A common origin of complex life cycles in parasitic flatworms: evidence from the complete mitochondrial genome of <Emphasis Type="Italic">Microcotyle sebastis</Emphasis> (Monogenea: Platyhelminthes)

Background  

The parasitic Platyhelminthes (Neodermata) contains three parasitic groups of flatworms, each having a unique morphology, and life style: Monogenea (primarily ectoparasitic), Trematoda (endoparasitic flukes), and Cestoda (endoparasitic tapeworms). The evolutionary origin of complex life cyles (multiple obligate hosts, as found in Trematoda and Cestoda) and of endo-/ecto-parasitism in these groups is still under debate and these questions can be resolved, only if the phylogenetic position of the Monogenea within the Neodermata clade is correctly estimated.     

Closing the mitochondrial circle on paraphyly of the Monogenea (Platyhelminthes) infers evolution in the diet of parasitic flatworms

Relationships between the three classes of Neodermata (parasitic Platyhelminthes) are much debated and restrict our understanding of the evolution of parasitism and contingent adaptations. The historic view of a sister relationship between Cestoda and Monogenea (Cercomeromorphae; larvae bearing posterior hooks) has been dismissed and the weight of evidence against monogenean monophyly has mounted. We present the nucleotide sequence of the complete mitochondrial (mt) genome of Benedenia seriolae (Monogenea: Monopisthocotylea: Capsalidae), the first complete non-gyrodactylid monopisthocotylean mt genome to be reported. We also include nucleotide sequence data for some mt protein coding genes for a second capsalid, Neobenedenia sp. Analyses of the new mt genomes with all available platyhelminth mt genomes provide new phylogenetic hypotheses, which strongly influence perspectives on the evolution of diet in the Neodermata. Our analyses do not support monogenean monophyly but confirm that the Digenea and Cestoda are each monophyletic and sister groups. Epithelial feeding monopisthocotyleans on fish hosts are basal in the Neodermata and represent the first shift to parasitism from free-living ancestors. The next evolutionary step in parasitism was a dietary change from epithelium to blood. The common ancestor of Digenea + Cestoda was monogenean-like and most likely sanguinivorous. From this ancestral condition, adult digeneans and cestodes independently evolved dietary specialisations to suit their diverse microhabitats in their final vertebrate hosts. These improved perspectives on relationships fundamentally enhance our understanding of the evolution of parasitism in the Neodermata and in particular, the evolution of diet.     

Ecological approach to the problem of monophyly of Neodermata (Platyhelminthes)

The ecological scenario of the evolution of main branches of Neodermata is described. The first neodermateans (= promonogeneans) were parasites of the gill lamellae of Paleozoic jawless vertebrates, which were microphagous suspension-feeding animals. The main apomorphic characters of the primary neodermateans are neodermis, cercomer (posterior hooked attachment organ) and swimming infective larva. All subsequent evolution of Neodermata was related with their acquisition new niches in hosts, which were intensively diverging in that time adapting to new food types and conquering new ecological niches. The acquisition of new microhabitats was accompanied by the development of morphological diversity in Neodermata especially in a structure of attachment and genital organs. Trematoda, Cestoda and Polyopisthocotylea comprise specialized evolutionary lineages and Monopisthocotylea is a basal taxon. Polyopisthocotylea is specialized to the blood feeding on fish gills. The common ancestors of the Trematoda and Cestoda inhabited walls of gills and pharyngeal cavities, from where they penetrated the digestive tract. The aspidogastridean multiloculate hold fast appears to be a highly specialized attachment organ of the monogenean ancestor, which inhabited muscular pharyngeal walls of Paleozoic vertebrates. The loss of cercomer hooks probably took place when mollusk-hosts have been involved in the aspidogastridean life cycle. The extinction of many chondrichthiean groups and progress of small plankton fishes (Teleostei) has led to the appearance Digenea. New vertebrate hosts needed a new infestation type and the cercaria appeared. Parthenogenesis has been developed in stages living in mollusks to counterbalance the loss of individuals at two transmission stages in the digenean cycle; this was resulted in a strong specificity to mollusk-host. Evolutionary tendencies of Trematoda and Cestoda show noticeable similarities.     

