Functional morphology of parasitic isopods: understanding morphological adaptations of attachment and feeding structures in Nerocila as a pre-requisite for reconstructing the evolution of Cymothoidae

Parasites significantly influence food webs and ecosystems and occur all over the world in almost every animal group. Within crustaceans there are numerous examples of ectoparasites; for example, representatives of the isopod group Cymothoidae. These obligatory parasitic isopods are relatively poorly studied regarding their functional morphology. Here we present new details of the morphological adaptations to parasitism of the cymothoiid ingroup Nerocila with up-to-date imaging methods (macro photography, stereo imaging, fluorescence photography, micro CT, and histology). Central aspects of the study were (1) the morphology of the mouthparts and (2) the attachment on the host, hence the morphology of the thoracopods. The mouthparts (labrum, mandibles, paragnaths, maxillulae, maxillae, maxillipeds) form a distinct mouth cone and are most likely used for true sucking. The mouthparts are tightly “folded” around each other and provide functional rails for the only two moving mouthparts, mandible and maxillula. Both are not moving in an ancestral-type median-lateral movement, but are strongly tilted to move more in a proximal-distal axis. New details concerning the attachment demonstrate that the angular arrangement of the thoracopods is differentiated to impede removal by the host. The increased understanding of morphological adaptation to parasitism of modern forms will be useful in identifying disarticulated (not attached to the host) fossil parasites.

Nervous system immunohistochemistry of the parasitic cnidarian Polypodium hydriforme at its free-living stage

Polypodium hydriforme, the only species in Polypodiozoa, which is currently considered a class of Cnidaria, and likely a sister group to Medusozoa (together with Myxozoa), is a cnidarian adapted to intracellular parasitism inside sturgeon oocytes. Free-living P. hydriforme lives on river bottoms; it walks on supporting tentacles and uses sensory tentacles to capture food and bring it to the mouth. The nervous system of free-living P. hydriforme was studied by confocal microscopy and immunohistochemistry using antibodies to FMRF-amide and α-tubulin combined with phalloidin-staining of F-actin fibres. A sensory FMRF-amide immunoreactive (IR) nerve net and an α-tubulin IR nerve net have been identified. The FMRF-amide IR nerve net underlies the epidermis along the tentacles and around the mouth; it consists of neurites emanating from epidermal sensory cells and basiepidermal ganglion cells, and it connects with cnidocytes. A deeper-lying α-tubulin IR nerve net occurs only in tentacles and looks like chains of different-sized beads crossing the mesoglea and entwining muscles. Anti-α-tubulin staining also reveals microtubules in muscle cells following the longitudinal muscle fibres or the thin circular F-actin fibres of the tentacles. Cnidocytes in the tentacles are embedded in a regular hexagonal non-neural network formed by the tubulin IR cytoskeleton of epidermal cells. Cnidocils of the cnidocytes around the mouth and in walking tentacles are identical, but those in sensory tentacles differ in length and width. The possible homology of the tubulin IR nerve net with motor nerve nets of cnidarians is discussed. The absence of a classic nerve ring around the mouth and the lack of specialised sense organs are considered to be plesiomorphic characters for Cnidaria.

Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent

Links between parasites and food webs are evolutionarily ancient but dynamic: life history theory provides insights into helminth complex life cycle origins. Most adult helminths benefit by sexual reproduction in vertebrates, often high up food chains, but direct infection is commonly constrained by a trophic vacuum between free-living propagules and definitive hosts. Intermediate hosts fill this vacuum, facilitating transmission to definitive hosts. The central question concerns why sexual reproduction, and sometimes even larval growth, is suppressed in intermediate hosts, favouring growth arrest at larval maturity in intermediate hosts and reproductive suppression until transmission to definitive hosts? Increased longevity and higher growth in definitive hosts can generate selection for larger parasite body size and higher fecundity at sexual maturity. Life cycle length is increased by two evolutionary mechanisms, upward and downward incorporation, allowing simple (one-host) cycles to become complex (multihost). In downward incorporation, an intermediate host is added below the definitive host: models suggest that downward incorporation probably evolves only after ecological or evolutionary perturbations create a trophic vacuum. In upward incorporation, a new definitive host is added above the original definitive host, which subsequently becomes an intermediate host, again maintained by the trophic vacuum: theory suggests that this is plausible even under constant ecological/evolutionary conditions. The final cycle is similar irrespective of its origin (upward or downward). Insights about host incorporation are best gained by linking comparative phylogenetic analyses (describing evolutionary history) with evolutionary models (examining selective forces). Ascent of host trophic levels and evolution of optimal host taxa ranges are discussed.

The Importance of Fossils in Understanding the Evolution of Parasites and Their Vectors

Knowledge concerning the diversity of parasitism and its reach across our current understanding of the tree of life has benefitted considerably from novel molecular phylogenetic methods. However, the timing of events and the resolution of the nature of the intimate relationships between parasites and their hosts in deep time remain problematic. Despite its vagaries, the fossil record provides the only direct evidence of parasites and parasitism in the fossil record of extant and extinct lineages. Here, we demonstrate the potential of the fossil record and other lines of geological evidence to calibrate the origin and evolution of parasitism by combining different kinds of dating evidence with novel molecular clock methodologies. Other novel methods promise to provide additional evidence for the presence or the life habit of pathogens and their vectors, including the discovery and analysis of ancient DNA and other biomolecules, as well as computed tomographic methods.

