Morphology and phylogeny of Turbellaria
The aim of the study is to clarify the phylogeny of lower bilateral animals (Acoela, Nemertodermatida and Xenoturbellida) as well as plathelminth turbellarians by examining the morphology of the nervous, muscular and reproductive systems. Immunocytochemistry, histochemistry, confocal and electron microscopy are used in search for new morphological characters. In recent years, the following results were achieved:
1. A new phylogenetic tree of Acoela was published introducing a new classification down to the level of the families. In the new system Diopisthoporidae (with the caudal mouth) proved to be the sister group of all other Acoela, the fact predicted long ago by our outstanding zoologists V.N. Beklemicshev and A.V. Ivanov.
2. Organization and development of the body wall muscles and the nervous system of several representatives of Nemertodermatida (Xenacoelomorpha) has been studied for the first time, giving a complete picture of the amazing plasticity of the neuromuscular systems in this small primitive group. Also for the first time mysterious broom organ of Flagellophora has been described.
3. Data on musculature of Convolutida (Acoela) formed the basis of the qualification paper by Frolova (2013). Data on the novel type of musculature with a distinct marginal muscle were published in ICIM3 materials in Berlin (2014) and new data on the nervous system of Convoluta and Anaperus have been recently obtained.
4. Currently we study the role of the nervous system characters of plathelminth turbellarians in testing phylogenetic hypotheses on the origin of Neodermata (Raikova, Kotikova, Frolova); this work is supported by the RFBR grant received in 2016. Preliminary data on the structure of the nervous system of the White Sea parasitic turbellarian Notentera ivanovi (Fecampiida) seem to falsify the hypothesis of close relationship of Fecampiida with Neodermata based on the ultrastructure of spermatogenesis.
Main publications:
Raikova O.I., Justine J.-L.,1999. Microtubular system during spermiogenesis and in the spermatozoon of Convoluta saliens (Platyhelminthes, Acoela): tubulin immunocytochemistry and electron microscopy. Molecular Reproduction and Development 52: 74–85.
Raikova, O.I. Reuter M., Jondelius U, Gustafsson M.K.S., 2000. An immunocytochemical and ultrastructural study of the nervous and muscular systems of Xenoturbella westbladi (Bilateria inc. sed). Zoomorphology, 120: 107–118.
Raikova O.I., Reuter M., Jondelius U, Gustafsson M.K.S., 2000. The brain of the Nemertodermatida (Platyhelminthes) as revealed by anti-5HT and anti-FMRFamide immunostainings Tissue & Cell. 32 (5): 358–365.
Raikova O.I., Reuter M., Gustafsson M.K.S, Maule A.G, Halton, D.W. and Jondelius U. 2004. Basiepidermal nervous system in Nemertoderma westbladi (Nemertodermatida): GYIRFamide immunoreactivity. Zoology, 107: 75–86.
Raikova O. I. Tekle, Y. I., Reuter, M. and Jondelius, U. 2006. Copulatory organ musculature in Childia (Acoela) as revealed by phalloidin fluorescence and confocal microscopy. Tissue and Cell. 38: 219–232.
Tekle Y. I., Raikova O.I, Justine J-L., Hendelberg J., Jondelius U. 2007. Ultrastructural and immunocytochemical investigation of acoel sperms with 9 + 1 axoneme structure: new sperm characters for unraveling phylogeny in Acoela. Zoomorphology, 126: 1–16.
Raikova O.I., Reuter M., Gustafsson M.K.S, Maule A.G, Halton, D.W. and Jondelius U. 2004. Evolution of the nervous system in Paraphanostoma (Acoela). Zoologica Scripta 33: 71–88.
Kotikova E.A., Raikova O. I., 2008 Architectonics of the central nervous system of Acoela, Plathelminthes, and Rotifera. Journal of Evolutionary Biochemistry and Physiology, 44 (1): 95-108.
Jondelius U., Wallberg A., Hooge M., Raikova O.I. 2011. . How the worm got its pharynx: phylogeny, classification and bayesian assessment of character evolution in Acoela. Systematic Biology 60 (6): 845–871.
Meyer-Wachsmuth I., Raikova O.I., Jondelius U. 2013. The muscular system of Nemertoderma westbladi and Meara stichopi (Nemertodermatida, Acoelomorpha). Zoomorphology, 132: 239-252.
Raikova O.I., Meyer-Wachsmuth I., Jondelius U. 2016. The plastic nervous system of Nemertodermatida. Organisms Diversity and Evolution, 16 (1): 85-104.
Raikova E.V., Raikova O.I. 2016. Nervous system immunohistochemistry of a parasitic cnidarian Polypodium hydriforme at its free–living stage. Zoology, 119 (2): 143–152