Vasyl V. Tkach

Phylogeny and classification of the Digenea (Platyhelminthes: Trematoda).

Complete small subunit ribosomal RNA gene (ssrDNA) and partial (D1-D3) large subunit ribosomal RNA gene (lsrDNA) sequences were used to estimate the phylogeny of the Digenea via maximum parsimony and Bayesian inference. Here we contribute 80 new ssrDNA and 124 new lsrDNA sequences. Fully complementary data sets of the two genes were assembled from newly generated and previously published sequences and comprised 163 digenean taxa representing 77 nominal families and seven aspidogastrean outgroup taxa representing three families. Analyses were conducted on the genes independently as well as combined and separate analyses including only the higher plagiorchiidan taxa were performed using a reduced-taxon alignment including additional characters that could not be otherwise unambiguously aligned. The combined data analyses yielded the most strongly supported results and differences between the two methods of analysis were primarily in their degree of resolution. The Bayesian analysis including all taxa and characters, and incorporating a model of nucleotide substitution (general-time-reversible with among-site rate heterogeneity), was considered the best estimate of the phylogeny and was used to evaluate their classification and evolution. In broad terms, the Digenea forms a dichotomy that is split between a lineage leading to the Brachylaimoidea, Diplostomoidea and Schistosomatoidea (collectively the Diplostomida nomen novum (nom. nov.)) and the remainder of the Digenea (the Plagiorchiida), in which the Bivesiculata nom. nov. and Transversotremata nom. nov. form the two most basal lineages, followed by the Hemiurata. The remainder of the Plagiorchiida forms a large number of independent lineages leading to the crown clade Xiphidiata nom. nov. that comprises the Allocreadioidea, Gorgoderoidea, Microphalloidea and Plagiorchioidea, which are united by the presence of a penetrating stylet in their cercariae. Although a majority of families and to a lesser degree, superfamilies are supported as currently defined, the traditional divisions of the Echinostomida, Plagiorchiida and Strigeida were found to comprise non-natural assemblages. Therefore, the membership of established higher taxa are emended, new taxa erected and a revised, phylogenetically based classification proposed and discussed in light of ontogeny, morphology and taxonomic history.

Advances and trends in the molecular systematics of the parasitic platyhelminthes

The application of molecular systematics to the parasitic Platyhelminthes (Cestoda, Digenea and Monogenea) over the last decade has advanced our understanding of their interrelationships and evolution substantially. Here we review the current state of play and the early works that led to the molecular-based hypotheses that now predominate in the field; advances in their systematics, taxonomy, classification and phylogeny, as well as trends in species circumscription, molecular targets and analytical methods are discussed for each of the three major parasitic groups. A by-product of this effort has been an ever increasing number of parasitic flatworms characterized genetically, and the useful application of these data to the diagnosis of animal and human pathogens, and to the elucidation of life histories are presented. The final section considers future directions in the field, including taxon sampling, molecular targets of choice, and the current and future utility of mitochondrial and nuclear genomics in systematic study.

Molecular phylogenetic analysis of the Microphalloidea Ward, 1901 (Trematoda: Digenea)

Phylogenetic interrelationships of 32 species belonging to 18 genera and four families of the superfamily Microphalloidea were studied using partial sequences of nuclear lsrDNA analysed by Bayesian inference and maximum parsimony. The resulting trees were well resolved at most nodes and demonstrated that the Microphalloidea, as represented by the present data-set, consists of three main clades corresponding to the families Lecithodendriidae, Microphallidae and Pleurogenidae + Prosthogonimidae. Interrelationships of taxa within each clade are considered; as a result of analysis of molecular and morphological data, Floridatrema Kinsella & Deblock, 1994 is synonymised with Maritrema Nicoll, 1907, Candidotrema Dollfus, 1951 with Pleurogenes Looss, 1896, and Schistogonimus Lühe, 1909 with Prosthogonimus Lühe, 1899. The taxonomic value of some morphological features, used traditionally for the differentiation of genera within the Lecithodendriidae and Prosthogonimidae, is reconsidered. Previous systematic schemes are discussed from the viewpoint of present results, and perspectives of future studies are outlined.

