The cestode Atractolytocestus huronensis (Caryophyllidea) continues to spread in Europe: new data on the helminth parasite of the common carp

Fish tapeworms (Cestoda) in the molecular era: achievements, gaps and prospects

The tapeworms of fishes (Chondrichthyes and Actinopterygii) account one-third (1670 from around 5000) of the total tapeworm (Platyhelminthes: Cestoda) species diversity. In total 1186 species from 9 orders occur as adults in elasmobranchs (sharks, rays and chimaeras), and 484 species from 8 orders mature in ray-finned fishes (referred to here as teleosts). Teleost tapeworms are dominated by freshwater species (78%), but only 3% of elasmobranch tapeworms are known from freshwater rays of South America and Asia (Borneo). In the last 2 decades, vast progress has been made in understanding species diversity, host associations and interrelationships among fish tapeworms. In total, 172 new species have been described since 2017 (149 from elasmobranchs and 23 from teleosts; invalidly described taxa are not included, especially those from the Oriental region). Molecular data, however, largely limited to a few molecular markers (mainly 28S rDNA, but also 18S and cox1), are available for about 40% of fish tapeworm species. They allowed us to significantly improve our understanding of their interrelationships, including proposals of a new, more natural classification at the higher-taxonomy level (orders and families) as well as at the lower-taxonomy level (genera). In this review, we summarize the main advances and provide perspectives for future research.

Molecular evidence of a putative new Atractolytocestus Anthony, 1958 (Cestoda: Caryophyllidea) species parasitic on common carp (Cyprinus carpio) in the People's Republic of China

Three tapeworm species belonging to the genus Atractolytocestus Anthony, 1958 (Cestoda: Caryophyllidea) were reported from the common carp, Cyprinus carpio L., 1758 in the People's Republic of China so far: Atractolytocestus sagittatus; Atractolytocestus tenuicollis; and Atractolytocestus huronensis. In this study, we identified a putatively new tapeworm species in common carp from the Danjiangkou Reservoir in Central China (Hubei Province). The species is morphologically similar to A. sagittatus, but it differed conspicuously in sequences of three molecular markers. Ribosomal internal transcribed spacers 2 sequence identity of the new species was 94.7–95.5%, 91.8–92.7% and 80.0–83.9% with A. huronensis, A. tenuicollis and A. sagittatus, respectively. cytochrome c oxidase and NADH dehydrogenase subunit 3 sequence identity with these three species was lower than 92%. We conclude that this is a new tapeworm species, and we named it after the locality: Atractolytocestus danjiangkouensis n. sp.

Diversity of monogeneans and tapeworms in cypriniform fishes across two continents

Cypriniformes, which exhibit a wide geographical distribution, are the most species-rich group of freshwater fishes. Despite considerable research on their parasites, no reliable estimates of their parasite diversity on a large geographical scale are available. In the present review, we analyse species richness of two parasitic flatworm groups (monogeneans and tapeworms) reported from cypriniform fishes in the two most intensively studied parts of the Holarctic region, Europe and North America. We also review knowledge on parasite speciation and host-parasite coevolution, and emphasise the risk of parasite co-introduction resulting from transfers of cypriniforms among different continents. As parasite diversity in European cypriniforms has been more intensively explored, we predicted a lower level of knowledge on parasite diversity in North American fishes, despite North America having a higher diversity of cypriniforms than Europe. Our data revealed a higher mean species richness of monogeneans and tapeworms per cypriniform species in Europe compared with North America. We showed that species richness of both parasite taxa in both continents is strongly affected by sample size, but that fish traits also play an important role in determining monogenean and tapeworm species richness in European cyprinoids. We recorded higher host specificity for cypriniform parasites in North America, even within parasite genera shared by cypriniforms on both continents. The host range of monogeneans parasitising cyprinoids on both continents was affected by phylogeny, indicating an effect of parasite life history on host specificity. The difference in parasite host range between the two continents could potentially be explained by either the low overall level of sampling activity in North America or an underestimation of parasite diversity in Europe. We suggest that future research efforts be focussed on cypriniforms in order to obtain reliable data for robust assessments of parasite species richness and phylogenies, to assess host-parasite coevolution and to reveal fish biogeography.

