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

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