Vitellogenesis of basal trematode Aspidogaster limacoides(Aspidogastrea: Aspidogastridae)

The vitellogenesis of the trematode Aspidogaster limacoides (Aspidogastrea: Aspidogastridae), a parasite of cyprinid fishes, is described here using transmission electron microscopy. Four different stages of vitellocytes are differentiated: immature vitellocytes, early maturing vitellocytes, advanced maturing vitellocytes and mature vitellocytes. The process follows the same general pattern already described in other free-living neoophorans and parasitic flatworms (i.e. Trematoda, Monogenea and Cestoda): differentiation into mature vitelline cells involves the development of mitochondria, granular endoplasmic reticulum, Golgi complexes, lipid droplets and shell-globules. Mature vitellocytes of A. limacoides are composed of numerous shell-globule clusters, few lipid droplets and glycogen granules. They differ from those of another aspidogastrean Rugogaster hydrolagi in that they possess numerous globules tightly packed and by the presence of only one type of vitelline material. The interstitial tissue of vitelline follicles of A. limacoides contains a peripheral nucleus and long cytoplasmic projections extending between vitelline cells. Since aspidogastreans are considered as an archaic group of parasitic flatworms and thus have a strategic phylogenetic position, future works needs to pay special attention to the ultrastructural and chemical composition of mature vitellocytes within this basal group of trematodes.     

First evidence of the presence of microtriches in the Gyrocotylidea

The Gyrocotylidea, a small and enigmatic group of intestinal parasites of chimaeras, has been considered to be related either to the Monogenea, or, more frequently, to the most primitive monozoic tapeworms (Cestoda), i.e., the Amphilinidea and Caryophyllidea. The present study, based on transmission electron microscopical observations of a species of Gyrocotyle from the rabbit fish, Chimaera monstrosa, in the North Atlantic, demonstrates for the first time the presence of microtriches as surface structures of gyrocotylideans. Because microtriches are considered to be an autapomorphy of tapeworms (Cestoda), in which they differ from other Neodermata (Monogenea and Trematoda), the present data represent another source of evidence in support of a close relationship between the gyrocotylideans and the tapeworms sensu stricto (Eucestoda). Simple morphology, small size, and shape uniformity of the microtriches of Gyrocotyle sp. may indicate they represent an original (plesiomorphic) form that then evolved in more derived cestode groups into a variety of types present mainly on the scolex. The microtriches of Gyrocotyle sp. resemble those found in caryophyllidean, spathebothriidean, pseudophyllidean, and trypanorhynch cestodes, which are considered to represent the most basal groups of the Eucestoda.     

Homoplasy or plesiomorphy? Reconstruction of the evolutionary history of mitochondrial gene order rearrangements in the subphylum Neodermata

Recent mitogenomic studies have exposed a gene order (GO) shared by two classes, four orders and 31 species (‘common GO’) within the flatworm subphylum Neodermata. There are two possible hypotheses for this phenomenon: convergent evolution (homoplasy) or shared ancestry (plesiomorphy). To test those, we conducted a meta-analysis on all available mitogenomes to infer the evolutionary history of GO in Neodermata. To improve the resolution, we added a newly sequenced mitogenome that exhibited the common GO, Euryhaliotrema johni (Ancyrocephalinae), to the dataset. Phylogenetic analyses conducted on two datasets (nucleotides of all 36 genes and amino acid sequences of 12 protein coding genes) and four algorithms (MrBayes, RAxML, IQ-TREE and PhyloBayes) produced topology instability towards the tips, so ancestral GO reconstructions were conducted using TreeREx and MLGO programs using all eight obtained topologies, plus three unique topologies from previous studies. The results consistently supported the second hypothesis, resolving the common GO as a plesiomorphic ancestral GO for Neodermata, Cestoda, Monopisthocotylea, Cestoda + Trematoda and Cestoda + Trematoda + Monopisthocotylea. This allowed us to trace the evolutionary GO scenarios from each common ancestor to its descendants amongst the Monogenea and Cestoda classes, and propose that the common GO was most likely retained throughout all of the common ancestors, leading to the extant species possessing the common GO. Neodermatan phylogeny inferred from GOs was largely incongruent with all 11 topologies described above, but it did support the mitogenomic dataset in resolving Polyopisthocotylea as the earliest neodermatan branch. Although highly derived GOs might be of some use in resolving isolated taxonomic and phylogenetic uncertainties, we conclude that, due to the discontinuous nature of their evolution, they tend to produce artefactual phylogenetic relationships, which makes them unsuitable for phylogenetic reconstruction in Neodermata. Wider and denser sampling of neodermatan mitogenomic sequences will be needed to infer the evolutionary pathways leading to the observed diversity of GOs with confidence.     

