Apicomplexans, a group of
intracellular parasites, have
life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an
apicomplexan organism is typified by a ''cellular variety'' with a distinct
morphology and
biochemistry
Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...
.
Not all apicomplexa develop all the following cellular varieties and division methods. This presentation is intended as an outline of a hypothetical generalised apicomplexan organism.
Methods of asexual replication
Apicomplexans (sporozoans) replicate via ways of
multiple fission (also known as
schizogony). These ways include , and , although the latter is sometimes referred to as schizogony, despite its general meaning.
Merogony is an
asexually reproductive process of apicomplexa. After infecting a host cell, a trophozoite (
see glossary below) increases in size while repeatedly replicating its
nucleus and other
organelles. During this process, the organism is known as a or .
Cytokinesis next subdivides the
multinucleated schizont into numerous identical daughter cells called merozoites (
see glossary below), which are released into the blood when the host cell ruptures. Organisms whose life cycles rely on this process include ''
Theileria'', ''
Babesia'', ''
Plasmodium'', and ''
Toxoplasma gondii.''
Sporogony is a type of sexual and asexual reproduction. It involves
karyogamy, the formation of a
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
, which is followed by
meiosis and multiple fission. This results in the production of sporozoites.
Other forms of replication include and .
Endodyogeny is a process of
asexual reproduction, favoured by parasites such as ''
Toxoplasma gondii''. It involves an unusual process in which two daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation.
Endopolygeny is the division into several organisms at once by internal
budding.
Glossary of cell types
Infectious stages
A (ancient Greek ', seed + ', animal) is the cell form that infects new hosts. In ''
Plasmodium'', for instance, the sporozoites are cells that develop in the mosquito's salivary glands, leave the mosquito during a blood meal, and enter
liver cells (
hepatocytes), where they multiply. Cells infected with sporozoites eventually burst, releasing merozoites into the bloodstream. Sporozoites are motile and they move by
gliding.
A (G. ', part
f a series+', animal) is the result of
merogony that takes place within a host cell. During this stage, the parasite infects the host's cells and then replicates its own nucleus and induces cell segmentation in a form of asexual reproduction. In
coccidiosis, merozoites form the first phase of the internal life cycle of coccidian. In the case of ''
Plasmodium'', merozoites infect
red blood cells and then rapidly reproduce asexually. The red blood cell host is destroyed by this process, which releases many new merozoites that go on to find new blood-borne hosts. Merozoites are motile. Before
schizogony, the merozoite is also known as the schizozoite.
A (
G. ', partner + ', cell) is a name given to a parasite's
gamete
A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
-forming cells. A male gametocyte divides to give many flagellated
microgametes, whereas the female gametocyte differentiates to a
macrogamete.
An (G. ', egg + ', motile) is a fertilised
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
capable of moving spontaneously. It penetrates epithelial cells lining the midgut of
mosquito
Mosquitoes, the Culicidae, are a Family (biology), family of small Diptera, flies consisting of 3,600 species. The word ''mosquito'' (formed by ''Musca (fly), mosca'' and diminutive ''-ito'') is Spanish and Portuguese for ''little fly''. Mos ...
es to form a thick-walled structure known as an oocyst under the mosquito's outer gut lining. Ookinetes are motile and they move by
gliding.
A (G. ', nourishment + ', animal) is the activated, intracellular feeding stage in the apicomplexan life cycle. After gorging itself on its host, the trophozoite undergoes schizogony and develops into a schizont, later releasing merozoites.
A hypnozoite (G. ', sleep + ', animal) is a quiescent parasite stage that is best known for its "... probable association with latency and relapse in human malarial infections caused by ''Plasmodium ovale'' and ''P. vivax''". Hypnozoites are directly sporozoite-derived.
A (G. ', slow + ', animal) is a sessile, slow-growing form of
zoonotic microorganism
A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s such as ''
Toxoplasma gondii'', among others responsible for parasitic infections. In chronic (latent)
toxoplasmosis, bradyzoites microscopically present as clusters enclosed by an irregular crescent-shaped wall (
cysts) in infected muscle and brain tissues. Also known as a bradyzoic merozoite.
A (G. ', fast + ', animal), contrasting with a bradyzoite, is a form typified by rapid growth and replication. Tachyzoites are the motile forms of those
coccidians which form tissue
pseudocysts, such as ''
Toxoplasma'' and ''
Sarcocystis''. Typically infecting cellular
vacuole
A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...
s, tachyzoites divide by endodyogeny and endopolygeny. Also known as a tachyzoic merozoite (same journal reference as for "bradyzoic merozoite", above).
An (G. ', egg + ', bladder) is a hardy, thick-walled spore, able to survive for lengthy periods outside a host. The
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
develops within the spore, which acts to protect it during transfer to new hosts. Organisms that create oocysts include ''
Eimeria'', ''
Isospora'', ''
Cryptosporidium'', and ''
Toxoplasma''.
File:Babesia life cycle human en.svg, Life cycle of the '' Babesia'' parasite
File:Eimeria life cycle usda.jpg, Life cycle of the '' Eimeria'' parasite
File:Toxoplasmosis life cycle en.svg, Life cycle of the '' Toxoplasma'' parasite
Genome size
The dynamics of
gene
In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
loss was studied in 41 apicomplexan
genomes.
Loss of genes employed in
amino acid metabolism and
steroid biosynthesis could be explained by
metabolic redundancy with the host.
Also,
DNA repair genes tend to be lost by apicomplexans with reduced
proteome size, probably reflecting a reduced need for DNA repair of genomes with smaller information content.
Reduced DNA repair may help explain the elevated
mutation rates in pathogens with reduced genome size.
See also
*
Trematode life cycle stages
References
{{reflist
Apicomplexa
Reproduction