Advances and Trends in the Molecular Systematics of the Parasitic Platyhelminthes

Macroderoides texanus n. sp. (Digenea: Macroderoididae) from alligator gar, Atractosteus spatula in Texas

Macroderoides texanus n. sp. is described based on 16 specimens collected from the intestine of the North American alligator gar, Atractosteus spatula. Of the five established species of Macroderoides, the new species is morphologically most similar to Macroderoides spiniferus and Macroderoides trilobatus. M. texanus n. sp. differs from M. spiniferus by having the ovary situated immediately posterior to the cirrus sac rather than at mid-way between the cirrus sac and anterior testis, the ventral sucker situated further posteriorly, and the vitelline fields extending somewhat posterior to posterior testis rather than to the middle of posterior testis. M. texanus n. sp. differs from M. trilobatus by having the ovary positioned immediately adjacent to, or overlapping the cirrus sac rather than at some distance posterior to it, and by having significantly larger eggs. Additionally, the new species has two distinctive rows of spines on the postero-ventral surface of the oral sucker that are lacking in M. spiniferus and M. trilobatus. Comparison of approximately 2,700-base-pair sequences of nuclear rDNA (partial 18S, complete ITS region and partial 28S) from M. texanus n. sp., M. spiniferus and M. trilobatus, strongly supports the status of M. texanus n. sp. as a new species.

Description of a new species of Crassicutis Manter, 1936, parasite of Cichlasoma beani Jordan (Osteichthyes: Cichlidae) in Mexico, based on morphology and sequences of the ITS1 and 28S ribosomal RNA genes

A new species of Crassicutis Manter, 1936 is described from the Sinaloan cichlid Cichlasoma beani (Jordan) (Osteichthyes: Cichlidae) in the upper Río Santiago basin. Crassicutis choudhuryi n. sp. differs from most of the other nominal species by having testes located in a symmetrical position. The only other species of the genus that includes some specimens exhibiting this trait is Crassicutis intermedius (Szidat 1954), a species found in 5 species of siluriforms and 1 species of characiform in South America. However, this species differs from Cr. choudhuryi n. sp. by having testes almost half of the size, and vitelline follicles extending anteriorly to the region between the acetabulum and the intestinal bifurcation. The new species is morphologically very similar to Crassicutis cichlasomae Manter, 1936, but clearly differs from this species because of the constantly symmetrical position of the testes. Additionally, Cr. choudhuryi n. sp. is found in the Santiago River basin on the Pacific slope of Mexico, parasitizing specifically the endemic Ci. beani that does not co-occur with any other cichlid. Cr. cichlasomae exhibits more hosts (about 25 species of cichlids only in Mexico) and a wider distribution range that extends from northeastern Mexico southward to Central America, Cuba, and Brazil. To corroborate that our specimens were not conspecific with Cr. cichlasomae, sequences of the internal transcribed spacer (ITS1) and the 28S ribosomal RNA genes of individuals from several populations (recently collected in southeastern Mexico) were obtained and compared to the species described herein. Sequence divergence (1.3% for the 28S and 4.0% for the ITS1) gives further support to the erection of a new species.

Sequence analysis of ribosomal and mitochondrial genes of the giant liver fluke Fascioloides magna (Trematoda: Fasciolidae): intraspecific variation and differentiation from Fasciola hepatica

Complete sequences of ribosomal and mitochondrial genes of the giant liver fluke Fascioloides magna are presented. In particular, small subunit (18S) and internal transcribed spacers (ITS1 and ITS2) of the ribosomal gene (rDNA), as well as cytochrome c oxidase subunit I (cox1) and nicotinamide dehydrogenase subunit I (nad1) of the mitochondrial DNA (mtDNA), were analyzed. The 18S and ITS sequences were compared with previously published sequences of the liver fluke Fasciola hepatica. Fixed interspecific genetic differences were determined that allow molecular differentiation of F. magna and F. hepatica using either the PCR-RFLP method or PCR amplification of species-specific DNA regions. Additionally, intraspecific sequence polymorphism of the complete cox1 and nad1 mitochondrial genes in geographically distinct F. magna populations was determined. Based on the sequence divergences, short (< 500 bp) variable regions suitable for broader biogeographical studies of giant liver fluke were designed.

