Ultrastructural aspects of spermatogenesis, testes, and vas deferens in the parthenogenetic tapeworm Atractolytocestus huronensis Anthony, 1958 (Cestoda: Caryophyllidea), a carp parasite from Slovakia

Ultrastructure and Cytochemistry of the Mature Spermatozoon of Khawia Armeniaca (Cholodkovsky, 1915) (Caryophyllidea: Lytocestidae), a Parasite of Capoeta Capoeta Sevangi (De Filippi, 1865) (Teleostei, Cyprinidae)

The mature spermatozoon of Khawia armeniaca, a monozoic caryophyllidean parasite of templar fish Capoeta capoeta sevangi (De Filippi, 1865) from the Lake Sevan, Armenia, has been studied using transmission electron microscopy and cytochemical technique of Thiéry (1967) for the first time. The mature spermatozoon of K. armeniaca consists of a single axoneme with the 9+'1' trepaxonematan structure, cortical microtubules and nucleus which are situated parallel to the longitudinal axis of the spermatozoon, and a moderately electrondense cytoplasm with glycogen particles. The cortical microtubules are arranged in one continuous semicircle beneath the plasma membrane in Region II and anterior part of Region III of the mature spermatozoon. The two opposite rows of cortical microtubules are observed in the remaining nuclear and at the beginning of the postnuclear part (Regions III, IV) of the male gamete The number of cortical microtubules is remarkably variable in the spermatozoa of various Khawia species. K. armeniaca exhibits the highest number of cortical microtubules in comparison with K. sinensis and K. rossittensis. Glycogen was detected in the cytoplasm of prenuclear (II), nuclear (III) and postnuclear (IV) regions with different ultrastructural organization of the mature spermatozoon of K. armeniaca. Variations of sperm ultrastructural characters within caryophyllideans and other cestodes are discussed.

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.

How to become a successful invasive tapeworm: a case study of abandoned sexuality and exceptional chromosome diversification in the triploid carp parasite Atractolytocestus huronensis Anthony, 1958 (Caryophyllidea: Lytocestidae)

Background

A cytogenetic analysis of the new local triploid population of the caryophyllidean tapeworm Atractolytocestus huronensis, a unique parthenogenetic species with the ability to colonise new regions, was performed to understand the inner structure of its chromosome complement.

Methods

A karyotype analysis was carried out using classical Giemsa staining and C-banding combined with fluorescent DAPI staining. A hypothesis that triplets are composed from three homologue chromosomes of approximately the same length and same centromere position was tested statistically for multiple dependent variables using a non-parametric Friedman’s ANOVA. The chromosomal location of ribosomal DNA clusters within the nucleolar organization region (NORs) and telomeric (TTAGGG)_n sequences were detected by fluorescent in situ hybridization (FISH). Chromosomes were subjected to AgNO₃ staining in order to determine whether the rDNA sites represent active NORs.

Results

The cytogenetic analysis confirmed the karyotype composed from eight chromosome triplets (3n = 24) as well as the existence of a pair of NORs located on each chromosome of the second triplet. Six NORs varied their activity from cell to cell, and it was reflected in the numbers of nucleoli (from 1 to 5). A huge morphological diversification of homologue chromosomes was originally detected in six out of eight triplets; the homologue elements differed significantly either in length and/or morphology, and some of them carried discernible interstitial telomeric sequences (ITSs), while the end telomeres were minute. The heterochromatin bands with high AT content varied irregularly, and the course of aberrant spermatogenesis was evident.

Conclusions

Diversification of homologues is a unique phenomenon very likely caused by the long-term absence of a recombination and consequential accumulation of chromosome rearrangements in the genome of A. huronensis during species evolution. Unalterable asexual reproduction of the tapeworm, along with international trade in its host (carp), is facilitating its ongoing spread.

Tour around the globe: The case of invasive tapeworm Atractolytocestus huronensis (Cestoda: Caryophyllidea), a parasite of common carp

The monozoic tapeworm Atractolytocestus huronensis Anthony, 1958 (Cestoda: Caryophyllidea), an intestinal parasite of the common carp, is characterized by its invasive character and potential to colonize new territories. It was initially described from North America and has also been found in several European countries. The most recent findings of A. huronensis originated from China and South Africa; however, no data on genetic relationships of these populations were available. The current study provides the first molecular characterisation of A. huronensis from South Africa and China using a partial sequence of mitochondrial cytochrome c oxidase subunit 1 (cox1) and a complete ribosomal ITS2 spacer. Ribosomal and mitochondrial data were applied for phylogenetic analyses in order to assess the genetic interrelationships among global A. huronensis populations. Divergent intragenomic copies of ribosomal ITS2 were detected in all analysed specimens; the structure and frequency of the ITS2 variants of tapeworms from China and South Africa corresponded with the data on ITS2 paralogues observed previously in A. huronensis from Slovakia, the United States and the United Kingdom. The phylogenetic analysis of cox1 indicated that A. huronensis exist in two slightly differentiated clusters; one cluster was supported by all phylogenetic approaches (NJ, ML, BI) and was represented by samples from China, the USA and the UK. A second cluster was represented by tapeworms from continental Europe (Slovakia, Hungary, Romania, Croatia) and South Africa. Haplotype network analysis revealed that the highest population diversity occurs in China. The results provide useful pilot information about the interrelationships of A. huronensis on four continents and indicate that China, or the eastern Palaearctic, served as the original source population for the global expansion of this invasive tapeworm. Data on the origin and distribution of the common carp, the only specific host of A. huronensis, are also discussed.

