Biology 332 - Protistology Term 2 - 2002-2003

Sporozoa - Apicomplexa

A. General Properties

This is a large group (30,000 species) of parasitic non-photosynthetic protists that wholly lack flagellated stages with the exception of male gametes.

1. Common cellular features of the Group

These organisms all have an apical complex at some point in their life cycle and are homogenous in cellular organization. Their organization includes the following common features:

a. The apical complex is a set of secretory and cytoskeletal structures that enables the young parasite to enter a host cell. It consists of several components:

two apical rings
a conoid - an open cone of spiral rod-like elements
polar wing of dense granular material that connects the ends of the longitudinal cytoskeletal microtubules
rhoptries and micronemes. These are modified secretory vesicles.

The rhoptries are sac-shaped structures that may be either long or short depending on the species, while micronemes are elongated thread-like structures. Rhoptries and micronemes are believed to be part of the same secretory system by some researchers. They are involved in secretion of enzymes and other proteins that are involved in removing extracellular material and allowing the parasite to enter the host cell.

apiComplex.jpg (160641 bytes)Detailed drawing of apical complex showing components of the apical complex and the microtubular cytoskeleton. Click on image for high resolution picture.

b. Micropore - is a simple cytostome  consisting of an invagination of the plasma membrane. It is very similar in structure to the flagellar pits that are occur in many other protist groups. Flagellar pits are sites of endo and exocytosis. In apicomplexans material is taken in through micropores to form endosomes or food vacuoles.

api-807.gif
(164024 bytes)    Structure of a micropore showing the relation of plasma membrane and alveolar membranes.

Similar structures occur in other protist groups and are called flagellar pits. In what groups do they occur?

c. Longitudinal microtubular skeleton. The longitudinal microtubules are one of the principle components of the cytoskeleton. The microtubules terminate at the polar rings at the anterior and posterior ends of the cell, They do not extend completely to the poles of the cell. The polar rings are composed of a dense protein material that binds the ends of the skeletal microtubules together.

d. Cortical alveoli. Beneath the plasma membrane lies one or more flattened membrane sacs, the cortical alveoli. This arrangement of three surface membranes (Plasma Membrane and two Alveolar membranes) is a common features of ciliates dinoflagelates and apicomplexans and is one of the reasons that these three groups have been placed in a common 'supergroup', the alveolates. The alveolar membranes line the cell surface with the exception of micropores and small regions at the apical and antapical ends.

api-802.gif (325018 bytes)Apical complex of a coccidian showing arrangement of membranes and cytoskeletal structures. Click to enlarge.

e. The mitochondria have tubular cristae (surprise! - NOT).

f. Mitotic spindle is internal and nuclear envelope does not break down during mitosis. Again, this is a common feature in many protist groups.

The storage products appear to be lipids and glycogen (alpha 1-4 glucan).

2. Life cycle - general features.

a. A peculiar pattern of cell division.  In these organisms growth and reproduction are separated. First the cell grows (often to a huge size) and them proceeds to undergo a process of multiple division to produce many progeny cells. The typical pattern is that nuclei divide first through several rounds of mitosis without cell division. The nuclei typically are arranged around the periphery of the large cell. The cytoplasm then cleaves around each of the nuclei. The cytoskeleton for each daughter cell is assembled from the anterior end of the cell towards the posterior. As this occurs, the typically banana-shaped daughter cells are cut off of the parental cell mass. The association of a nucleus with each of the forming cells suggests that the main microtubule organizing centre for the cytoskeleton is associated with the nucleus. The apical complex is the first part of the daughter cell to form.

sporoz1.gif (4211 bytes)  Reproduction in apicomplexans: Cell growth (A and B) occurs prior to division. Division takes place through several rounds of mitosis without cytokinesis (C) followed by multiple cleavage (D) resulting in the simultaneous production of a set of progeny cells (E).

b. The progeny produced by multiple fission may be either vegetative infections cells or gametes. There are three positions in the life cycle at which the multiple fission process may occur: Merogony, gamogony and sporogony.

Apicomplexan.JPG (31197 bytes)Typical apicomplexan life cycle. The three points at which multiple fission occurs (Merogony, Gamogony and Sporogony) are indicated in boxes. The principal stages are shown in bold type.

Gamogony is the process where by a gamont gives rise to many gametes.

Sporogony is the process whereby a single zygote gives rise to many spores or sporozoites. Sporozoites are small cells that lie within the thickwalled resistant spore.

Merogony is a process that increases the number of infective cells.  A single large schizont gives rise to a large number of small merozoites that infect other host cells.