Morphological and molecular differentiation between Dicrocoelium dendriticum (Rudolphi, 1819) and Dicrocoelium chinensis (Sudarikov and Ryjikov, 1951) Tang and Tang, 1978 (Platyhelminthes: Digenea)

Dicrocoelium dendriticum (Rudolphi, 1819) and Dicrocoelium hospes (Looss, 1907) are recognised to affect the liver of domestic and wild ruminants. A third species, Dicrocoelium orientalis which was described from musk deer in the Baikal region of the former Soviet Union and re-named to Dicrocoelium chinensis (Sudarikov and Ryjikov, 1951) Tang and Tang, 1978 was isolated from other species of deer in Asian countries and from mouflon and roe deer in Europe. Scant information is available for D. chinensis, including the range of species that act as definitive and intermediate hosts. To provide morphological and molecular evidences differentiating D. chinensis versus D. dendriticum, 239 Dicrocoelium spp. specimens were collected from sheep, cattle and sika deer from different localities in Austria, Germany and Italy. Specimens were morphologically identified based on the testes orientation, overall size, and level of maximum body width and other morphometric measurements. From this sample, 10 specimens of D. chinensis and 25 of D. dendriticum from different hosts and geographical localities were characterized molecularly through sequencing of partial 18S rDNA (approximately 1400 bp) and ITS-2 (including the 5.8S and 28S flanking regions; approximately 600 bp). Interspecific differences between D. dendriticum and D. chinensis of 0.14% and 3.8% were recorded in 18S rRNA and ITS-2 sequences, respectively. Phylogenetic analyses via Bayesian inference were conducted using sequences of ITS-2 (276 bp) and partial 28S (221 bp) of the above species of Dicrocoelium together with 20 species belonging to the Xiphidiata within the Plagiorchiida available in GenBank. Both gene regions were strongly concordant in differentiating the Dicrocoeliidae, Gorgoderidae and Plagiorchiidae and were in agreement with their current classification. Morphological and molecular characterization clearly differentiate D. dendriticum and D. chinensis as two distinct digeneans infecting ruminants. The implications on the separate status of D. chinensis on the etiology, biology and diagnosis of dicrocoeliosis are discussed.

The biology of gyrodactylid monogeneans: the "Russian-doll killers"

This article reviews the history of gyrodactylid research focussing on the unique anatomy, behaviour, ecology and evolution of the viviparous forms while identifying gaps in our knowledge and directions for future research. We provide the first summary of research on the oviparous gyrodactylids from South American catfish, and highlight the plesiomorphic characters shared by gyrodactylids and other primitive monogeneans. Of these, the most important are the crawling, unciliated larva and the spike sensilla of the cephalic lobes. These characters allow gyrodactylids to transfer between hosts at any stage of the life cycle, without a specific transmission stage. We emphasise the importance of progenesis in shaping the evolution of the viviparous genera and discuss the relative extent of progenesis in the different genera. The validity of the familial classification is discussed and we conclude that the most significant division within the family is between the oviparous and the viviparous genera. The older divisions into Isancistrinae and Polyclithrinae should be allowed to lapse. We discuss approaches to the taxonomy of gyrodactylids, and we emphasise the importance of adequate morphological and molecular data in new descriptions. Host specificity patterns in gyrodactylids are discussed extensively and we note the importance of host shifts, revealed by molecular data, in the evolution of gyrodactylids. To date, the most closely related gyrodactylids have not been found on closely related hosts, demonstrating the importance of host shifts in their evolution. The most closely related species pair is that of G. salaris and G. thymalli, and we provide an account of the patterns of evolution taking place in different mitochondrial clades of this species complex. The host specificity of these clades is reviewed, demonstrating that, although each clade has its preferred host, there is a range of specificity to different salmonids, providing opportunities for complex patterns of survival and interbreeding in Scandinavia. At the same time, we identify trends in systematics and phylogeny relevant to the G. salaris epidemics on Atlantic salmon in Norway, which can be applied more generally to parasite epidemiology and evolution. Although much of gyrodactylid research in the last 30 years has been directed towards salmonid parasites, there is great potential in using other experimental systems, such as the gyrodactylids of poeciliids and sticklebacks. We also highlight the role of glacial lakes and modified river systems during the ice ages in gyrodactylid speciation, and suggest that salmon infecting clades of G. salaris first arose from G. thymalli in such lakes, but failed to spread fully across Scandinavia before further dispersal was ended by rising sea levels. This dispersal has been continued by human activity, leading to the appearance of G. salaris as a pathogen in Norway. We review the history and current status of the epidemic, and current strategies for elimination of the parasite from Norway. Finally, we consider opportunities for further spread of the parasite within and beyond Europe.