Transcriptome data reveal Syndermatan relationships and suggest the evolution of endoparasitism in Acanthocephala via an epizoic stage

The taxon Syndermata comprises the biologically interesting wheel animals ("Rotifera": Bdelloidea + Monogononta + Seisonidea) and thorny-headed worms (Acanthocephala), and is central for testing superordinate phylogenetic hypotheses (Platyzoa, Gnathifera) in the metazoan tree of life. Recent analyses of syndermatan phylogeny suggested paraphyly of Eurotatoria (free-living bdelloids and monogononts) with respect to endoparasitic acanthocephalans. Data of epizoic seisonids, however, were absent, which may have affected the branching order within the syndermatan clade. Moreover, the position of Seisonidea within Syndermata should help in understanding the evolution of acanthocephalan endoparasitism. Here, we report the first phylogenomic analysis that includes all four higher-ranked groups of Syndermata. The analyzed data sets comprise new transcriptome data for Seison spec. (Seisonidea), Brachionus manjavacas (Monogononta), Adineta vaga (Bdelloidea), and Paratenuisentis ambiguus (Acanthocephala). Maximum likelihood and Bayesian trees for a total of 19 metazoan species were reconstructed from up to 410 functionally diverse proteins. The results unanimously place Monogononta basally within Syndermata, and Bdelloidea appear as the sister group to a clade comprising epizoic Seisonidea and endoparasitic Acanthocephala. Our results support monophyly of Syndermata, Hemirotifera (Bdelloidea + Seisonidea + Acanthocephala), and Pararotatoria (Seisonidea + Acanthocephala), rejecting monophyly of traditional Rotifera and Eurotatoria. This serves as an indication that early acanthocephalans lived epizoically or as ectoparasites on arthropods, before their complex lifecycle with arthropod intermediate and vertebrate definite hosts evolved.

Classification of the acanthocephala

In 1985, Amin presented a new system for the classification of the Acanthocephala in Crompton and Nickol's (1985) book 'Biology of the Acanthocephala' and recognized the concepts of Meyer (1931, 1932, 1933) and Van Cleave (1936, 1941, 1947, 1948, 1949, 1951, 1952). This system became the standard for the taxonomy of this group and remains so to date. Many changes have taken place and many new genera and species, as well as higher taxa, have been described since. An updated version of the 1985 scheme incorporating new concepts in molecular taxonomy, gene sequencing and phylogenetic studies is presented. The hierarchy has undergone a total face lift with Amin's (1987) addition of a new class, Polyacanthocephala (and a new order and family) to remove inconsistencies in the class Palaeacanthocephala. Amin and Ha (2008) added a third order (and a new family) to the Palaeacanthocephala, Heteramorphida, which combines features from the palaeacanthocephalan families Polymorphidae and Heteracanthocephalidae. Other families and subfamilies have been added but some have been eliminated, e.g. the three subfamilies of Arythmacanthidae: Arhythmacanthinae Yamaguti, 1935; Neoacanthocephaloidinae Golvan, 1960; and Paracanthocephaloidinae Golvan, 1969. Amin (1985) listed 22 families, 122 genera and 903 species (4, 4 and 14 families; 13, 28 and 81 genera; 167, 167 and 569 species in Archiacanthocephala, Eoacanthocephala and Palaeacanthocephala, respectively). The number of taxa listed in the present treatment is 26 families (18% increase), 157 genera (29%), and 1298 species (44%) (4, 4 and 16; 18, 29 and 106; 189, 255 and 845, in the same order), which also includes 1 family, 1 genus and 4 species in the class Polyacanthocephala Amin, 1987, and 3 genera and 5 species in the fossil family Zhijinitidae.

Meeting report: Spontaneous lesions and diseases in wild, captive-bred, and zoo-housed nonhuman primates and in nonhuman primate species used in drug safety studies

The combination of loss of habitat, human population encroachment, and increased demand of select nonhuman primates for biomedical research has significantly affected populations. There remains a need for knowledge and expertise in understanding background findings as related to the age, source, strain, and disease status of nonhuman primates. In particular, for safety/biomedical studies, a broader understanding and documentation of lesions would help clarify background from drug-related findings. A workshop and a minisymposium on spontaneous lesions and diseases in nonhuman primates were sponsored by the concurrent Annual Meetings of the American College of Veterinary Pathologists and the American Society for Veterinary Clinical Pathology held December 3-4, 2011, in Nashville, Tennessee. The first session had presentations from Drs Lowenstine and Montali, pathologists with extensive experience in wild and zoo populations of nonhuman primates, which was followed by presentations of 20 unique case reports of rare or newly observed spontaneous lesions in nonhuman primates (see online files for access to digital whole-slide images corresponding to each case report at http://www.scanscope.com/ACVP%20Slide%20Seminars/2011/Primate%20Pathology/view.apml). The minisymposium was composed of 5 nonhuman-primate researchers (Drs Bradley, Cline, Sasseville, Miller, Hutto) who concentrated on background and spontaneous lesions in nonhuman primates used in drug safety studies. Cynomolgus and rhesus macaques were emphasized, with some material presented on common marmosets. Congenital, acquired, inflammatory, and neoplastic changes were highlighed with a focus on clinical, macroscopic, and histopathologic findings that could confound the interpretation of drug safety studies.

