Molecular phylogeny of the Piroplasmida: challenges from two potentially novel species found in the platypus (Ornithorhynchus anatinus)

The order Piroplasmida (Phylum: Apicomplexa) includes three main genera, Babesia, Theileria, and Cytauxzoon that are vector-borneprotozoan haemoparasites, some of which have clinical relevance both in humans and animals. With exceptions, Babesia is typically differentiated from Theileria based largely on morphology, serology, and several life-cycle peculiarities, within the tick vectors and vertebrate hosts. Despite these differences, and their global socio-economic importance, piroplasms often present conflicting or uncertain molecular classifications, nomenclature, and intergenic relationships. For instance, based on the small ribosomal subunit RNA gene (18S rDNA), the taxon Piroplasmida includes at least nine mono- and para-phyletic clades, which are sometimes populated by relatively distant members of the same genus, or by closely related species of different genera1 . As part of a study into the health and ecology of the platypus (Ornithorhynchus anatinus; Order: Monotremata), blood samples were collected for haematological and biochemical analysis, from wild-trapped individuals in Tasmania. Blood, together with ectoparasites removed from each individual, was also evaluated for piroplasm infections by blood film microscopy, and molecular analysis of the 18S rDNA. Moreover, phylogenetic analyses were performed on a subset of samples. Pleomorphic organisms with occasional tetrads and intra-leukocytic forms, thought to be Theileria schizonts, were observed by microscopy, and were phenotypically consistent with Theileria ornithorhynchi, a piroplasm of the platypus named in previous studies2 . However, molecularly, the parasite-derived DNA belonged to two potentially novel piroplasm species, forming one monophyletic clade, clearly separated from other known marsupial-derived Theileria spp.3 . The high prevalence (100%), apparent lack of clinical signs, and distinct phylogenetic position of the parasite, likely reflect the unique ecology and evolutionary history of its ancient vector-host system