Morphological and Phylogenetic Study of Protococcidians Sheds Light on the Evolution of Epicellular Parasitism in Sporozoa (Apicomplexa), with the Description of Eleutheroschizon planoratum sp. nov

Abstract

The order Protococcidiida is one of the most poorly studied basal groups of Sporozoa (Apicomplexa sensu stricto). To date, the phylogenetic unity of protococcidians and their relationship with other sporozoans are understudied. Only the protococcidian Eleutheroschizon duboscqi has molecular evidence of a sister position to “true” coccidians (Eimeria, Sarcocystis, Toxoplasma). E. duboscqi is characterized by epicellular development in the so-called parasitophorous sac of the host cell origin. The unusual localization of Eleutheroschizon is comparable to that of Cryptosporidium. We describe a new species of the genus, E. planoratum ex Naineris quadricuspida polychaete from the White Sea, using light and electron microscopy. The morphology of attachment apparatus, phylogenetic analyses of concatenated DNA sequences of the nuclear ribosomal operon (SSU rDNA, ITS1, 5.8S rDNA, ITS2, and LSU rDNA), and compensatory base changes in ITS2 secondary structures of both protococcidians confirm the new species. The resulting phylogenies also confirm that Eleutheroschizon is sister to eimeriid coccidians, while Cryptosporidium tends to be grouped with gregarines. We discuss a new type of endoparasitism among sporozoans—the closed epicellular parasitism that evolved convergently in Eleutheroschizon and Cryptosporidium. The diagnosis of the new species and the emended diagnoses of the species E. duboscqi and the genus Eleutheroschizon are presented.