[Cytological aspects of similarity and difference of Myxozoa and Cnidaria]

A comparative cytomorphological analysis of Myxozoa and parasitic Cnidaria Polypodium hydriforme has been carried out in view of the Weill (1938) hypothesis, which regards Myxozoa as a reduced Cnidaria. The question on the relation of Myxozoa and Cnidaria was arising several times with the application of some new methods during the Myxozoa studies. At present the idea on their phylogenetic relationships has appeared again in connection with an absolutely new understanding of the myxozoan life cycle (Wolf, Markiw, 1984), as well as with the application of molecular-biological methods for their phylogenetic studies. The latter, however, provided some diverse results. So far no comparative cytomorphological analysis of Myxozoa and Polypodium has been carried out. The present paper is to fill the gap on the basis of accumulated facts. According to Weill (1938), the features of similarity of parasitic Cnidaria and Myxozoa are the following: 1) the presence in both of extrusomes (nematocysts and polar capsules) whose structure and development are surprizingly similar; 2) the nuclear dimorphism and somato-generative segregation; 3) the presence of a somatic nutritional cell, surrounding the multiplying generative cells; at present it is known that polyploidy of somatic nuclei and the absence of parasitophorous vacuole are characteristic of trophamnion of Polypodium and trophozoite of Myxozoa; 4) the presence of radial symmetry in both groups; 5) the construction of a diblastic organism made of a cluster of endodermal cells and a few ectodermal cells; 6) the similarity of their cell contacts (Grassé, 1970). At present it is possible to add to Weill's (1938) list of features common for parasitic Cnidaria and Myxozoa the number of important similarities between Polypodium and Myxozoa, some of which being not characteristic of Cnidaria: 1) the "cell in cell" organization of all Polypodium parasitic stages and all Myxozoa life cycle stages; 2) the presence of gametophore supplied with extrusomes; 3) both organisms have haplophase in their life cycles preceded by two-step meiosis; 4) there are mitochondria with tubular cristae in both organisms; 5) the absence of spermatozoa and eggs in both organisms; 6) the similarity of Polypodium cnidocile structure and the cone-like formation situated at the anterior end of polar capsule of actinospore (Lom. Dykova, 1997); 7) the participation of MTOC in the formation of extrusomes in both animals. In spite of the obvious similarity between Myxozoa and parasitic Cnidaria (including Polypodium) it is, however, necessary to take into account differences between them, the main being as follows: the absence in Myxozoa of flagellated stages, centrioles, tissues and organs, true blastophylla, planula-like larvae, gastrulation; the presence of low cell integrations in Myxozoa; Cnidaria and Myxozoa have different types of mitosis, their life cycles and the discharge mechanism of their stinging apparatus being also different. We consider as quite valid a suggestion by Siddall et al. (1995) that parasitic Cnidaria could present an early separated branch of the cnidarian evolution. Further studies of Myxozoa life cycle may show their more definite relation to parasitic Cnidaria. The problem has not yet been solved completely since the available molecular-biological data are rather contradictory and moreover there is no distinct idea as to the Eumetazoa ancestor so far. A further thorough investigation is badly needed in the feelds of developmental cycle, cytomorphology and molecular biology of the variety of narcomedusae and representatives of Myxozoa. This may help to find some transitional forms and stages of the animals and to understand whether we deal with a regressive evolution of parasitic Cnidaria or with a parallel evolution of taxa originated from the common ancestor.