New data on spermiogenesis and trepaxonematan axoneme in basal tapeworms (Cestoda, Caryophyllidea, Lytocestidae) parasitizing cyprinid fishes.

Monozoic caryophyllidean cestodes, intestinal parasites of cyprinid fishes, represent a group of tapeworms with an unclear evolutionary history. As spermatology may provide phylogenetically important data, the spermiogenesis and ultrastructure of the mature spermatozoon have been investigated using an integrative approach combining transmission electron microscopy, cytochemistry and electron tomography in Khawia rossittensis (Szidat, 1937). The process of spermatid formation is accompanied by the presence of ultrastructural characters not described in traditional models of spermiogenesis, e.g., apical electron-dense material, the two striated roots situated unusually opposite each other, branching of typical striated roots, an intercentriolar body comprising five electron-dense and four electron-lucent layers, rotation of both free flagella and flagellar buds to the median cytoplasmic process at 90°, and a complete proximodistal fusion. The synchronous rotation of both flagellar buds and growing free flagella is an evolutionarily linked pattern favouring the hypothesis that the Caryophyllidea are not ancestral but are secondarily derived from polyzoic forms. Electron tomography analysis has revealed a unique feature of two helicoidal tubular structures in the central electron-dense core of the axoneme of mature spermatozoon. These data provide new insights into the architecture of the 9 + '1' axoneme, which is shared by male gametes of all trepaxonematan Platyhelminthes.

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 Brunanská 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.