First record of the American cestode Proteocephalus ambloplitis (Leidy, 1887) (Proteocephalidae) in Europe

Here, we report the first record of pleroceroids of the Nearctic tapeworm Proteocephalus ambloplitis (Leidy, 1887) in European fresh waters. The plerocercoids were found encapsulated in the internal organs of the invasive pumpkinseed Lepomis gibbosus (Linnaeus, 1758) in a pond in Bègles Plage, Bordeaux, France. This parasite species was not observed in L. gibbosus collected from two nearby localities, Bordeaux Lac and a pond in the Parc de Fontaudin. Species identification was confirmed using molecular data and morphological characteristics.

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.

The complete mitochondrial DNA of three monozoic tapeworms in the Caryophyllidea: a mitogenomic perspective on the phylogeny of eucestodes

Background

External segmentation and internal proglottization are important evolutionary characters of the Eucestoda. The monozoic caryophyllideans are considered the earliest diverging eucestodes based on partial mitochondrial genes and nuclear rDNA sequences, yet, there are currently no complete mitogenomes available. We have therefore sequenced the complete mitogenomes of three caryophyllideans, as well as the polyzoic Schyzocotyle acheilognathi, explored the phylogenetic relationships of eucestodes and compared the gene arrangements between unsegmented and segmented cestodes.

Results

The circular mitogenome of Atractolytocestus huronensis was 15,130 bp, the longest sequence of all the available cestodes, 14,620 bp for Khawia sinensis, 14,011 bp for Breviscolex orientalis and 14,046 bp for Schyzocotyle acheilognathi. The A-T content of the three caryophyllideans was found to be lower than any other published mitogenome. Highly repetitive regions were detected among the non-coding regions (NCRs) of the four cestode species. The evolutionary relationship determined between the five orders (Caryophyllidea, Diphyllobothriidea, Bothriocephalidea, Proteocephalidea and Cyclophyllidea) is consistent with that expected from morphology and the large fragments of mtDNA when reconstructed using all 36 genes. Examination of the 54 mitogenomes from these five orders, revealed a unique arrangement for each order except for the Cyclophyllidea which had two types that were identical to that of the Diphyllobothriidea and the Proteocephalidea. When comparing gene order between the unsegmented and segmented cestodes, the segmented cestodes were found to have the lower similarities due to a long distance transposition event. All rearrangement events between the four arrangement categories took place at the junction of rrnS-tRNA^Arg (P1) where NCRs are common.

Conclusions

Highly repetitive regions are detected among NCRs of the four cestode species. A long distance transposition event is inferred between the unsegmented and segmented cestodes. Gene arrangements of Taeniidae and the rest of the families in the Cyclophyllidea are found be identical to those of the sister order Proteocephalidea and the relatively basal order Diphyllobothriidea, respectively.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2245-y) contains supplementary material, which is available to authorized users.

Asian fish tapeworm, Khawia japonensis (Yamaguti, 1934), has expanded its European invasive range

The invasive fish tapeworm, Khawia japonensis (Yamaguti, 1934) originally described in Japan, is reported for the first time in Slovakia. The tapeworm was found in farmed common carp (Cyprinus carpio L.) from East-Slovakian breeding fish pond. This finding is registered 4 years after the first announcement of this parasite in Europe (Po River Basin in Italy) in 2010 and increases its distribution area within this continent. Morphological characterization of K. japonensis supplemented with DNA sequences of cox1 and ribosomal lsrDNA genes is provided. Specimens from Slovakia phenotypically corresponded with those from feral and farmed carps from China, Vietnam, and Italy. Moreover, 100 and 98.7% identity of partial ribosomal lsrDNA gene and mitochondrial cox1 genes, respectively, were detected with K. japonensis from Japan. The invasive and pathogenic potential of K. japonensis in commercial breeding fisheries and its possible further spread in natural habitats is difficult to estimate for now. As yet, K. japonensis appears to be without a major impact on commercial breeding fisheries, but calls for more attention to the problem of biological invasions.

