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Abstract

The spermiogenesis and the mature spermatozoon of Schistocephalus solidus (Cestoda: Diphyllobothriidea) are described using transmission electron microscopy. Spermiogenesis in S. solidus begins with the formation in the spermatid of a differentiation zone surrounded by cortical microtubules and delimited by arching membranes. This conical area presents two centrioles associated with striated rootlets and a median cytoplasmic extension between them. The centrioles are separated by an intercentriolar body composed of three electron-dense plates dividing four electron-lucent plates. The centrioles give rise to two flagella that undergo a rotation and later fuse proximodistally with the median cytoplasmic expansion. The presence of an electron-dense material in the distal part of the differentiation zone is observed in the early stage of spermiogenesis. This pattern corresponds to Type I spermiogenesis according to the classification proposed by Baˆ and Marchand (Me´moires du Muse´um National d’Histoire Naturelle 1995; 166: 87). The mature spermatozoon of S. solidus presents the Type I pattern defined by Levron et al. (Biological Reviews 2010; 85: 523). It consists of five regions that exhibit two axonemes, parallel cortical microtubules, nucleus and electron-dense zones. The anterior tip of the spermatozoon possesses only a few singlets. The axonemes are of a 9 + ’1’ trepaxonematan pattern and do not reach the posterior extremity of the mature spermatozoon.