Deep sequencing of small RNAs confirms an annelid affinity of Myzostomida

Myzostomida comprise a group of marine worms associated mainly with echinoderms since the Carboniferous. Due to their unusual morphology the phylogenetic position in relation to other Lophotrochozoa is discussed since their description. According to different morphological and molecular markers the Myzostomida are either close to Platyzoa or Annelida. Here we investigated small non-coding RNAs of Myzostoma cirriferum to infer the phylogenetic position of myzostomids. Based on transcriptomic data collected by Illumina Deep Sequencing we analyzed the microRNA (miRNA) families occurring in M. cirriferum. Phylogenetic analysis revealed the presence of 13 miRNA-families exclusively shared by Annelida (including Sipuncula) and Myzostomida, as such highly significantly supporting an annelid origin of myzostomids. Furthermore, using a mapping-approach and secondary structure models we predicted several miRNA-candidates unique for myzostomids.

The nomenclature of the recent Pentastomida (Crustacea), with a list of species and available names

The taxonomy of the Recent members of the crustacean subclass Pentastomida is based on nine accepted family names derived from 12 available names, 24 generic names derived from 37 available names (plus two incorrect subsequent spellings and one nomen nudum) and 124 accepted species names derived from 183 available names of which six remain incertae sedis as to their generic assignment. Compilation of this list has revealed that existing catalogues have included misspellings, wrong attributions and dates of the authors of taxa, and incorrectly nominated type-species. These are corrected here with reference to the original descriptions and diagnoses. Notably, all families except one were erected much earlier and by authors other than Fain (1961), who was credited by Martin & Davis (2001) and other authors before and afterwards with seven of the nine families they recognised. Other significant taxonomic anomalies are revealed. Raillietiellidae Sambon, 1922 is a senior synonym of Cephalobaenidae Heymons, 1922, the name in popular usage for the family including Cephalobaena Heymons, 1922 and Raillietiella Sambon, in Vaney & Sambon, 1910; here the two genera are placed in separate families following Almeida & Christoffersen (1999). Heymonsia Hett, 1934, considered a junior synonym of Raillietiella, is a nomen nudum. Raillietiella geckonis (Diesing, 1850) is a potential senior synonym of several SE Asian species of this genus. Raillietiella frenata Ali, Riley & Self, 1981 is a widely used species name but is a subjective junior synonym of R. hebitihamata Self & Kuntz, 1960 according to its own authors. Morphological and molecular evidence suggest that R. indica Gedoelst, 1921 is a subjective senior synonym of both species. The priority of Linguatulidae Haldeman, 1851 over Linguatulida Vogt, 1851, erected as a family in the same year, is established by applying the First Reviser rule. Linguatula serrata Frölich, 1789 is herein selected as the type-species of Prionoderma, making it an objective synonym of Linguatula Frölich, 1789. The priority of L. serrata over Taenia rhinaris Meyer, 1789 and T.capraea Abildgaard, 1789, all published in the same year, is established by applying the First Reviser rule. The purported synonymy of Netrorhynchus Zenker, 1827, also misspelled Nettorhynchus, with Armillifer Sambon, 1922 would seem to be ill-founded and without popular support. Armillifer australis Heymons, 1935, published as a subspecies of A. moniliformis (Diesing, 1836), is both a senior synonym and a homonym of A. australis Riley & Self, 1981. Humboldt (1812) is confirmed as the author of Porocephalus Humboldt, 1812 and P. crotali Humboldt, 1812. Pentastomidae Shipley, 1909 is an older family name than its subjective synonym Porocephalinae Sambon, 1922, but prevailing usage allows the latter to be retained as a family name. Cayerina mirabilis Kishida, 1927 is a genus and species from a Japanese frog that has not appeared in the more recent pentastome literature. Sebekia minor (Wedl, 1861) is an objective senior synonym of the more widely used S. wedli Gigioli, in Sambon, 1922. The importance of the many junior synonyms will become evident should refined morphological and molecular evidence reveal cryptic species or greater host-specificity than presently recognised.

Massive visceral pentastomiasis in a long-tailed macaque - an incidental finding

BACKGROUND

An unusual case of visceral pentastomiasis in a male adult long-tailed macaque imported from China is reported.

METHODS

The monkey was part of a toxicologic study. A massive accumulation of C-shaped parasites in various visceral organs was found post-mortem.

RESULTS

Based on the morphology of the nymphs, pentastomiasis was diagnosed etiopathologically. The pentastome genus and species was identified as Armillifer agkistrodontis by PCR and respective sequencing.

CONCLUSION

Molecular diagnostic methods are necessary tools to determine the exact species involved.

Morphology, distribution, and evolution of apical structure of nematocysts in hexacorallia

Cnidae are complex intracellular capsules made by all cnidarians. The most diverse of these capsules are nematocysts, which are made by all members of the phylum; spirocysts and ptychocysts are made only by members of some lineages, and they show less functional and structural diversity. In nematocysts, the apex has been shown to be either a hinged cap (operculum) or three flaps that flex outward during discharge. The operculum is known only from medusozoan nematocysts; flaps are known only from nematocysts of members of the hexacorallian order Actiniaria, although they have been inferred to be characteristic of Anthozoa, the group to which Actiniaria belongs. Using scanning and transmission electron microscopy, we discover a third apical morphology in nematocysts, an apical cap, which we find in all nonactiniarian anthozoans examined. This apical cap is identical structurally to the apical cap of spirocysts, and it resembles the apical structure of ptychocysts, whose apex is documented here for the first time. Additionally, a full survey of nematocysts from all body structures of two actiniarians demonstrates that a particular type of nematocyst, the microbasic p-mastigophore of the mesenterial filaments, does not have apical flaps. The observed variation does not correspond to conventional categorization of capsule morphology and raises questions about the function and structure of capsules across Cnidaria. Despite some ambiguity in optimization of ancestral states across cnidae, we determine that the apical cap is the plesiomorphic structure for anthozoan cnidae and that apical flaps are a synapomorphy of Actiniaria. At present, the operculum is interpreted as a synapomorphy for Medusozoa, but either it or an apical cap is the ancestral state for nematocysts.