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.

Cytogenetics and chromosomes of tapeworms (Platyhelminthes, Cestoda)

Tapeworms (Cestoda, Platyhelminthes) are a highly diversified group of parasites that can have significant veterinary importance as well as medical impact as disease agents of human alveococcosis, hydatidosis, taeniosis/cysticercosis/neurocysticercosis, hymenolepidosis or diphyllobothriasis. Because of their great diversity, there has been keen interest in their phylogenetic relationships to other obligate parasitic platyhelminthes, as well as within the group itself. Recent phylogenetic analyses of cestodes, however, have focused on morphological, molecular, life cycle, embryology and host-specificity features and conspicuously omitted inclusion of karyological data. Here we review the literature from 1907 to 2010 and the current status of knowledge of the chromosomes and cytogenetics within all of the cestode orders and place it within an evolutionary perspective. Karyological data are discussed and tabulated for 115 species from nine eucestode orders with ideograms of 46 species, and a comparison of cytogenetic patterns between acetabulate and bothriate cestode lineages is made. Attention is drawn to gaps in our knowledge for seven remaining orders and cestodarian groups Gyrocotylidea and Amphilinidea. Among the cytogenetic aspects covered are: chromosome number, triploidy, classical karyotype cytogenetics (banding patterns, karyotype asymmetry, secondary constrictions), as well as advanced karyotype techniques allowing location of genes on chromosomes by fluorescence in situ hybridization. We demonstrate that further progress in cestode karyosystematics rests with new molecular approaches and the application of advanced cytogenetic markers facilitating intimate karyotype analysis.

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

Spermatogenesis, testes, and vas deferens in the parthenogenetic monozoic tapeworm Atractolytocestus huronensis Anthony, 1958 (Cestoda: Caryophyllidea) from Slovakia, parasitizing the carp Cyprinus carpio L., have been investigated by means of transmission electron microscopy for the first time. The present results show that helminths with parthenogenetic and normal reproduction may share some common spermatology features, e.g., dense cytoplasm of the peripherally localized spermatogonia or a rosette type of spermatogenesis. In contrast to tapeworms with normal reproduction, the most prominent ultrastructural characteristic of the spermatocytes of A. huronensis is fragmentation of their nuclei. This clear feature of cell degeneration might be a consequence of the aberrant first meiotic division. Peripheral cortical microtubules and a single centriole, indicators of the ongoing spermiogenesis, were observed only very rarely in the early spermatids. Characteristics of normal spermiogenesis, i.e., apical dense material in the zone of differentiation in early stages of spermiogenesis, flagellar rotation, and proximo-distal fusion, were never found in the present study. The testes follicles are surrounded by a thin cytoplasmic sheath underlined by a basal lamina. Vas deferens is lined by flat epithelium with numerous surface lamellae and cilia. Mature, functional spermatozoa were not observed in the vas deferens of A. huronensis from Slovakia.

Tapeworm Khawia sinensis: review of the introduction and subsequent decline of a pathogen of carp, Cyprinus carpio

The Asian tapeworm Khawia sinensis Hsü, 1935 (Cestoda: Caryophyllidea) is a large-sized (body length up to 11.5 cm) monozoic (unsegmented) parasite of common carp (Cyprinus carpio L.) that may cause mortality of young fish (fry). Since the 1960s, this cestode successfully colonized a large part of Europe, including the British Isles, North America and Japan. However, a review of published records provides evidence that the tapeworm K. sinensis, invasive parasite of carp, has become less common during the last two decades. Decline of K. sinensis may have been related to the recent introduction of another invasive tapeworm, the caryophyllidean Atractolytocestus huronensis Anthony, 1958 to Europe. Other factors that may have caused that K. sinensis is much less common than previously are also briefly discussed. A comparison of K. sinensis from feral and cultured carp, published to date, with those recently found for the first time in wild populations of carp in Slovakia did not reveal any marked differences in their morphology or measurements.