Dactylogyrus species parasitizing European Barbus species: morphometric and molecular variability

The aims of the study were (1) to describe the Dactylogyrus communities living on selected South European Barbus species, (2) to analyse morphometric variability of their attachment and reproductive organs, and (3) to perform molecular phylogenetic analyses, in order to investigate the mode of speciation in Dactylogyrus species parasitizing congeneric hosts. In Bulgaria, Dactylogyrus crivellius, D. dyki and D. petenyi were found on B. balcanicus, and D. dyki on B. cyclolepis. In Spain, Dactylogyrus carpathicus and D. dyki were detected on B. meridionalis. Morphometric analyses of D. dyki revealed significant differences in the attachment and reproductive organs when individuals from different Barbus species were compared. Two monophyletic groups were recognized from the molecular phylogenetic analyses: the first included D. carpathicus and D. crivellius which have large body size and anchors, with a weakly supported basal position of D. malleus from B. barbus; the second included D. dyki and D. petenyi which have small body and anchor sizes. The comparison of host and parasite phylogenies did not indicate the intrahost speciation. Intraspecific molecular variability was found between individuals of D. dyki and D. carpathicus from different Barbus species, suggesting the need for a taxonomic revision for these species.

Making the most of mitochondrial genomes--markers for phylogeny, molecular ecology and barcodes in Schistosoma (Platyhelminthes: Digenea)

An increasing number of complete sequences of mitochondrial (mt) genomes provides the opportunity to optimise the choice of molecular markers for phylogenetic and ecological studies. This is particularly the case where mt genomes from closely related taxa have been sequenced; e.g., within Schistosoma. These blood flukes include species that are the causative agents of schistosomiasis, where there has been a need to optimise markers for species and strain recognition. For many phylogenetic and population genetic studies, the choice of nucleotide sequences depends primarily on suitable PCR primers. Complete mt genomes allow individual gene or other mt markers to be assessed relative to one another for potential information content, prior to broad-scale sampling. We assess the phylogenetic utility of individual genes and identify regions that contain the greatest interspecific variation for molecular ecological and diagnostic markers. We show that variable characters are not randomly distributed along the genome and there is a positive correlation between polymorphism and divergence. The mt genomes of African and Asian schistosomes were compared with the available intraspecific dataset of Schistosoma mansoni through sliding window analyses, in order to assess whether the observed polymorphism was at a level predicted from interspecific comparisons. We found a positive correlation except for the two genes (cox1 and nad1) adjoining the putative control region in S. mansoni. The genes nad1, nad4, nad5, cox1 and cox3 resolved phylogenies that were consistent with a benchmark phylogeny and in general, longer genes performed better in phylogenetic reconstruction. Considering the information content of entire mt genome sequences, partial cox1 would not be the ideal marker for either species identification (barcoding) or population studies with Schistosoma species. Instead, we suggest the use of cox3 and nad5 for both phylogenetic and population studies. Five primer pairs designed against Schistosoma mekongi and Schistosoma malayensis were tested successfully against Schistosoma japonicum. In combination, these fragments encompass 20-27% of the variation amongst the genomes (average total length approximately 14,000bp), thus providing an efficient means of encapsulating the greatest amount of variation within the shortest sequence. Comparative mitogenomics provides the basis of a rational approach to molecular marker selection and optimisation.

An annotated list of species of the Proteocephalus Weinland, 1858 aggregate sensu de Chambrier et al. (2004) (Cestoda: Proteocephalidea), parasites of fishes in the Palaearctic Region, their phylogenetic relationships and a key to their identification

A list and key to the identification of valid species of tapeworms of the Proteocephalus Weinland, 1858 aggregate sensu de Chambrier et al. (2004), i.e. species of the genus occurring in fresh- and brackish-water fishes in the Palaearctic Region, are provided, with data on their hosts and geographical distribution. Instead of 32 taxa listed by Schmidt (1986) and subsequent authors, only the following 14 species are considered to be valid: P. ambiguus (Dujardin, 1845) (type-species); P. cernuae (Gmelin, 1790); P. filicollis (Rudolphi, 1802); P. fluviatilis Bangham, 1925; P. gobiorum Dogiel & Bychowsky, 1939; P. longicollis (Zeder, 1800); P. macrocephalus (Creplin, 1825); P. midoriensis Shimazu, 1990; P. percae (Müller, 1780); P. plecoglossi Yamaguti, 1934; P. sagittus (Grimm, 1872); P. tetrastomus (Rudolphi, 1810); P. thymalli (Annenkova-Chlopina, 1923); and P. torulosus (Batsch, 1786). An analysis of sequences of the nuclear genes (ITS2 and V4 region of 18S rDNA) revealed the following phylogenetic relationships for these taxa: P. torulosus ((P. midoriensis, P. sagittus) (P. fluviatilis (P. filicollis, P. gobiorum, P. macrocephalus)) (P. cernuae, P. plecoglossi, P. tetrastomus ((P. longicollis, P. percae) (P. ambiguus, P. thymalli)))). P. pronini Rusinek, 2001 from grayling Thymallus arcticus nigrescens is synonymised with P. thymalli. P. esocis La Rue, 1911 is apparently invalid but its conspecificity with either P. percae or P. longicollis could not be confirmed due to the absence of the scolex in the holotype and the unavailability of other material for morphological and molecular studies. P. osculatus (Goeze, 1782) has recently been transferred to Glanitaenia de Chambrier, Mariaux, Vaucher & Zehnder, 2004. The validity of the genus is supported by the position of G. osculata within the Proteocephalidea, based on molecular data, as well as its morphology and nature of the definitive host (the European wels Silurus glanis). P. hemispherous Rahemo & Al-Niaeemi, 2001, described from S. glanis in Iraq, is transferred to Postgangesia Akhmerov, 1960 as Postgangesia hemispherous (Rahemo & Al-Niaeemi, 2001) n. comb.