The many roads to parasitism: a tale of convergence

Parasitic organisms account for a large portion of living species. They have arisen on multiple independent occasions in many phyla, and thus encompass a huge biological diversity. This review uses several lines of evidence to argue that this vast diversity can be reduced to a few evolutionary end points that transcend phylogenetic boundaries. These represent peaks in the adaptive landscape reached independently by different lineages undergoing convergent evolution. Among eukaryotic parasites living in or on animals, six basic parasitic strategies are identified based on the number of hosts used per parasite generation, the fitness loss incurred by the host, and the transmission routes used by the parasites. They are parasitoids, parasitic castrators, directly transmitted parasites, trophically transmitted parasites, vector-transmitted parasites and micropredators. These show evidence of convergence in morphology, physiology, reproduction, life cycles and transmission patterns. Parasite-host body size ratios, and the relationship between virulence and intensity of infection, are also associated with the different parasitic strategies, but not consistently so. At the population level, patterns of parasite distribution among hosts are not uniform across all parasitic strategies, but are distinctly different for parasitoids and castrators than for other parasites. To demonstrate that the above six strategies defined for animal parasites are universal, comparisons are made with parasites of plants, in particular, plant-parasitic nematodes and parasitic angiosperms; these are shown to follow the same evolutionary trajectories seen among animal parasites, despite huge physiological and ecological differences between animals and plants. Beyond demonstrating the inevitable convergence of disparate lineages across biological hyperspace towards a limited set of adaptive strategies, this synthesis also provides a unifying framework for the study of parasitism.

Phylogenetic analysis of dicyemid mesozoans (phylum Dicyemida) from innexin amino acid sequences: dicyemids are not related to Platyhelminthes

Dicyemid mesozoans are endoparasites, or endosymbionts, found only in the renal sac of benthic cephalopod molluscs. The body organization of dicyemids is very simple, consisting of usually 10 to 40 cells, with neither body cavities nor differentiated organs. Dicyemids were considered as primitive animals, and the out-group of all metazoans, or as occupying a basal position of lophotrochozoans close to flatworms. We cloned cDNAs encoding for the gap junction component proteins, innexin, from the dicyemids. Its expression pattern was observed by whole-mount in situ hybridization. In adult individuals, the innexin was expressed in calottes, infusorigens, and infusoriform embryos. The unique temporal pattern was observed in the developing infusoriform embryos. Innexin amino acid sequences had taxon-specific indels which enabled identification of the 3 major protostome lineages, i.e., 2 ecdysozoans (arthropods and nematodes) and the lophotrochozoans. The dicyemids show typical, lophotrochozoan-type indels. In addition, the Bayesian and maximum likelihood trees based on the innexin amino acid sequences suggested dicyemids to be more closely related to the higher lophotrochozoans than to the flatworms. Flatworms were the sister group, or consistently basal, to the other lophotrochozoan clade that included dicyemids, annelids, molluscs, and brachiopods.

Multi-host model-based identification of Armillifer agkistrodontis (Pentastomida), a new zoonotic parasite from China

BACKGROUND

Pentastomiasis is a rare parasitic infection of humans. Pentastomids are dioecious obligate parasites requiring multiple hosts to complete their lifecycle. Despite their worm-like appearance, they are commonly placed into a separate sub-class of the subphylum Crustacea, phylum Arthropoda. However, their systematic position is not uncontested and historically, they have been considered as a separate phylum.

METHODOLOGY/PRINCIPAL FINDINGS

An appraisal of Armillifer agkistrodontis was performed in terms of morphology and genetic identification after its lifecycle had been established in a multi-host model, i.e., mice and rats as intermediate hosts, and snakes (Agkistrodon acutus and Python molurus) as definitive hosts. Different stages of the parasite, including eggs, larvae and adults, were isolated and examined morphologically using light and electron microscopes. Phylogenetic and cluster analysis were also undertaken, focusing on the 18S rRNA and the Cox1 gene. The time for lifecycle completion was about 14 months, including 4 months for the development of eggs to infectious larvae in the intermediate host and 10 months for infectious larvae to mature in the final host. The main morphological difference between A. armillatus and Linguatula serrata is the number of abdominal annuli. Based on the 18S rRNA sequence, the shortest hereditary distance was found between A. agkistrodontis and Raillietiella spp. The highest degree of homology in the Cox 1 nucleic acid sequences and predicted amino acid sequences was found between A. agkistrodontis and A. armillatus.

CONCLUSION

This is the first time that a multi-host model of the entire lifecycle of A. agkistrodontis has been established. Morphologic and genetic analyses supported the notion that pentastomids should be placed into the phylum Arthropoda.