The morphology and systematic status of Khawia rossittensis (Szidat, 1937) and K. parva (Zmeev, 1936) (Cestoda: Caryophyllidea), parasites of cyprinid fishes

The morphology of the two little-known fish cestodes of the genus Khawia Hsü, 1935, K. rossittensis (Szidat, 1937) and K. parva (Zmeev, 1936) from cyprinid fishes, were studied on the basis of newly collected specimens from goldfish Carassius auratus auratus (L.) and museum specimens, respectively. This paper provides the first detailed species diagnosis for K. rossittensis from Slovakia, which is compared with specimens from different geographical regions and K. parva, a somewhat similar Far Eastern species from the same host. The two species differ in scolex morphology, anterior extent of the vitelline follicles, shape of the ovary and size of the eggs. Based on these differences, K. rossittensis and K. parva are considered to be separate taxa. K. parva, listed among the "species incertae sedis" by Mackiewicz (1972) and even within Caryophyllaeus Gmelin, 1790 by Schmidt (1986), should be considered a valid species of Khawia. The results support the previous conclusions of Kulakovskaya (1961), Dubinina (1971) and Protasova et al. (1990).

Hosts and parasites as aliens

Over the past decades, various free-living animals (hosts) and their parasites have invaded recipient areas in which they had not previously occurred, thus gaining the status of aliens or exotics. In general this happened to a low extent for hundreds of years. With variable frequency, invasions have been followed by the dispersal and establishment of non-indigenous species, whether host or parasite. In the literature thus far, colonizations by both hosts and parasites have not been treated and reviewed together, although both are usually interwoven in various ways. As to those factors permitting invasive success and colonization strength, various hypotheses have been put forward depending on the scientific background of respective authors and on the conspicuousness of certain invasions. Researchers who have tried to analyse characteristic developmental patterns, the speed of dispersal or the degree of genetic divergence in populations of alien species have come to different conclusions. Among parasitologists, the applied aspects of parasite invasions, such as the negative effects on economically important hosts, have long been at the centre of interest. In this contribution, invasions by hosts as well as parasites are considered comparatively, revealing many similarities and a few differences. Two helminths, the liver fluke,Fasciola hepatica,of cattle and sheep and the swimbladder nematode,Anguillicola crassus,of eels are shown to be useful as model parasites for the study of animal invasions and environmental global change. Introductions ofF. hepaticahave been associated with imports of cattle or other grazing animals. In various target areas, susceptible lymnaeid snails serving as intermediate hosts were either naturally present and/or were introduced from the donor continent of the parasite (Europe) and/or from other regions which were not within the original range of the parasite, partly reflecting progressive stages of a global biota change. In several introduced areas,F. hepaticaco-occurs with native or exotic populations of the congenericF. gigantica, with thus far unknown implications. Over the fluke's extended range, in addition to domestic stock animals, wild native or naturalized mammals can also serve as final hosts. Indigenous and displaced populations ofF. hepatica, however, have not yet been studied comparatively from an evolutionary perspective.A. crassus, from the Far East, has invaded three continents, without the previous naturalization of its natural hostAnguilla japonica, by switching to the respective indigenous eel species. Local entomostrac crustaceans serve as susceptible intermediate hosts. The novel final hosts turned out to be naive in respect to the introduced nematode with far reaching consequences for the parasite's morphology (size), abundance and pathogenicity. Comparative infection experiments with Japanese and European eels yielded many differences in the hosts' immune defence, mirroring coevolution versus an abrupt host switch associated with the introduction of the helminth. In other associations of native hosts and invasive parasites, the elevated pathogenicity of the parasite seems to result from other deficiencies such as a lack of anti-parasitic behaviour of the naïve host compared to the donor host which displays distinct behavioural patterns, keeping the abundance of the parasite low. From the small amount of available literature, it can be concluded that the adaptation of certain populations of the novel host to the alien parasite takes several decades to a century or more. Summarizing all we know about hosts and parasites as aliens, tentative patterns and principles can be figured out, but individual case studies teach us that generalizations should be avoided.