Added resolution among ordinal level relationships of tapeworms (Platyhelminthes: Cestoda) with complete small and large subunit nuclear ribosomal RNA genes

The addition of large subunit ribosomal DNA (lsrDNA) to small subunit ribosomal DNA (ssrDNA) has been shown to add resolution to phylogenies at various taxonomic levels for a diversity of phyla. We added nearly complete lsrDNA (4057-4593bp) sequences to ssrDNA (1940-2228bp) for 26 ingroup and 3 outgroup taxa in an attempt to provide an improved ordinal phylogeny for the Cestoda. Ten lsrDNA and seven ssrDNA sequences were generated from new taxa and 13 existing partial lsrDNA sequences were sequenced to completion. The majority of phylogenetic signal in the combined analysis came from lsrDNA (69.6% of parsimonious informative sites, as opposed to 30.4% obtained from ssrDNA), resulting in almost identical topologies for lsrDNA and lsr+ssrDNA (pairwise symmetric distance=6) in model-based analyses. Topology testing found trees based on partial lsrDNA (domains D1-D3)+ssrDNA and complete lsr+ssrDNA to differ significantly; the addition of lsrDNA domains D4-D12 had a significant effect on topology. Overall nodal support was greatest in the combined analysis and weakest for ssrDNA only. Our molecular phylogenies differed significantly from those based on morphology alone. Acetabulate lineages form a monophyletic group, with the Tetraphyllidea being paraphyletic. Support for the combined data was high for the following topology: (Litobothriidea (Lecanicephalidea (Rhinebothrium/Rhodobothrium (Clistobothrium (Pachybothrium(Acanthobothrium Proteocephalidea) (Mesocestoididae, Nippotaeniidea, Cyclophyllidea, Tetrabothriidea)))))); all genus names refer to tetraphyllidean lineages. Although the interrelationships among the four most derived taxa remain uncertain, overall ambiguity of the acetabulate interrelationships was reduced. The Pseudophyllidea were recovered as polyphyletic, with support for a sister-group relationship between Diphyllobothriidae and Haplobothriidea. The monophyly of the Trypanorhyncha was recovered for the first time based on molecular data. The positions of the Trypanorhyncha, Diphyllidea and "Bothriocephaliidea" in relation to other orders remains ambiguous. Higher congruence was found between trees based on model-based phylogenetic methods than with those constructed under the parsimony criterion. Although some uncertainties remain, the addition of lsrDNA D4-D12 has provided an overall more resolved and better supported cestode phylogeny, which further promotes the utility of complete lsrDNA as phylogenetic marker where ssrDNA alone proves inadequate.

Cryptic species of Didymobothrium rudolphii (Cestoda: Spathebothriidea) from the sand sole, Solea lascaris, off the Portuguese coast, with an analysis of their molecules, morphology, ultrastructure and phylogeny

Didymobothrium rudolphii (Cestoda: Spathebothriidea) was collected seasonally from the sand sole, Solea lascaris, off the northern, central and southern areas of the Portuguese coast. Morphological and molecular analyses were conducted in order to examine the possible existence of cryptic species and to facilitate the circumscription of their morphological boundaries. Data were compared between D. rudolphii specimens from each of the 3 geographical areas and 4 seasons, and principal components analysis of 18 morphological characters was used to detect differences. Two distinct genotypes were present with sequence divergences of 1.9% and 2.1% in the large subunit (lsrDNA) and second internal transcribed spacer (ITS-2) of ribosomal DNA (rDNA), respectively. The less common 'central' genotype was present only off the central area from summer to winter, whereas the 'common' genotype was present throughout the year off the northern and southern areas, but only during spring in the central area. No sequence variation was found within each genotype. The presence of 2 distinct genetic entities was supported by morphological analyses, which showed the 'central' genotype specimens to be more slender and elongate, although morphometric ranges overlapped considerably for most characters. Molecular phylogenetic analysis of 4 of the 5 known genera of the Spathebothriidea showed Spathebothrium to be the earliest branching lineage and the 2 genotypes of Didymobothrium formed a sister group to Cyathocephalus. The concordance of genetic differences with variation in host diet according to season and locality could account for sympatric speciation occurring in the central region of the Portuguese coast.