Phylogenetic analysis of the suborder plagiorchiata (Platyhelminthes, Digenea) based on partial lsrDNA sequences

The phylogenetic relationships and systematic position of the members of the suborder Plagiorchiata, one of the derived and most diverse groups of Digenea, have always been controversial. Here, we present a phylogeny of this group based on the analysis of partial sequences of the lsrDNA in 28 species representing 13 families of Plagiorchiata, as well as four outgroups. Our results show that the Plagiorchiata, as considered by most authors, is not monophyletic, and that the superfamilies Opecoeloidea, and most probably Dicrocoelioidea and Gorgoderoidea, may have to be removed from this suborder. According to our results, the Plagiorchiata includes only parasites of terrestrial vertebrates. We find the Plagiorchiata to be composed of two well-supported clades which can be ranked as superfamilies: (1) Plagiorchioidea, including the Plagiorchiidae, Haematoloechidae, Telorchiidae, Brachycoeliidae and Leptophallidae; and (2) Microphalloidea containing the Microphallidae, Prosthogonimidae, Lecithodendriidae and Pleurogenidae. The genetic analysis also allowed revision of the position of several taxa of Plagiorchiata, including: (1) a confirmation of the position of the Brachycoeliidae within the Plagiorchiata; (2) a close phylogenetic relationships of Macrodera with Paralepoderma, Leptophallus and Metaleptophallus; (3) the grouping of Opisthioglyphe and Telorchis within a distinct and strongly supported clade; and (4) the placement of Allassogonoporus amphoraeformis within the Pleurogenidae, and not close to Lecithodendriidae. Some systematic changes, corresponding to these results, are proposed.

Molecular and morphological differentiation between species of the Plagiorchis vespertilionis group (Digenea, Plagiorchiidae) occurring in European bats, with a re-description of P. vespertilionis (Müller, 1780).

The taxonomic history of the species of Plagiorchis Lühe, 1899 occurring in European bats has been very confused because of high morphological similarity between different forms/species and the inadequate initial description of P. vespertilionis(Müller, 1780). As morphological data alone have not provided enough convincing arguments to solve the problem, the sequences from the nuclear rDNA ITS region (ITS1, 5.8S and ITS2) of three species of the P. vespertilionis group (P. vespertilionis, P. muelleri Tkach & Sharpilo, 1990 and P. koreanus Ogata, 1938) occurring in European bats were used to test the validity of these species and evaluate some of the morphological characters used for the species differentiation within this group. P. elegans from birds was used as the outgroup in the analysis. All three ingroup species were clearly distinguishable using ITS sequences. Among them, P. koreanus occupied a basal position, while P. vespertilionis and P. muelleri appeared as a cluster of two closely related, derived species. ITS sequences of the specimens obtained from different hosts and/or geographical areas did not exhibit any intraspecific variability. Morphological study of the material in collections revealed characters which enable the species of Plagiorchis from bats in Europe to be distinguished. Taking into account that the type-material of P. vespertilionis, described during the 18th Century, has been lost, for nomenclatural stability, a neotype is established and described. An amended description of the type-material of P. muelleri and keys for the determination of Plagiorchis spp. from European bats are presented.

Phylogenetic and biogeographical relationships among some holarctic frog lung flukes (Digenea: Haematoloechidae)

A phylogenetic study of 8 North American and European species of frog lung flukes belonging to Haematoloechus was conducted using approximately 850 to 1,000 bases of the internal transcribed spacer region (ITS 1 + 5.8S + ITS 2) and 1,250 bases of the large subunit (LSU) of the nuclear ribosomal DNA. Adequate phylogenetic resolution could not be obtained from 5.8S or ITS 2 data. Analysis of ITS 1 data produced 2 equally parsimonious trees that differed only in the position of Haematoloechus breviplexus relative to H. medioplexus and H. varioplexus. Single, identical trees were produced by analysis of both LSU sequence data and a data set comprised of all ITS and LSU data. All trees demonstrated 3 distinct evolutionary lineages within the Holarctic Haematoloechus examined. The results confirmed the taxonomic validity of H. abbreviatus and demonstrated that the presence or absence of extracecal uterine loops is not a character meaningful to the recognition of evolutionary lineages or differentiation of genera. Examination of ITS sequence data revealed almost no intraspecific variation within 5 species of Haematoloechus and demonstrated an approximately 150-base indel common to the North American H. longiplexus and the European H. asper. Two of 3 clades revealed by the phylogenetic analyses are comprised of both European and North American species, indicating that lineages of Haematoloechus arose before the breakup of Laurasia and radiated after Eurasia and North America split. Within each of 3 evolutionary lineages, members share similar patterns of arthropod host specificity distinct from patterns found in the other lineages. This suggests that second intermediate host specificity may be a trait that has been conserved through evolutionary time.

A Review of Polylekithum Arnold 1934 and Its Familial Affinities Using Morphological and Molecular Data, with description of Polylekithum catahoulensis sp. nov.