Why do you think that this might be of benefit to a parasite? Merogony is not present in all life histories. The relative importance of increase in cell numbers at gamogony and at sporogony also varies between different  groups of apicomplexans and between species. While there is a common general life cycle pattern, there is lots of variation in life history.

There are two major groups of apicomplexans, Gregarines and Coccidia.

B. Gregarines

Gregarines re generally parasites of body cavities of invertebrates. they do little tissue-level damage and their development takes place nearly entirely outside of host cells.

Life history and morphology.

1. the sporozoite is released from a spore tht is typically eaten by the host. The sporozoite invades a host epithelial cell and becomes a trophozoite.

2. The cell lyses, releasing the trophozoite which then attaches itself to the gut epithelium. A this stage is is differentiated into three sections:

epimerite - the apical hold-fast orgalelle
protomerite, the anterior body section
deuteromerite - the second, larger body section that contains the nucleus.

The proteromerite and the deuteromerite are separated by a fibrous, non-membranous septum. Both protomerite and deuteromerite are part of the same cell.

Not all gregarines are subdivided as described above. Those what are divided into protomerite and deuteromerite are called cephaline gregarines, those that are not are called acephaline gregarines. Monocystis, the main species tht you will study in the lab is an acephaline gregarine.

The surface of  trophozoite is covered by a series of deep longitudinal folds that are supported by cross-linked actin filaments. Waves of movement pass along these folds to move the cell forward over a bed of secreted visco-elastic slime. This peculiar gliding motion is called gregarine motion and is representative of the group. At a bio-mechanical level it is not fundamentally different in principle than the movement of a snail (gastropod mollusc) on slime or from the movement of pennate diatom All of these systems rely on peropdic contraction of the cell or organismal surface riding upon a bed of secreted visco-elastic material. These visco-elastic materials act like a glue to hold, the non-moving parts of the organismal surface, but transform to a fluid under   mechanical strain, and so allow the moving parts of the organismal surface to advance.

3. As the trophozoites develop they becme associated with individuals of complementary mating type (the other 'sex', as it were) to form stable pairs called syzygys. In the case of cephaline gregarines, cells in a syzygys are generally arranged in tandem. The anterior cell is called the primite and the posterior cell the satellite.

Trophozoites of some gregarine species can become very large (up to 2 mm). Eventually the trophozoites undergo gamogony.

4. Gamogony is the process of gamete formation. The cells that give rise to gametes are called gamonts. The pair of gamonts (still in syzygy) round up and secrete a common gamontocyst around themselves. The gamontocyst wall is a secreted non-cellular structure that serves to contain (like a cage or box) the gametes and later the zygotes that arise from the gamonts.

Within the gamontocyst each of the gamonts undergoes extensive multiple nuclear divisions and then simultaneour multiple fission to give rise to hundreds of small gametes. These gametes pinch off from the surface of the cell and leave behind a residual cytoplasmic mass that the remainder of the gamont.

The gamonts are of two types. One gamont gives rise to flagellated (male) gametes and the other to round non flagellated (female gametes). the flagellated gametes have a polarized cytoskeleton with a pointed anterior end, two posteriorly directed flagella, and a nucleus that located near the pointed end with the flagella. In the female gametes, the nucleus is centrally located.

5. The gametes find complementary partners within the narrow confines of the gamontocyst and fuse. As with all fertilization, the fusion first involves fusion of the cells, and later fusion of nuclei. In the laboratory material it is possible to find specimens at the pronuclear stage in which cells have funsed and possess two haploid pronuclei that have not yet fused to produce the zygote nucleus.

6. After fertilization the zygote secretes a spore wall and directly proceeds to undergo the first meiotic division. The spores have a 'football' or spindle and are quite different in appearance from the sphaerical early zygotes at stages prior to the secretion of the spore wall. Typical meiotic prophase stages are recognizable in many spores.   After conmpletion of the first meiotic division the spore will contain two nuclei, and after the second division, four.

7. Meiosis is followed by a third sporogenic division (mitosis) to produce eight small nuclei within the single cytoplasmic mass. The cytoplasm then cleaves to produce eight sporozoites. After the cytoplasmic division has taken place the nuclei re usually located ner the middle of the cells, producing a ring of eight nuclei around the middle of the spore. At earlier stages prior to cell division the nucvleu have a more random arrangement.

After the spore walls are formed the gamontocyst may disintegrate releasing the spores. In species that live in the gut of the host, the spores are eliminated with the feces. In those that live in the reporductive tract, the spores are released with gametes. In those organisms in which the gregarines live in the coelomic cavity, the spores are released on the death of the host.

In the life history described above merogony does not occur. What is the chief reproductive stage? There are some gregarines in which merogony does occur (schizogregarines) and in these organisms the relative degree of reporduction occuring during gamogony and sporogony is substantially less than that in gregarines that do not undergo merogony.

