Spermatological characters of the digenean Rubenstrema exasperatum (Rudolphi, 1819) (Plagiorchioidea, Omphalometridae).

We present the first ultrastructural description of spermiogenesis and of the spermatozoon of Rubenstrema exasperatum (Omphalometridae), an intestinal parasite of Sorex araneus (Soricidae). Spermiogenesis begins with the formation of the differentiation zone delimited at the base by the ring of the arched membranes and bordered by cortical microtubules. This area contains two centrioles associated with striated rootlets and with an intercentriolar body. It also contains the nucleus and numerous mitochondria. The intercentriolar body is made up of seven electron-dense layers. The two centrioles give rise to two free flagella that grow orthogonally to the median cytoplasmic process. Additionally, flagellar rotation is followed by the proximodistal fusion of the flagella with the median cytoplasmic process, while the nucleus and mitochondria migrate along the spermatid. The constriction of the ring of arched membranes gives rise to the young spermatozoon. The mature spermatozoon of R. exasperatum shows several ultrastructural characters found in digenean spermatozoa such as two axonemes, mitochondrion, two bundles of parallel cortical microtubules, nucleus, and granules of glycogen. External ornamentation of the plasma membrane and spinelike bodies are also present in the spermatozoon of this species. In the present study, we produced additional spermatological data concerning a previously unexplored family, and we also compare our data to the existent ultrastructural descriptions within the Digenea in order to assess the use of several characters proposed as phylogenetic tools.