Achromatorida is an order of non-pigmented intraerythrocytic parasitic alveolates belonging to the subclass Haemosporidiasina. The order was created by Jacques Euzéby in 1988.[1]
The taxonomy of these organisms has been one of some controversy. Weylon in 1926 grouped many of these genera into the genus Babesia - given what is now known about these genera this was probably an error. Agreement on the organisation of these genera probably cannot be regarded as being settled.
The apical complex has a polar ring and rhoptries. A conoid is lacking and most species lack the associated pellicular microtubules.
Flagellae are lacking. The trophozoite stage is separated from erythrocyte by single membrane (in the other groups there usually 2 or more). Vectors include ticks and leeches.
Evolution
A Bayesian analysis based on the nuclear genes suggests a date of divergence between Babesia and Theileria of 56.48 million years ago ( 95% credible interval 28.17 million years ago–86.87 million years ago)[2]
Species in this family have rhoptries, a polar ring and subpellicular tubules in its apical complex. With the exception of the genus Tunetella which infects tortoises, species in this family infect in warm-blooded vertebrates and are transmitted by ticks.
This family includes species that infect marine animals. A notable exception is the genus Serpentoplasma which infects reptiles. Very little is known about most of these genera.
The genus Nephromyces may also belong to this family.[3]
Suborder Theileriina
These are parasites of erythrocytes and diverse white blood cells with sexual reproduction by exoerythrocytic or by exo- and endoerythrocytic schizogony.
There are also a number of genera of uncertain placement that earlier were provisionally included in this taxon:
Elleipsisoma Franca 1912: This genus is currently placed in the Eimeriorina. Its placement there is unusual given that species in this genus infect erythrocytes.
The genus Nicollia Franca 1910 is now regarded as a synonym for Babesia.[4]
Cytauxzoon replicates in macrophages rather than in lymphocytes. For this reason may be moved to a new family at some point.
Pirhemocyton was described as a protozoan infecting lizards but the intraerythrocytic inclusions have since been shown to be due to a viral infection.
An additional genus - Chelonplasma - has been described but its taxonomic status is unclear.
Synonyms
The classification of the species in this group is difficult. Over the passage of time several genera have been described only to re classified as being synonyms of previously described genera.
Currently recognised synonyms of the genus Babesia include:
Achromaticus Dionisi 1899
Apiosoma
Babesiella Mesnil 1919
Francaiella Yakimoff 1926
Haematococcus Babes 1888
Luhsia Dschunkowsky 1938
Microsoma Sohns 1918
Nicollia Franca 1910
Patonella Ray & Idnani 1943
Piroplasma Patton 1895
Pyrosoma Smith 1893
Rosiella Nuttall 1912
Sogdianella Schurenkova 1939
Recognised synonyms of the genus Nicollia - which is now itself regarded as a synonym of Babesia - include:
Nuttallia Franca 1909
Smithia Franca 1910
Recognised synonyms of the genus Theileria include:
Gonderia
Notes
Apiosoma is currently the name of a genus of ciliated protozoa also known as Glossatella. Although this genus is pathogenic to fish it has not related to the parasites in this taxon.
Nuttallia Dall 1898 is a genus of sunset clams (Psammobiidae)
References
^Euzéby, J. (1988). Apicomplexa, 2: Haemosporidioses, Part 1: Plasmodiids, Haemoproteids, "Piroplasms" (general characters). Comparative Medical Protozoology. Vol. 3.
^Levine ND (1984). "Nomenclatural corrections and new taxa in the apicomplexan protozoa". Trans Am Microsc Soc. 103 (2): 195–204. doi:10.2307/3226242. JSTOR3226242.