Utility of complete large and small subunit rRNA genes in resolving the phylogeny of the Neodermata (Platyhelminthes): implications and a review of the cercomer theory

We combined nearly complete sequences of large (LSU) and small (SSU) subunit rDNA from 32 flatworm species to estimate the phylogeny of the Platyhelminthes using maximum parsimony, maximum likelihood and Bayesian inference methods. Rooted against the Catenulida, combined evidence trees offered no support for the Revertospermata, which was also rejected by constraint analysis. Generally, nodal support was higher for groupings estimated from the combined data partitions and all methods of analysis provided congruent estimates of phylogeny. The Monogenea and Proseriata were resolved as monophyletic, rejecting previous suggestions of paraphyly based on SSU and partial LSU data sets and thus supporting widely accepted morphological synapomorphies. Monophyly of the Neodermata was supported and its sister group was a clade of neoophoran 'turbellarians' to the exclusion of the Proseriata which in turn was more basal. Taxa with similar spermatology to the Neodermata ( Ichthyophaga , Notentera , Urastoma and Kronborgia ) were the sister group to Tricladida + Prolecithophora, which in turn were sister to the Rhabdocoela. Polycladida + Macrostomida + Lecithoepitheliata was the earliest divergent offshoot of the Rhabditophora. Among the Neodermata, the Cercomeromorphae (Cestoda + Monogenea) was not supported, whereas Cestoda + Trematoda was well supported. Although there is no known synapomorphy for this latter grouping, our data highlight problems associated with the 'cercomer theory' and we reject putative homologies regarding neodermatan 'cercomers' that have been sustained in the literature without careful scrutiny.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78, 155–171.     

How to catch a parasite: Parasite Niche Modeler (PaNic) meets Fishbase

Parasite Niche Modeler (PaNic) is a free online software tool that suggests potential hosts for fish parasites. For a particular parasite species from the major helminth groups (Acanthocephala, Cestoda, Monogenea, Nematoda, Trematoda), PaNic takes data from known hosts (maximum body length, growth rate, life span, age at first maturity, trophic level, phylogeny, and biogeography) and hypothesizes similar fish species that might serve as hosts to that parasite. Users can give varying weights to host attributes and create custom models. In addition to suggesting plausible hosts (with varying degrees of confidence), the models indicate known host species that appear to be outliers in comparison to other known hosts. These unique features make PaNic an innovative tool for addressing both theoretical and applied questions in fish parasitology. PaNic can be accessed at < http://purl.oclc.org/fishpest >.     

Ultrastructure of the protonephridial system, of Anoplodiscus cirrusspiralis (Monogenea Monopisthocotylea).

The flame bulb is formed by a terminal cell and a proximal canal cell. The weir consists of interdigitating ribs all of which form one circle, i.e. alternating ribs do not have distinctly 'internal' or 'external' positions. Cytoplasmic cords are absent and all ribs, i.e. those continuous with the proximal canal cell and those continuous with the terminal cell, form external leptotriches. At least some external leptotriches have interconnected branches extending along the flame bulb. Internal leptotriches are not branched and arise from the basal perikaryon of the terminal cell. In the cytoplasmic cylinder at the tip of the flame bulb, structures resembling incomplete septate junctions were seen. However, neither the cytoplasmic cylinder nor the small protonephridial capillaries contain complete septate junctions as found in all other Monogenea Polyopisthocotylea, Monogenea Monopisthocotylea, Trematoda Aspidogastrea and Trematoda Digenea examined to date. In the lack of a septate junction, Anoplodiscus resembles Udonella, Amphilinidea, Gyrocotylidea and Eucestoda. However, the presence in this species of rudimentary septate junctions in the small capillaries and of complete junctions in larger ones indicates that complete junctions have been secondarily lost. Anoplodiscus resembles the Monogenea and Trematoda in the presence of lamellae in the larger protonephridial ducts. For the first time in a monogenean, the ultrastructure of the excretory bladder is described. A nucleated convoluted duct opens through a narrow connecting duct into the bladder, which in turn opens through a narrow connecting duct into the excretory pore lined by tegument. Convoluted duct, connecting ducts and bladder are lined by a lamellated epitheliu.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hox genes from the Polystomatidae (Platyhelminthes, Monogenea)

