Ultrastructure of the spermatozoon of the trematode Notocotylus noyeri (Digenea: Notocotylidae), a parasite of Microtus arvalis (Rodentia: Cricetidae)

Abstract

In the present paper, we describe the ultrastructure of the spermatozoon of the notocotylid Notocotylus noyeri (Joyeux, 1922) by means of transmission electron microscopy. The mature spermatozoon of N. noyeri exhibits the general pattern described in the majority of digeneans: two axonemes of the 9 + "1" pattern of the Trepaxonemata, nucleus, mitochondria, parallel cortical microtubules, spine-like bodies and ornamentation of the plasma membrane. The glycogenic nature of the electron-dense granules was evidenced applying the test of Thiéry. The ultrastructural features of the spermatozoon of N. noyeri present some differences in relation to those of the Pronocephalidea described until now, but confirm a general pattern for the Notocotylidae, namely a spermatozoon with two mitochondria and an anterior region with ornamentation of the plasma membrane associated with spine-like bodies. The posterior extremity of the spermatozoon exhibits only some microtubules after the disorganisation of the second axoneme. The present study confirms that some ultrastructural characters of the sperm cell such as the presence or absence of lateral expansions, the number of mitochondria and the morphology of both anterior and posterior spermatozoon extremities are useful for phylogenetic purposes within the Pronocephaloidea. Thus, unlike notocotylids, pronocephalids exhibit external ornamentation and a lateral expansion in the anterior spermatozoon region. Moreover, notocotylid spermatozoa present two mitochondria, whereas pronocephalid spermatozoa exhibit a single mitochondrion. Finally, pronocephalids are characterised by a type 2 posterior spermatozoon extremity, whereas notocotylids exhibit a type 3 posterior spermatozoon extremity.