Chave de identificação de Gastrotricha de águas continentais e marinhas do Brasil

Os Gastrotricha são microinvertebrados de vida livre, caracterizados por um estilo de vida meiobentônico em ambientes marinhos e de água doce. Apesar de este grupo ser abundante e ecologicamente importante nas teias alimentares, o seu conhecimento no Brasil é muito subestimado. Com este estudo, considerando a importância dos Gastrotricha nas comunidades bênticas intersticiais, esperamos ajudar e estimular novas pesquisas com este grupo no Brasil. Assim, esse trabalho fornece uma chave dicotômica para todas as espécies previamente reportadas em águas continentais brasileiras. Além disso, apresentamos informações sobre métodos de coleta, técnicas de preservação e de fotografias dos Gastrotricha. As terminologias, em português, das principais características morfológicas utilizadas para distinguir as espécies são apresentas na forma de um glossário.

Support for the monophyletic origin of Gnathifera from phylogenomics

The monophyletic origin of Spiralia within the metazoan tree of life is supported by many large-scale phylogenomic data. While there is now substantial molecular evidence for Lophotrochozoa being a monophyletic taxon within Spiralia, the phylogenetic affiliations of many other spiralian phyla remain unclear. Here we focus on the question of a monophyletic taxon Gnathifera, which was originally characterized by jaw morphology as comprising the taxa Rotifera, Acanthocephala and Gnathostomulida. Based on a large-scale molecular sequence dataset of 11,146 amino acid residues, we reconstructed phylogenetic trees of spiralian phyla using maximum-likelihood and Bayesian approaches. We obtain the first phylogenomic evidence for the clade Gnathifera, linking Syndermata (Rotifera+Acanthocephala) with Gnathostomulida. Furthermore, our data support recent findings concerning the paraphyly of Eurotatoria.

EST based phylogenomics of Syndermata questions monophyly of Eurotatoria

BACKGROUND

The metazoan taxon Syndermata comprising Rotifera (in the classical sense of Monogononta+Bdelloidea+Seisonidea) and Acanthocephala has raised several hypotheses connected to the phylogeny of these animal groups and the included subtaxa. While the monophyletic origin of Syndermata and Acanthocephala is well established based on morphological and molecular data, the phylogenetic position of Syndermata within Spiralia, the monophyletic origin of Monogononta, Bdelloidea, and Seisonidea and the acanthocephalan sister group are still a matter of debate. The comparison of the alternative hypotheses suggests that testing the phylogenetic validity of Eurotatoria (Monogononta+Bdelloidea) is the key to unravel the phylogenetic relations within Syndermata. The syndermatan phylogeny in turn is a prerequisite for reconstructing the evolution of the acanthocephalan endoparasitism.

RESULTS

Here we present our results from a phylogenomic approach studying i) the phylogenetic position of Syndermata within Spiralia, ii) the monophyletic origin of monogononts and bdelloids and iii) the phylogenetic relations of the latter two taxa to acanthocephalans. For this analysis we have generated EST libraries of Pomphorhynchus laevis, Echinorhynchus truttae (Acanthocephala) and Brachionus plicatilis (Monogononta). By extending these data with database entries of B. plicatilis, Philodina roseola (Bdelloidea) and 25 additional metazoan species, we conducted phylogenetic reconstructions based on 79 ribosomal proteins using maximum likelihood and bayesian approaches. Our findings suggest that the phylogenetic position of Syndermata within Spiralia is close to Platyhelminthes, that Eurotatoria are not monophyletic and that bdelloids are more closely related to acanthocephalans than monogononts.

CONCLUSION

Mapping morphological character evolution onto molecular phylogeny suggests the (partial or complete) reduction of the corona and the emergence of a retractable anterior end (rostrum, proboscis) before the separation of Acanthocephala. In particular, the evolution of a rostrum might have been a key event leading to the later evolution of the acanthocephalan endoparasitism, given the enormous relevance of the proboscis for anchoring of the adults to the definitive hosts' intestinal wall.

First maxillae suction discs in Branchiura (Crustacea): development and evolution in light of the first molecular phylogeny of Branchiura, Pentastomida, and other "Maxillopoda"

The fish ectoparasites Branchiura (Crustacea) display two different ways of attachment to the fish surface as adults: the first maxillae are either hooks (Dolops) or suction discs (Argulus, Chonopeltis, and Dipteropeltis). In larval Argulus foliaceus the first maxillae are hooks. With the first molecular phylogeny of the Branchiura as a background, the present paper discusses the evolutionary scenarios leading to hooks versus suction discs. Specific homologies exist between larval Argulus foliaceus hooks and adult Dolops ranarum hooks. These include the presence of a comparable number of segments/portions and a distal segment terminating in a double structure: a distal two-part hook (in Argulus) or one hook and an associate spine-like structure (in Dolops). In the phylogenetic reconstruction based on three molecular markers (mitochondrial 16S rRNA, nuclear 18S and 28S rRNA), Dolops ranarum is found to be in a sister group position to all other Branchiura, which in this analysis include six Argulus and one Chonopeltis sequences. Based on the molecular phylogeny a likely evolutionary scenario is that the ancestral branchiuran used hooks (on the first maxilla) for attachment, as seen in Dolops, of which the proximal part was subsequently modified into suction discs in Argulus and Chonopeltis (and Dipteropeltis). The sister group relationship of the Branchiura and Pentastomida is confirmed based on the most comprehensive taxon sampling until now. No evidence was found for a branchiuran in-group position of the Pentastomida.