Hox genes form a multigenic family that play a fundamental role during the early stages of development. They are organised in a single cluster and share a 60 amino acid conserved sequence that corresponds to the DNA binding domain, i.e. the homeodomain. Sequence conservation in this region has allowed investigators to explore Hox diversity in the metazoan lineages. Within parasitic flatworms only homeobox sequences of parasite species from the Cestoda and Digenea have been reported. In the present study we surveyed species of the Polyopisthocotylea (Monogenea) in order to clarify Hox identification and diversification processes in the neodermatan lineage. From cloning of degenerative PCR products of the central region of the homeobox, we report one ParaHox and 25 new Hox sequences from 10 species of the Polystomatidae and one species of the Diclidophoridae, which extend Hox gene diversity from 46 to 72 within Neodermata. Hox sequences from the Polyopisthocotylea were annotated and classified from sequence alignments and Bayesian inferences of 178 Hox, ParaHox and related gene families recovered from all available parasitic platyhelminths and other bilaterian taxa. Our results are discussed in the light of the recent Hox evolutionary schemes. They may provide new perspectives to study the transition from turbellarians to parasitic flatworms with complex life-cycles and outline the first steps for evolutionary developmental biological approaches within platyhelminth parasites.     

Phylogenetic analysis of the Rhabdocoela (Platyhelminthes) with emphasis on the Neodermata and relatives

Phylogenetic systematic analysis of 24 taxa representing the rhabdocoel platyhelminths, based on a suite of 89 morphological characters, produced two equally parsimonious trees, 181 steps long, with a consistency index (CI) of 0.69 and a rescaled consistency index (RCI) of 0.56, differing only with respect to that portion of the tree containing Umagillidae, Acholadidae, Graffillinae, Pseudograffillinae, Pterastericolidae and Hypoblepharinidae. Our results accommodate all previously proposed sister taxa to the Neodermata in a single clade in which ((Dalyelliidae + Temnocephalida) Typhloplanidae) is the sister group of ((Fecampiidae +  Urastoma ) ( Udonella ((Aspidogastrea + Digenea) (Monogenea (Gyrocotylidea (Amphilinidea + Eucestoda)))))). Bootstrap and jackknife analyses indicate that the groupings of ((Dalyelliidae + Temnocephalida) Typhloplanidae) and of ((Fecampiidae +  Urastoma ) ( Udonella ((Aspidogastrea + Digenea) (Monogenea (Gyrocotylidea (Amphilinidea + Eucestoda)))))) are highly robust, with the latter clade having a CI of 90% and RCI of 82%. Disagreements among previous analyses of these taxa have been due to the influence of missing data for critical characters in key taxa and differences in the taxa analysed, rather than any inherent weakness in the morphological data. Non-phylogenetic systematic approaches to homology assessment and misconceptions regarding phylogenetic systematic methodology are discussed. Recent analyses combining sequence data with a subset of approximately 60% of the morphological characters should be re-assessed using the entire morphological database. Even if Udonella is a monogenean, it is most parsimonious to suggest that the common ancestor of the Neodermata had a vertebrate–arthropod two-host life cycle.     