Parasites of commercially important fish from Lake Naivasha, Rift Valley, Kenya

In Lake Naivasha, the common carp Cyprinus carpio L. 1758 was accidentally introduced from fish farms adjacent to River Malewa in 1999 and now forms the bulk of the total fish caught. Since its introduction, no study has been made on its parasitic community nor are there any reports on ectoparasites from other fish species in this lake to the best of our knowledge. Therefore, the aim of this study was to describe the parasitic community of C. carpio and two other commercially important fish species: Oreochromis leucostictus and Tilapia zillii. Additionally, the abundant Barbus paludinosus was included in the study. A total of 286 fish (145C. carpio, 56 O. leucostictus, 18 T. zillii, and 67 B. paludinosus) were collected during the year 2011 and examined. Ten taxa of parasites were recovered from C. carpio dominated by the monogenean Dactylogyrus minutus, occurring with a prevalence (p) of 99.3%. Thirteen taxa of parasites were identified from O. leucostictus dominated by monogeneans Cichlidogyrus spp. (p = 91.1%). T. zillii harbored nine taxa of parasites with the digenean Tylodelphys sp. (p = 83.3%) being dominant and B. paludinosus harbored 11 taxa of parasites dominated by an unidentified monogenean of the genus Dactylogyrus (p = 83.6%). C. carpio had the lowest helminth species diversity and richness while monogenetic trematodes, which have never been reported from fish in Lake Naivasha, were the most prevalent parasites recovered.

Spermatological characters of monozoic tapeworms (Cestoda: Caryophyllidea), including first data on a species from the Indomalayan catfish

The ultrastructure of spermiogenesis and mature spermatozoon in Lytocestus indicus (Cestoda: Lytocestidae) is described; this is the first representative of this group of monozoic, presumably most basal, tapeworms (Eucestoda) from the Indomalayan region to be documented in this manner. Similarly, as in other caryophyllideans, its spermiogenesis involves the formation of a conical differentiation zone with 2 centrioles associated with striated roots and an intercentriolar body. In the course of the process, 1 of the centrioles develops a free flagellum, which fuses with a cytoplasmic protrusion, whereas the other remains oriented in a cytoplasmic bud. Spermiogenesis is also characterized by the presence of electron-dense material in the early stages of spermiogenesis and a slight rotation of the flagellar bud. The mature spermatozoon of L. indicus is a filiform cell tapered at both extremities that lacks mitochondria; its nucleus has parallel disposition to the axoneme and does not reach up to the posterior extremity of the spermatozoon, which is typical for spermatozoa of the type III pattern. The new data confirm that caryophyllideans share the same type of spermiogenesis that is considered to be plesiomorphic in the Eucestoda. The existing information on spermatological ultrastructure of 8 members for 3 of 4 caryophyllidean families from different host groups (cyprinids and catostomids, both Cypriniformes, and mochokids and clariids, both Siluriformes) from 4 zoogeographical regions (Palearctic, Neotropic, Ethiopian, and Indomalayan regions) demonstrates great uniformity in spermiogenesis and sperm ultrastructure, which does not reflect different taxonomic position of the species studied.

Early intrauterine embryonic development in Khawia sinensis Hsü, 1935 (Cestoda, Caryophyllidea, Lytocestidae), an invasive tapeworm of carp (Cyprinus carpio): an ultrastructural study

Intrauterine embryonic development in the caryophyllidean tapeworm Khawia sinensis has been investigated using transmission electron microscopy and cytochemical staining with periodic acid-thiosemicarbazide-silver proteinate for glycogen. Contrary to previous light microscopy findings that reported the release of non-embryonated eggs of K. sinenesis to the external environment, the present study documents various stages of embryonation (ovoviviparity) within the intrauterine eggs of this cestode. At the initial stage of embryonic development, each fertilised oocyte is accompanied by several vitellocytes that become enclosed within the operculate, electrondense shell. Cleavage divisions result in formation of blastomeres (up to about 24 cells) of various sizes. Mitotic divisions and apparent rosette arrangment of the blastomeres, the latter atypical within the Eucestoda, are observed for the first time in the intrauterine eggs of K. sinenesis. The early embryo enclosed within the electrondense shell is surrounded by a thin membraneous layer which in some enlarged regions shows presence of nuclei. Simultaneously to multiplication and differentiation, some of the blastomeres undergo deterioration. A progressive degeneration of the vitellocytes within eggs provides nutritive reserves, including lipids, for the developing embryo. The possible significance of this atypical timing of the intrauterine embryonic development to (1) the ecology of K. sinensis and that of a recent introduction of another invasive tapeworm, the caryophyllidean Atractolytocestus huronensis Anthony, 1958 to Europe; and (2) the affiliation of caryophyllideans with other lower cestodes, are discussed.