Comparative spermatology of selected polyclad flatworms (platyhelminthes)

Sperm ultrastructure of four acotylean (Idioplana atlantica, Armatoplana leptalea, Styloplanocera fasciata, Melloplana ferruginea) and three cotylean polyclads (Pseudoceros bicolor, Phrikoceros mopsus, Enchiridium evelinae) was investigated. All spermatozoa are biflagellate, exhibiting a 9+"1" axoneme pattern. All acotylean axonemes originate and extend within the sperm shaft, and once exiting the shaft, remain attached to it. The flagella of all cotylean spermatozoa exit the shaft immediately and remain free. Structures shared by all species include: an elongated nucleus, in acotyleans located only in the posterior part of the shaft, whereas in cotyleans it extends along the entire sperm body; mitochondria along with small and large dense bodies arranged in a specific pattern; and a ring of microtubules that extends along the entire sperm shaft just beneath the cell membrane. A unique spermatozoon has been found in E. evelinae, where round vesicle-like structures fill the anterior part of the nucleus, and a different type of large dense bodies is present. The spermatozoa of all studied species exhibit numerous characters (axoneme/flagella position, distribution and position of large and small dense bodies, of mitochondria, presence of nuclear vesicles) that may be of phylogenetic value at the family and higher taxonomic levels.

Sacculogenesis of Buddenbrockia plumatellae (Myxozoa) within the invertebrate host Plumatella repens (Bryozoa) with comments on the evolutionary relationships of the Myxozoa

Members of the phylum Myxozoa are obligate parasites, primarily of aquatic organisms. Their phylogeny has remained problematic, with studies placing them within either the Bilateria or Cnidaria. The discovery that the enigmatic Buddenbrockia plumatellae is a myxozoan that possesses distinct bilaterian features appeared to have finally resolved the debate. B. plumatellae is described as a triploblastic 'worm-like' organism, within which typical myxozoan malacospores form. Using EM we examined the early development of the B. plumatellae 'worms' within the bryozoan host Plumatella repens. The initial development involved numerous unicellular, amoeboid pre-saccular stages that were present within the basal lamina of the host's body wall. These stages migrate immediately beneath the peritoneum where a significant host tissue reaction occurs. The stages aggregate, initiating the formation of a 'worm'. The base of a developing 'worm' forms a pseudosyncytium which resolves into an ectoderm surrounding a mesendoderm. The pseudosyncytium is directly anchored into neighbouring host cells via masses of striated fibres. The replication of the ectodermal and mesendodermal cells extends the developing 'worm' into the coelom of the host. The mesendoderm resolves to form a mesoderm and an endoderm. Myogenesis appears to be initiated from the anchored end of the 'worm' and develops along the mesoderm. The aggregation and differentiation of amoeboid pre-saccular stages to initiate the 'worm' draws analogies to the sacculogenesis observed for Tetracapsuloides bryosalmonae, B. plumatellae's sister taxon within the class Malacosporea. The development of a multicellular, spore forming organism, from single cells does not correlate to any bilaterian or cnidarian species. Current phylogenies indicate the Myxozoa are basal bilaterians along with the Acoela and Mesozoa. Comparison with these other basal groups may help to resolve the placement of Myxozoa within the tree of life.

Divergent location of ribosomal genes in chromosomes of fish thorny-headed worms, Pomphorhynchus laevis and Pomphorhynchus tereticollis (Acanthocephala)

We studied distribution of ribosomal DNA (rDNA) sequences along with chromosomal location of the nucleolar organizer regions (NORs) in males of two fish parasites, Pomphorhynchus laevis and Pomphorhynchus tereticollis (Acanthocephala). Fluorescence in situ hybridization with 18S rDNA probe identified two clusters of rDNA in each species, but revealed a remarkable difference in their location on chromosomes. In P. laevis, the rDNA-FISH signals were found in long arms of the first chromosome pair and in short arms of the second pair. Whereas in P. tereticollis, rDNA clusters were located in long arms of both the first and second chromosome pairs. The divergent location of rDNA clusters in the chromosome No. 2 supports current classification of P. tereticollis, previously considered a synonym of P. laevis, as a separate species. A possible scenario of the second chromosome rearrangement during karyotype evolution of the two species involves two successive pericentric inversions. In both species, one or two prominent nucleoli were apparent within interphase nuclei stained with either silver nitrate or a fluorescent dye YOYO-1. However, a single large nucleolus was observed in early stages of mitosis and meiosis I regardless the number of rDNA clusters. Nevertheless, two bivalents with silver-stained NORs in diakinesis and two silver-stained sites in early prophase II nuclei indicated that all NORs are active. This means that each Pomphorhynchus NOR generates a nucleolus, but the resulting nucleoli have a strong tendency to associate in a large body.