The type material of Polylekithum ictaluri, P. halli, and Maculifer chandleri was examined from the United States National Parasite Museum, and we determined that the material was conspecific, making P. halli and M. chandleri junior subjective synonyms of P. ictaluri. Polylekithum catahoulensis sp. nov. was described from material collected from catfishes at the Catahoula Wildlife Refuge, LaSalle Parish, Louisiana, USA, and compared with P. ictaluri collected from catfishes in Reelfoot Lake, Obion County, Tennessee, USA, and the Pearl River, Hancock County, Mississippi, USA. Polylekithum catahoulensis had smaller eggs (77–88 μm long by 51–63 μm wide vs. 94–108 μm by 52–76 μm) and a longer forebody (35–41% of overall body length vs. 29–34%). Comparison of more than 2,400 bp long fragments of nuclear ribosomal DNA (complete ITS and partial 28S regions) strongly supported the status of P. catahoulensis as a new species. Molecular phylogenetic analysis of 28S rDNA gene sequences from Polylekithum as well as representative species from Allocreadiidae, Atractotrematidae, Brachycoeliidae, Callodistomidae, Dicrocoeliidae, Encyclometridae, Gorgoderidae, Haploporidae, Opecoelidae, Plagiorchiidae, and Telorchiidae rooted by Monorchiidae and Lissorchiidae demonstrated that of the families tested, Polylekithum was most closely related to Encyclometridae as a gorgoderoid and not to Allocreadiidae as previously reported. Morphological features of three South American allocreadiids, Allocreadium patagonicum, P. percai, and A. pichi were inconsistent with generic diagnoses of Allocreadium and Polylekithum, so we suggested they belonged in a single unnamed genus similar to Creptotrema. Polylekithum catlai from India was assessed from the description and failed to conform to the generic diagnosis of Polylekithum. Morphology of Caudouterina suggested a close relationship with Polylekithum and not Allocreadiidae.

The Nematode Genus Rhabdias (Nematoda: Rhabdiasidae) from Amphibians and Reptiles of the Nearctic

Abstract Data on the representatives of the nematode family Rhabdiasidae in the Nearctic are summarized from the literature, study of new material, and examination of museum specimens. A description of Rhabdias tarichae sp. nov. and redescriptions of Rhabdias joaquinensis, Rhabdias ambystomae, Rhabdias ranae, and Rhabdias americanus are given. New host and geographical records are provided for several Rhabdias species. Rhabdias sp., probably representing another species new to science, was found among museum specimens from the California newt Taricha torosa; however, the poor state of the specimens did not permit description. Keys for determination of Nearctic Rhabdias species are provided, and the applicability of some morphological characters for differentiation of Rhabdias species is discussed.

Two new species of Rhabdias (Nematoda: Rhabdiasidae) from the marine toad, Bufo marinus (L.) (Lissamphibia: Anura: Bufonidae), in Central America.

Two new Rhabdias species are described from the lungs of the cane toad Bufo marinus (L.) from Costa Rica and Nicaragua. Rhabdias alabialis n. sp. differs from other known species of the genus by the remarkable morphology of its head end, i.e., the absence of lips or pseudolabia, the slitlike oral opening, and the triangular shape of the buccal capsule in apical view. Rhabdias pseudosphaerocephala n. sp. is identified as a form previously known in Central and South America as Rhabdias sphaerocephala Goodey, 1924, a species initially described from toads in Europe. The new species is differentiated from R. sphaerocephala based on head-end morphology and sequences of nuclear rDNA.

A NEW SPECIES OF RHABDIAS FROM LUNGS OF THE WOOD FROG, RANA SYLVATICA, IN NORTH AMERICA: THE LAST SIBLING OF RHABDIAS RANAE ?

Rhabdias bakeri n. sp. is described from specimens found in lungs of the wood frog, Rana sylvatica, from North Dakota. The new species has previously been mistakenly identified as Rhabdias ranae Walton, 1929, a common parasite of the leopard frog, Rana pipiens. The new species differs from R. ranae and Rhabdias joaquinensis Ingles, 1935 by the shape and size of pseudolabia, shape and size of buccal capsule, and wider esophageal bulb. Molecular analysis based on the partial sequences of nuclear 18S rDNA gene, complete sequences of internal transcribed spacer region, and partial sequences of 28S gene demonstrates clear differences between Rhabdias from Ra. sylvatica and Ra. pipiens, and supports the status of R. bakeri as a new species.