Diplomonadida

Based mainly on:

Kulda, J. and Nohynkova, E. (1978). Intestinal Flagellates. Parasitic protozoa II (Kreier, J.P., ed.) Academic P, Inc., New York, 2-139

Siddall, M., E., Hong, H. and Desser, S., S. (1992) Phylogenetic analysis of the Diplomonadida (Weyon, 1926) Brugerolle, 1975: Evidence for Heterochromy in Protozoa and Against Giardia lamblia as a "Missing Link". Journal of Protozoology. 39(3), 361-367

This order is usually split in two groups enteromonads and diplomonads according to their single- or double- karyomastigont organisation respectively. Various authors assign different taxonomic levels to these groups. A result of cladistic analysis (Siddall et all., 1992) indicates that splitting of this order is groundless.

Enteromonadidae

This family comprises three genera: Enteromonas, Trimitus and Caviomonas. These small flagellates live as harmless commensals in the intestine of various animals, both vertebrates and invertebrates. Enteromonas hominis inhabits the cecum of man. Enteromonas, the only genus studied with electron microscopy, shows a striking homology with Hexamita, and the family Enteromonadidae, although their members possess only one karyomastigont, belongs taxonomically to the order Diplomonadida. Both trophozoites and cysts occur in Enteromonas. Only trophozoites have been described for Trimitus and Caviomonas. Enteromonadidae are very small flagellates, frequently measuring less than 5µm in length. Their body is pyriform, attenuated posteriorly and flattened ventrally. An oval, vesicular nucleus is situated in the anterior extremity of the cell. Apposed to the ventral pole of the nucleus lies a complex of kinetosomes giving rise to flagella and funis. The number of flagella is characteristic of the genera: four for Enteromonas, three for Trimitus and one for Caviomonas. In Enteromonas and Trimitus one of the flagella is recurrent. The cytostome was detected by electron microscopy in Enteromonas. Light microscopic observations suggest that similar cytostomes might be present also in Trimitus and Caviomonas. Food vacuoles are frequent and relatively large, many of them containing ingested bacteria. Cisternae of the rough endoplasmic reticulum are situated in the cell periphery. Mitochondria, Golgi apparatus, and microbody-like organelles era absent. Cyst have been found only in the genus Enteromonas.

Diplomonadidae

Members of the family Diplomonadidae are axially symmetrical with eight flagella, two nuclei, and a double set of accessory organelles. The family includes six genera. Two are free living (Trepomonas, Trigonomonas), one contains both parasitic and free living species (Hexamita), and three are exclusively parasitic (Spironucleus, Octomitus and Giardia). Certain species of Hexamita and Giardia, and most species of Spironucleus are pathogenic or potentially pathogenic. Genus Giardia comprises species pathogenic to man and some mammal species (dog, cattle, sheep, rabbit, chinchilla, goat). Trophozoites of Diplomonadidae have pyriform, ovoid or oval body with two anteriorly situated nuclei and two sets of mastigont organelles arranged in binary axial symmetry. In the genus Giardia a major part of the ventral surface of trophozoites is modified into a large adhesive disc. There are eight flagella originating in two opposed kinetosomal complexes. Typically, six flagella are anterior, the remaining two are recurrent, passing backwards through the body and emerging posteriorly with a free-trailing portion. In all genera each mastigont bears two kinetosomes interconnected by osmiophilic filaments and grouped in two pairs . The anterior pair comprises the kinetosomes of anterior flagella (K1, K2), the posterior pair the kinetosomes of the third anterior flagellum (K3), and recurrent flagellum (R). Ribbons composed of single layer of associated microtubules are accessory to each mastigont in all diplomonad genera. Topologically three types are recognised: supranuclear microtubules, infranuclear microtubules and funis. In Giardia are supranuclear microtubules modified in the striated disc, the subpelicular skeleton supporting the domed chamber of the ventral adhesive disc. Striated root fibrils modified in arched lamellae are present in all genera except Giardia. They are joined to kinetosome R and extend backwards up to the posterior extremity, usually expanding in the posterior part of the body. The marginal plate in Giardia is a periodic structure located in the ventrolateral flange surrounding the adhesive disc and is associated with axonemes. The ventrolateral flange is probably contractile and functionally important in the attachment of Giardia. Cytostomes are characteristic of Diplomonadidae but are absent in Giardia and Octomitus. In Hexamita and Spironucleus, cytostomes are formed by tubelike invagination of the cytoplasmic membrane surrounding recurrent flagella. Cisternae of the rough endoplasmic reticulum are abundant . Digestive vacuoles, ribosomes, and rosettes of granules, interpreted as glycogen, occur in the cytoplasm. Mitochondria, Golgi apparatus, and microbody-like organelles are absent. Cysts of Diplomonadiae are oval bodies surrounded by a thick wall. All genera form cysts but with the exception of Giardia, little is known on their structure. Ripe cysts are four-nucleate probably in all genera. Trophozoites of Diplomonadidae multiply by longitudinal binary fission. Binary division at the cyst stage is a regular process occurring probably in all diplomonadid genera.