First Molecular Data on the Phylum Loricifera – An Investigation into the Phylogeny of Ecdysozoa with Emphasis on the Positions of Loricifera and Priapulida

Recent progress in molecular techniques has generated a wealth of information for phylogenetic analysis. Among metazoans all but a single phylum have been incorporated into some sort of molecular analysis. However, the minute and rare species of the phylum Loricifera have remained elusive to molecular systematists. Here we report the first molecular sequence data (nearly complete 18S rRNA) for a member of the phylum Loricifera, Pliciloricus sp. from Korea. The new sequence data were analyzed together with 52 other ecdysozoan sequences, with all other phyla represented by three or more sequences. The data set was analyzed using parsimony as an optimality criterion under direct optimization as well as using a Bayesian approach. The parsimony analysis was also accompanied by a sensitivity analysis. The results of both analyses are largely congruent, finding monophyly of each ecdysozoan phylum, except for Priapulida, in which the coelomate Meiopriapulus is separate from a clade of pseudocoelomate priapulids. The data also suggest a relationship of the pseudocoelomate priapulids to kinorhynchs, and a relationship of nematodes to tardigrades. The Bayesian analysis placed the arthropods as the sister group to a clade that includes tardigrades and nematodes. However, these results were shown to be parameter dependent in the sensitivity analysis. The position of Loricifera was extremely unstable to parameter variation, and support for a relationship of loriciferans to any particular ecdysozoan phylum was not found in the data.

Further use of nearly complete 28S and 18S rRNA genes to classify Ecdysozoa: 37 more arthropods and a kinorhynch

This work expands on a study from 2004 by Mallatt, Garey, and Shultz [Mallatt, J.M., Garey, J.R., Shultz, J.W., 2004. Ecdysozoan phylogeny and Bayesian inference: first use of nearly complete 28S and 18S rRNA gene sequences to classify the arthropods and their kin. Mol. Phylogenet. Evol. 31, 178-191] that evaluated the phylogenetic relationships in Ecdysozoa (molting animals), especially arthropods. Here, the number of rRNA gene-sequences was effectively doubled for each major group of arthropods, and sequences from the phylum Kinorhyncha (mud dragons) were also included, bringing the number of ecdysozoan taxa to over 80. The methods emphasized maximum likelihood, Bayesian inference and statistical testing with parametric bootstrapping, but also included parsimony and minimum evolution. Prominent findings from our combined analysis of both genes are as follows. The fundamental subdivisions of Hexapoda (insects and relatives) are Insecta and Entognatha, with the latter consisting of collembolans (springtails) and a clade of proturans plus diplurans. Our rRNA-gene data provide the strongest evidence to date that the sister group of Hexapoda is Branchiopoda (fairy shrimps, tadpole shrimps, etc.), not Malacostraca. The large, Pancrustacea clade (hexapods within a paraphyletic Crustacea) divided into a few basic subclades: hexapods plus branchiopods; cirripedes (barnacles) plus malacostracans (lobsters, crabs, true shrimps, isopods, etc.); and the basally located clades of (a) ostracods (seed shrimps) and (b) branchiurans (fish lice) plus the bizarre pentastomids (tongue worms). These findings about Pancrustacea agree with a recent study by Regier, Shultz, and Kambic that used entirely different genes [Regier, J.C., Shultz, J.W., Kambic, R.E., 2005a. Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic. Proc. R. Soc. B 272, 395-401]. In Malacostraca, the stomatopod (mantis shrimp) was not at the base of the eumalacostracans, as is widely claimed, but grouped instead with an euphausiacean (krill). Within centipedes, Craterostigmus was the sister to all other pleurostigmophorans, contrary to the consensus view. Our trees also united myriapods (millipedes and centipedes) with chelicerates (horseshoe crabs, spiders, scorpions, and relatives) and united pycnogonids (sea spiders) with chelicerates, but with much less support than in the previous rRNA-gene study. Finally, kinorhynchs joined priapulans (penis worms) at the base of Ecdysozoa.

A modern approach to rotiferan phylogeny: combining morphological and molecular data

The phylogeny of selected members of the phylum Rotifera is examined based on analyses under parsimony direct optimization and Bayesian inference of phylogeny. Species of the higher metazoan lineages Acanthocephala, Micrognathozoa, Cycliophora, and potential outgroups are included to test rotiferan monophyly. The data include 74 morphological characters combined with DNA sequence data from four molecular loci, including the nuclear 18S rRNA, 28S rRNA, histone H3, and the mitochondrial cytochrome c oxidase subunit I. The combined molecular and total evidence analyses support the inclusion of Acanthocephala as a rotiferan ingroup, but do not support the inclusion of Micrognathozoa and Cycliophora. Within Rotifera, the monophyletic Monogononta is sister group to a clade consisting of Acanthocephala, Seisonidea, and Bdelloidea-for which we propose the name Hemirotifera. We also formally propose the inclusion of Acanthocephala within Rotifera, but maintaining the name Rotifera for the new expanded phylum. Within Monogononta, Gnesiotrocha and Ploima are also supported by the data. The relationships within Ploima remain unstable to parameter variation or to the method of phylogeny reconstruction and poorly supported, and the analyses showed that monophyly was questionable for the families Dicranophoridae, Notommatidae, and Brachionidae, and for the genus Proales. Otherwise, monophyly was generally supported for the represented ploimid families and genera.

