Abstract
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.
Highlights
Caryophyllidean tapeworms are monozoic, i.e., without external and internal proglottidation, and contain only a single set of reproductive organs; there are 42 genera and 122 valid species distributed in four families[1]
This area contains cortical microtubules, apical dense material and two centrioles associated with striated roots, which are oriented tangential to the apex of the nucleus (Figs 1A and 2a)
The formation of free flagella and flagellar buds is accompanied by their rotation towards the prolonging median cytoplasmic process (MCP) (Figs 1D,F,G and 2c1–d2)
Summary
Caryophyllidean tapeworms are monozoic, i.e., without external and internal proglottidation, and contain only a single set of reproductive organs; there are 42 genera and 122 valid species distributed in four families[1] This group apparently played a key role in the evolution of cestodes[2] and represents a basal or nearly basal group of tapeworms; phylogenetic relationships within the order Caryophyllidea and its evolutionary interrelationships with the most relative group of tapeworms remain unsolved[3,4]. Www.nature.com/scientificreports using electron tomography analysis in one-axoneme spermatozoa of the evolutionarily more derived tapeworm Nippotaenia mogurndae[17]. These interesting findings call for further detailed electron tomography studies of other caryophyllidean cestodes and cestode groups to elucidate the basic sperm structure of type 9 + ‘1’ in the Trepaxonemata. The spermatological characteristics of Khawia rossittensis, another lytocestid tapeworm and parasite of Carassius gibelio, have been examined to shed more light on the cytodifferentiation and fine structural architecture of the male gametes of caryophyllidean cestodes
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