Abstract
Spermatological characters of Dictyocotyle coeliaca Nybelin, a unique endoparasitic monogenean from the body cavity of the ray Amblyraja radiata (Elasmobranchii: Rajidae), was investigated by means of scanning and transmission electron microscopy for the first time. The process of the spermatozoon formation begins with the appearance of the differentiation zone which contains two centrioles. Subsequently developed two free flagella rotate to lie parallel before their fusing. After fusion, both the mitochondrion and nucleus migrate alongside the axonemes. Spermatids are formed within parallel cytoplasmic canals. During late spermiogenesis, an electron-dense spiral end-piece is formed proximally to the centriolar region of some spermatids. At the end of spermiogenesis, this end-piece is separated by a plasma membrane from the late spermatid and remains in the residual cytoplasm when spermatozoa are released into the testis lumen. The mature spermatozoon of D. coeliaca contains two parallel axonemes with the 9 + “1” trepaxonematan pattern, mitochondrion, nucleus, parallel cortical microtubules, and electron-dense granules. Especially the presence of electron-dense granules, occurring frequenly in the spermatozoa of endoparasitic flatworms, may indicate an adaptation to endoparasitic lifestyle in D. coeliaca. The anterior extremity of the male gamete contains one centriolar derivative, a small component of the second centriolar derivative and anterior extremity of the mitochondrion. The posterior extremity of the mature spematozoon exhibits tubular elements of the disorganized axoneme. Variations of the spermatozoon ultrastructural characters within monocotylid monogeneans and possible evolutionary significance of glycogen in sperm to the endoparasitic habit are discussed.
Acknowledgements
The authors would like to specially thank Dr. Willy Hemmingsen, University of Troms⊘ (Norway), and the staff of the RV ‘Johan Ruud’, of the University for their help with the fishing and material collection. The authors are grateful to the staff of the Centre of Electron Microscopy, I.D. Papanin Institute for the Biology of Inland Waters, Russian Academy of Sciences, Borok, Russia, for technical assistance. We thank an anonymous reviewer for reference to A. paronaperugiae. The present study was supported by the Russian Foundation for Fundamental Research (Project no. 15-04-02890a to LGP) and the Grant Agency of the Slovak Republic VEGA (Project no. 2/0104/16 to MB). The work was realized within a frame of Centre of Excellence for Parasitology (Code ITMS: 26220120022) based on the support of the Operational Programme “Research & Development” funded from the European regional Development Fund (rate 0.4).
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