Revisiting caryophyllidean type of spermiogenesis in the Eucestoda based on spermatozoon differentiation and ultrastructure of Caryophyllaeus laticeps (Pallas, 1781).

Spermiogenesis of the monozoic cestode Caryophyllaeus laticeps (Pallas, 1781) (Caryophyllidea: Caryophyllaeidae), a parasite of Abramis brama (Pisces: Cyprinidae), has been investigated by means of transmission electron microscopy and cytochemical staining with periodic acid-thiosemicarbazide-silver proteinate for glycogen. The process of spermatozoon formation corresponds in the basic pattern to that of Khawia armeniaca as described by Bruňanská and Poddubnaya (Parasitol Res 99:449-454, 2006). The dense material at the early stages of spermiogenesis, an intercentriolar body, the formation of a free flagellum and flagellar bud, the penetration of the nucleus into the spermatid body when the fusion of the free flagellum with the median cytoplasmic process has started, and a complete proximodistal fusion have been determined. In contrast to previous data on the caryophyllidean type spermiogenesis, the latter more recent observations show the presence and the rotation of a free flagellum and a flagellar bud. This pattern indicates clearly a derived stage of spermiogenesis in the Caryophyllidea, when the second shorter flagellum is greatly reduced comparing with that in the Spathebothriidea, Diphyllobothriidea, or Bothriocephalidea, thus forming a flagellar bud. The flagellar bud occurs in all stages of spermiogenesis and represents an evolved character in the Caryophyllidea. The mature spermatozoon of C. laticeps consists of one axoneme of the 9 + "1" trepaxonematan structure, parallel cortical microtubules, and a nucleus. Cell components are situated in a moderately electrondense cytoplasm, containing glycogen in the principal regions (II, III, and IV) of the spermatozoon. A crested body is absent. Similarities and differences between spermatozoa of caryophyllideans as well as other Eucestoda are discussed.