A checklist of parasites recorded on sticklebacks (Actinopterygii: Gasterosteidae) from Poland

Parasites of the three-spined stickleback, Gasterosteus aculeatus (L.), and the nine-spined stickleback, Pungitius pungitius (L.), from brackish and freshwater habitats in Poland are listed. The following list was compiled from published records and own long-term studies. Parasites are listed alphabetically in meaningful groups of parasites with notes about their location on the host and distribution in Poland with references. A total of 51 species (taxa) of parasites were recorded on G. aculeatus (3 Apicomplexa, 1 Microsporidia, 3 Peritrichia, 1 Myxozoa, 7 Digenea, 2 Monogenea, 10 Cestoda, 10 Nematoda, 7 Acanthocephala, 3 Copepoda, 1 Branchiura, 2 Hirudinea, 1 Bivalvia) and 16 parasites on P. pungitius (1 Apicomplexa, 1 Microsporidia, 2 Peritrichia, 4 Digenea, 1 Monogenea, 2 Cestoda, 2 Nematoda, 1 Copepoda, 1 Branchiura, 1 Bivalvia).     

A fossil record for trematodes: extent and potential uses

Some extant parasitic flatworms (Phylum Platyhelminthes, Class Trematoda, Order Digenea) produce pits on the interior shell surface of their molluscan bivalve hosts. Based on comparisons of pits in Tertiary and Quaternary Gemma, Parastarte and Transennella (Family Veneridae), we establish a history of association between trematodes and these host genera spanning over 5 My in North America. This is the first report of a fossil record for the Trematoda. Similar pits are also present in many other bivalve genera (both fossil and living species), indicating that a substantial fossil record may exist for parasitic flatworms. The identification of parasitic infections from fossil shells may provide a new dimension for the study of the evolutionary biology, paleoecology and biogeography of host-parasite associations. □ Trematoda, trace fossils, Veneridae, paleoecology. life-history evolution, parasites.     

A Checklist of Metazoan Parasites from Rainbow Trout (Oncorhynchus mykiss)

An extensive literature survey on metazoan parasites from rainbow trout Oncorhynchus mykiss has been conducted. The taxa Monogenea, Cestoda, Digenea, Nematoda, Acanthocephala, Crustacea and Hirudinea are covered. A total of 169 taxonomic entities are recorded in rainbow trout worldwide although few of these may prove synonyms in future analyses of the parasite specimens. These records include Monogenea (15), Cestoda (27), Digenea (37), Nematoda (39), Acanthocephala (23), Crustacea (17), Mollusca (6) and Hirudinea (5). The large number of parasites in this salmonid reflects its cosmopolitan distribution.     

Diversity in the Monogenea and Digenea: does lifestyle matter?

If the cestodes are excluded, then the parasitic platyhelminths of fishes divide neatly into the external and monoxenous Monogenea and the internal and heteroxenous Digenea. Both groups have apparently had long associations of coevolution, host switching and adaptation with fishes and have become highly successful in their respective habitats. Current estimates of species richness for the two groups suggest that they may be remarkably similar. Here we consider the nature of the diversity of the Monogenea and Digenea of fishes in terms of richness of species and higher taxa to determine what processes may be responsible for observed differences. The Monogenea includes at least two super-genera (Dactylogyrus and Gyrodactylus) each of which has hundreds of species; no comparable genera are found in the Digenea. Possible reasons for this difference include the higher host specificity of monogeneans and their shorter generation time. If allowance is made for the vagaries of taxonomic 'lumping' and 'splitting', then there are probably comparable numbers of families of monogeneans and digeneans in fishes. However, the nature of the families differ profoundly. Richness in higher taxa (families) in the Digenea is explicable in terms of processes that appear to have been unimportant in the Monogenea. Readily identifiable sources of diversity in the Digenea are: recolonisation of fishes by taxa that arose in association with tetrapods; adoption of new sites within hosts; adoption of new diets and feeding mechanisms; adaptations relating to the exploitation of ecologically similar groups of fishes and second intermediate hosts; and adaptations relating to the exploitation of phylogenetic lineages of molluscs. In contrast, most higher- level monogenean diversity (other than that associated with the subclasses) relates principally to morphological specialisation for attachment by the haptor.     

Plagioporus sinitsini (Digenea : Opecoelidae): a one-host life cycle

Adult Plagioporus sinitsini occur within daughter sporocysts voided with the feces of prosobranch snails Elimia symmetrica in Basin Creek, North Carolina. These worms produced eggs containing active miracidia while still in the snail. Adults in snails and adults in rosyside dace, Clinostomus funduloides, collected from the same stream were indistinguishable morphometrically. Adults in snails develop from cotylocercous cercariae sequestered in daughter sporocysts that pass through the metacercaria stage. These observations, and previous study in Michigan, suggest that the life cycle of P. sinitsini has 3 potential pathways, i.e., a 3-host life cycle involving molluscan, arthropod, and piscine hosts, a 2-host life cycle involving only molluscan and piscine hosts, and a 1-host life cycle involving only the snail host. The truncated life cycles do not appear to be the result of paedomorphosis.     

