebrates. Members of this clade retain plastid genomes and the canonical apicomplexan plastid metabolism. Marosporidians contains the most reduced apicoplast genomes, which are sequenced to date, lack canonical plastidial RNA polymerase. So they provide reductive organelle evolution.