C. Coccidia

The other major group of apicomplexa are the coccidia. There are very successful parasites that have developed tissue-level parasitism, alternate hosts and other advanced features. They rely on merogony as the chief reproductive phase.

1. Protococcidia and the basic coccidian life cycle.

There are a few primitive coccidia that lack merogony (as do most of the gregarines). This makes comparison of the life cycles easy.  The chief differences are that

Trophozoites of both mating types grow separately to form gamonts (no syzygys here, please).
Gamonts of the two mating types are morphologically distinct. The macrogametocyte (female gamont) grows into a large egg-like macrogamete, while the microgametocyte undergoes multiple rounds of nucler division and then differentiated into many small flagellated microgametocytes.
After fertilization the zygote differentiates into an oocyst. This specialized cell has a thick secreted wall. Inside it the cytoplasm undergoes multiple fission to form many spores.
Many spores typically from from a single zygote, rather than one as in the gregarines. These spores are contained within the oocyst. Each spore contains a small number of sporozoites (typically two or four).

2. Eucoccidia and tissue parasitism.

The eucoccidia differ from the protococcidia primarily by the development of tissue-level parasitism and merogony. In these organisms nearly all stages of the life cycle are spent as intracellular parasites.

Trophozoites grow in host cells and different into schizonts. Schizonts undergo multiple rounds of nuclear division and then simultaneously divide to give rise to a large number of merozoites. This process is called merogony. The merozoites then infect other cells and the cycle can be repeated many times

Eventually, at somepoint in the infection process a physiological threshold is crossed and trophozoites differentiate into gamonts rather than into schizonts.

The gamonts (macrogametocytes and microgametocytes) differentiate into gametes (macrogametes and microgametes, respectively).

Fertilization occurs ad the fertilized macrogamete becomes an oocyst. The oocyst wall is formed from exocytosis of secretory granules that are produced during development of the gamont. Later the oocyst undergoes sporogony to produce a number of spores with several sporozoites each.

A number of the more advanced coccidia have developed alternate hosts. The primary host is the ancestral host for the organism and is the site of the sexual phase of the life cycle (gamogony and sporogony). The secondary host (the more recent evolutionarily) is the site of thevegetative reproductive stages (merogony).

Think about what is required for this to work. Many organisms could ingest spores. If these spores could grow in the new host and if there were some way for the parasite to get back into its primary host, then it would be possible for the parasite to complete its life cycle. In the simple, presumably more primitive cases this transfer from the secondary to the primary host occurs when the primary host eats the secondary host and thus acquires a load of parasites that rapidly differentiate into gamonts and complete the life cycle. Presumably the primary host provides essential physiological physical cues that enable the parasite to complete its life history.

A bit trickier are cases where the primary host is an insect that transfers the parasite to a second host through feeding and then later retrieves the parasite by the same means. This is what happens with Plasmodium, the malarial parasite. The primary host is a mosquito.

Sporozoites are injected with the saliva when the mosquito bites the secondary host (bird or mammal).
In the secondary host the parasite undergoes several rounds of merogony, finally culminating in a merogony in circulating blood cells. When a certain stage in the infection is reached, gamonts are produced but do not differentiate into gametes in the secondary host.
When a mosquito then feeds on the blood of the host, gamonts are sucked into the stomach of the mosquite where the rapidly (within a few minutes) differentiate into gametes, fertilization occurs and the oocyst migrates into the gut epithelium enters a cell and undergoes sporogony to form a large mass in the host cell to cause it to bulge into the haemocoel and eventually burst releasing hundreds of sporozoites.
These sporozoites then migrate through the haemocoel to the salivary gland where they enter the secretory cells and pass through them and accumulate in the lumen of the salivary duct, ready to be injected into a new host.

Just think of the nearly endless numbers of gamonts that can be produced by a cow relative to the small number that can be made by a mosquito!

3. Origins and phyletic relations of the Apicomplexa

wpeE9.jpg (24213 bytes)

Alveolate phylogeny. Click to enlarge

Apicomplexa are alveolates that appear to be closely related to dinoflagellates. There are a few 'free-living' apicomplexan- like organisms that include Perkinsus and Colpodella. These organisms have an apical complex, but also have rather dinoflagellate-like features. Riblsomal DNA sequencing studies place them at the bottom of the dino - apicomplexan clade

It is clear that the dinoflagellates and apicomplexans are very closely related. It is not clear whether early dinos gave rise to apicomplexans, or vice-versa, or whether both groups developed from a common radiation of early alveolates. It is clear, however that the apical complex arose very early and prior to the development of animal parasitism.

What do you make of the observation that the most primitive Dinos are the parasites?