The evolutionary expansion and host-parasite relationships of the Digenea

Relevant data on the Digenea extracted from a host-parasite data-base are analysed in relation to host-groups, host-specificity, speciation, radiation and geographical distribution. The classification, evolution, co-evolution, and co-speciation of the group are discussed. Principal components analyses indicated that 119 families formed 11 groups in relation to their vertebrate hosts and the 55 families with molluscan records formed 6 groups in relation to their molluscan hosts. The most prominent host-groups are the Fish and Mammals. Individual digenean families did not exhibit the host combinations Fish + Birds, Fish + Mammals, Herpetiles + Birds and Herpetiles + Mammals. Families with Fish hosts tended to use Prosobranch and, to a lesser extent Bivalve, molluscs, whereas families in Herpetiles, Birds and Mammals tended to use Pulmonates. Families using 3 or 4 mixed vertebrate groups tended to use mixed molluscan groups. Families using Herpetiles as the vertebrate host tend to be the most host-specific and the least speciose, whereas those using 3 to 4 mixed vertebrate groups are the most speciose. In a detailed examination of three zoogonid genera, few indications of co-evolution with their vertebrate hosts were detected, and geographical information from the data-base appeared to shed no light upon the geographical origins of the Digenea. Some of these findings are commented upon in relation to the evolution of the Digenea.     

Parasites of fishes from Laurel Creek, Ontario

Two-hundred and nine fish of 13 species from Laurel Creek, Ontario, were examined for parasites between May and October 1973. Eighty-four species of parasites (22 of Protozoa, 24 of Monogenea, 17 of Digenea, 11 of Cestoda, 4 of Nematoda, 1 of Hirudinea, 1 of glochidia, and 4 of Crustacea) were collected and are listed and discussed.     

Ecological-faunistical review of fish parasites in the Umbozero Lake (the Kola Peninsula)

Data on fish infestation by parasites in the Umbozero Lake are given. 90 species of parasites were identified including 16 species of Myxosporea, 2 Suctoria, 18 Peritricha, 12 Monogenea, 13 Cestoda, 18 Trematoda, 5 Nematoda, 2 Acanthocephala, 2 Hirudinea, and 2 species of Crustacea. Character of parasites' distribution in fishes of this lake and ecological peculiarities in the host-parasite system are demonstrated.     

Host-parasite relationships of longnose dace, Rhinichthys cataractae, from the Ford River, Michigan.

A total of 1,115 longnose dace, Rhinichthys cataractae (family Cyprinidae), were examined for parasites from May 1983 through October 1986 from 3 localities in the Ford River in Michigan's Upper Peninsula. Thirteen parasite species (1 Monogenea, 2 Digenea, 2 Cestoda, 4 Nematoda, 1 Acanthocephala, 3 Protozoa) infected dace. The parasite faunas of dace, taxonomically and in species number, were similar between localities. Posthodiplostomum minimum minimum, Neascus sp., and Rhabdochona canadensis were the most common helminths infecting dace from each locality. The first 2 species did not exhibit consistent seasonal infection patterns between years, whereas the prevalence and mean intensity of R. canadensis in dace from the downriver locality were higher in summer 1983, 1984, and 1985. The intensity of infection of each of these helminth species significantly increased with host length. The prevalences and mean intensities of P. m. minimum, Neascus sp., and R. canadensis as well as the helminth infracommunity diversity were highest in dace from the upriver locality. The major factors that influenced parasite intensity were environmental factors that occurred when and where a fish began its life, the sequence of events that occurred in each habitat the fish encountered during its life, and the length of exposure (age of fish). Dace have isolationist helminth infracommunities arising from factors including ectothermy, a simple enteric system, restricted vagility, and being gape-limited. Allogenic helminths with indirect life cycles predominate in the depauperate helminth fauna of dace.