Spermiogenesis and sperm ultrastructure inCalicotyle affinisScott, 1911 (Platyhelminthes, Monogenea, Monopisthocotylea, Monocotylidae)

Abstract

Summary Spermatological characteristics of Calicotyle affi nis Scott, 1911, an endoparasitic monocotylid monogenean from the cloaca of a holocephalan fi sh Chimaera monstrosa L, have been investigated by means of transmission electron microscopy for the fi rst time. Spermiogenesis exhibits features basically similar to those of the congeneric Calicotyle kroyeri and Calicotyle australiensis, but there are some new fi ndings with respect to the formation and fi ne structure of the spermatozoon including the remarkable complex end-piece (EP). Morphogenesis of the EP, which is located at the anterior (proximal) region of the late spermatid, includes two stages: (1) the centriolar region is continuous with a cytoplasmic mass of the zone of differentiation, the electron-dense surface of the spermatid undergoes signifi cant changes in the sculpturing and the inner core of developing spermatid is electron-lucent; (2) after central fusion of the arching membranes a defi nitive structure of the EP is subsequently evolved, fi nally comprising 3 -4 electron-dense discs attached to a central common electron-lucent column. The EP is considered as a synapomorphy of the genera Calicotyle + Dictyocotyle. The mature spermatozoon of C. affi nis comprises the EP, two parallel axonemes of almost equal lengths with the 9 + "1" trepaxonematan pattern, mitochondrion, nucleus, and a reduced number of parallel cortical microtubules (1 -3). The posterior (distal) extremity of the mature spematozoon contains a single tapering axoneme. Ultrastructural characteristics of the mature spermatozoon of C. affi nis coincide mostly with those of congeneric C. australiensis. Variations of the spermatological characters within the genus Calicotyle, between Calicotyle and enigmatic Dictyocotyle as well as other monocotylid monogeneans are discussed.