[The origin of heteroxeny in Sporozoa]

Hypothesis on the origin of Sporozoa from Spiromonadida ancestors is discussed on the basis of the data on their ultrastructure. The phylum Sporozoa comprises three large distinct groups of organisms as follows: Perkinsemorpha, Gregarinomorpha and Coccidiomorpha. Advanced Coccidiomorpha have not descended directly from Gregarinomorpha. Gregarinomorpha and Coccidiomorpha have common ancestors, Protospiromonadida. Heteroxeny is quite common among Coccidiomorpha. The formation of heteroxeny in Coccidiomorpha proceeded in different ways and at different time in different groups. Cystoisospora, Toxoplasma, Aggregata, Atoxoplasma, Schellackia have primary definitive hosts while Sarcocystis, Karyolysus, Haemogregarina, Hepatozoon, Plasmodium, Haemaproteus, Leucocytozoon, Akiba, Babesiosoma, Theileria, Babesia have primary intermediate hosts.