Insight into the role of cetaceans in the life cycle of the tetraphyllideans (Platyhelminthes: Cestoda)

Four types of tetraphyllidean larvae infect cetaceans worldwide: two plerocercoids differing in size, 'small' (SP) and 'large' (LP), and two merocercoids referred to as Phyllobothrium delphini and Monorygma grimaldii. The latter merocercoid larvae parasitize marine mammals exclusively and exhibit a specialised cystic structure. Adult stages are unknown for any of the larvae and thus the role of cetaceans in the life cycle of these species has been a long-standing problem. The SP and LP forms are thought to be earlier stages of P. delphini and M. grimaldii that are presumed to infect large pelagic sharks that feed on cetaceans. A molecular analysis of the D2 variable region of the large subunit ribosomal DNA gene based on several individuals of each larval type collected from three Mediterranean species of cetaceans showed consistent and unique molecular signatures for each type regardless of host species or site of infection. The degree of divergence suggested that LP, P. delphini and M. grimaldii larvae may represent separate species, whereas SP may be conspecific with M. grimaldii. In all host species, individuals of SP accumulated in the gut areas in which the lymphoid tissue was especially developed. We suggest therefore that these larvae use the lymphatic system to migrate to the abdominal peritoneum and mesenteries where they develop into forms recognizable as M. grimaldii. The plerocercoid stage of P. delphini remains unknown. In a partial phylogenetic tree of the Tetraphyllidea, all larvae formed a clade that included a representative of the genus Clistobothrium, some species of which parasitize sharks such as the great white which is known to feed on cetaceans. A bibliographic examination of tetraphyllidean infections in marine mammals indicated that these larvae are acquired mostly offshore. In summary, the evidence suggests that cetaceans play a significant role in the life cycle of these larvae. In addition, it seems clear that cetaceans act as natural intermediate hosts for P. delphini and M. grimaldii, as within these hosts they undergo development from the plerocercoid stage to the merocercoid stage. Because tetraphyllidean species use fish, cephalopods and other marine invertebrates as intermediate hosts, the inclusion of cetaceans in the life cycle would have facilitated their transmission to apex predators such as the large, lamnid sharks. The biological significance of infections of LP in cetaceans is unclear, but infections do not seem to be accidental as such larvae show high prevalence and abundance as well as a high degree of site specificity, particularly in the anal crypts and bile ducts.

Vitellocyte ultrastructure in the cestode Didymobothrium rudolphii (Monticelli, 1890): possible evidence for the recognition of divergent taxa within the Spathebothriidea

In the spathebothriidean tapeworm Didymobothrium rudolphii (Monticelli, 1890) the fine structure of the vitellocytes at different stages of their development within the vitelline follicles, vitelline ducts and uterus was studied for the first time using transmission electron microscopy. The vitellocyte inclusions of D. rudolphii are shell globule clusters containing tightly packed shell globules associated with a matrix of moderate electron density, glycogen granules, large electron-lucent lipid droplets (up to 3 μm in diameter), and, occasionally, a lipid droplet may occur in the nucleus of the vitellocytes. The diameter of the clusters ranges from 0.4 to 2.5 μm, the number of shell globules in the clusters varies from 8 to 45, and the size of the globules ranges from 0.12 to 0.25 μm and they are of approximately homogeneous sizes within a cluster. Most vitellocyte lipid droplets have a heterogeneous configuration with a ‘cavity’ inside them when they are within vitelline ducts and intrauterine eggs. Vitellocytes of the eggs contain dark concentric bodies and lipid droplets. The interstitial tissue has a syncytial structure. The morphological parameters of the diameter and shape of shell globule clusters, arrangement of shell globules in clusters, number and diameter of globules within clusters, types of lipid droplets and presence of dark concentric bodies are compared with those of two other spathebothriidean genera, Cyathocephalus and Diplocotyle. The comparative data demonstrate that vitelline material morphology has unique features in three spathenothriidean genera and may be used as evidence for the recognition of separate taxa.