ИСТИНА

Apicomplexans represent a very successful group of unicellular parasites consisting entirely of parasitic species infecting different vertebrates and invertebrates. Most of them are well known as agents causing human and animal diseases (e. g. malaria,toxoplasmosis,cryptosporidiosis), which are intensively studied in different aspects of biology and medicine. In contrast, deep-branching apicomplexans are generally considered of no practical importance and thus remain poorly investigated. These groups, however, are crucial in our understanding of evolutionary pathways of the phylum Apicomplexa. Here we present observations on several apicomplexan parasites from marine invertebrates of the White Sea: protococcidian Eleutheroschizon dubosqui and blastogregarine Siedleckia nematoides from intestine of polychaete Scoloplos armiger, agamococcidian Rhytidocystis sp. (presumably new species) from intestine of polychaete Travisia forbesii, eugregarines Urospora travisiae and U. ovalis parasitizing the body cavity of the same host. U. chiridotae inhabiting blood vessel of holothurian Chiridota laevis and intestinal archigregarine Selenidium sp. from polychaete Pygospio elegans. Studied parasites differ in their morphological aspects, localization and in the mode of movement: i. e. gliding (U. travisiae), metaboly (E. dubosqui and U. ovalis), nematode-like movement (Selenidium and Siedleckia). They seem to show parallel pathways of evolution realized by various morphological and probably functional adaptations. Combined morphological and molecular-phylogenetical analysis supports our hypothesis that there are several early emerging branches of Apicomplexa.