Phylogenetic relationships among Syndermata inferred from nuclear and mitochondrial gene sequences

Phylogenetic relationships among Syndermata have been extensively debated, mainly because the sister-group of the Acanthocephala has not yet been clearly identified from analyses of morphological and molecular data. Here we conduct phylogenetic analyses on samples from the 4 classes of Acanthocephala (Archiacanthocephala, Eoacanthocephala, Polyacanthocephala, and Palaeacanthocephala) and the 3 Rotifera classes (Bdelloidea, Monogononta, and Seisonidea). We do so using small-subunit (SSU) and large-subunit (LSU) ribosomal DNA and cytochrome c oxidase subunit 1 (cox 1) sequences. These nuclear and mitochondrial DNA sequences were obtained for 27 acanthocephalans, 9 rotifers, and representatives of 6 phyla that were used as outgroups. Maximum parsimony (MP), maximum likelihood (ML), and Bayesian analyses were conducted on the nuclear rDNA(SSU+LSU) and the combined sequence dataset(SSU+LSU+cox 1 genes). Phylogenetic analyses of the combined rDNA and cox 1 data uniformly provided strong support for a clade including rotifers plus acanthocephalans (Syndermata). Strong support was also found for monophyly of Acanthocephala in analyses of the combined dataset or rDNA sequences alone. Within the Acanthocephala the monophyletic grouping of the representatives of each class was strongly supported. Our results depicted Archiacanthocephala as the sister-group to the remaining acanthocephalans. Analyses of the combined dataset recovered a sister-group relationship between Acanthocephala and Bdelloidea by parsimony, likelihood, and Bayesian methods. Support for this clade was generally strong. Alternative topologies that depicted a different rotifer sister-group of Acanthocephala (or monophyly of Rotifera) were significantly worse. In this paraphyletic assemblage of rotifers, the relative positions of Seisonidea and Monogononta to the clade Bdelloidea+Acanthocephala were inconsistent among trees based on different inference methods. These results indicate that Bdelloidea is the free-living sister-group to acanthocephalans, which should prove key for comparative investigations of the morphological, molecular, and ecological changes accompanying the evolution of parasitism.

Parasite histories and novel phylogenetic tools: Alternative approaches to inferring parasite evolution from molecular markers

Parasitological research is often contingent on the knowledge of the phylogeny/genealogy of the studied group. Although molecular phylogenetics has proved to be a powerful tool in such investigations, its application in the traditional fashion, based on a tree inference from the primary nucleotide sequences may, in many cases, be insufficient or even improper. These limitations are due to a number of factors, such as a scarcity/ambiguity of phylogenetic information in the sequences, an intricacy of gene relationships at low phylogenetic levels, or a lack of criteria when deciding among several competing coevolutionary scenarios. With respect to the importance of a precise and reliable phylogenetic background in many biological studies, attempts are being made to extend molecular phylogenetics with a variety of new data sources and methodologies. In this review, selected approaches potentially applicable to parasitological research are presented and their advantages as well as drawbacks are discussed. These issues include the usage of idiosyncratic markers (unique features with presumably low probability of homoplasy), such as insertion of mobile elements, gene rearrangements and secondary structure features; the problem of ancestral polymorphism and reticulate relationships at low phylogenetic levels; and the utility of a molecular clock to facilitate discrimination among alternative scenarios in host-parasite coevolution.

Phylogenetic position of the Pentastomida and (pan)crustacean relationships

Pentastomids are a small group of vermiform animals with unique morphology and parasitic lifestyle. They are generally recognized as being related to the Arthropoda; however, the nature of this relationship is controversial. We have determined the complete sequence of the mitochondrial DNA (mtDNA) of the pentastomid Armillifer armillatus and complete or nearly complete mtDNA sequences from representatives of four previously unsampled groups of Crustacea: Remipedia (Speleonectes tulumensis), Cephalocarida (Hutchinsoniella macracantha), Cirripedia (Pollicipes polymerus) and Branchiura (Argulus americanus). Analyses of the mtDNA gene arrangements and sequences determined in this study indicate unambiguously that pentastomids are a group of modified crustaceans probably related to branchiurans. In addition, gene arrangement comparisons strongly support an unforeseen assemblage of pentastomids with maxillopod and cephalocarid crustaceans, to the exclusion of remipedes, branchiopods, malacostracans and hexapods.

Molecular phylogeny of the Platyhelminthes

The phylum Platyhelminthes has traditionally been considered the most basal bilaterian taxon. The main difficulty with this placement is the lack of convincing synapomorphies for all Platyhelminthes, which suggest that they are polyphyletic. Recent molecular findings based on 18S rDNA sequence data and number and type of Hox genes strongly suggest that the majority of Platyhelminthes are members of the lophotrochozoan protostomes, whereas the Acoelomorpha (Acoela + Nemertodermatida) fall outside of the Platyhelminthes as the most basal bilaterian taxon. Here we review phylum-wide analyses based on complete ribosomal and other nuclear genes addressed to answer the main issues facing systematics and phylogeny of Platyhelminthes. We present and discuss (i) new corroborative evidence for the polyphyly of the Platyhelminthes and the basal position of Acoelomorpha; (ii) a new consensus internal tree of the phylum; (iii) the nature of the sister group to the Neodermata and the hypotheses on the origin of parasitism; and (iv) the internal phylogeny of some rhabditophoran orders. Some methodological caveats are also introduced. The need to erect a new phylum, the Acoelomorpha, separate from the Platyhelminthes (now Catenulida + Rhabditophora) and based on present and new morphological and molecular characters is highlighted, and a proposal made.

Current advances in the phylogenetic reconstruction of metazoan evolution. A new paradigm for the Cambrian explosion?

The study of metazoan evolution has fascinated biologists for centuries, and it will certainly keep doing so. Recent interest on the origin of metazoan body plans, early metazoan evolution, genetic mechanisms generating disparity and diversity, molecular clock information, paleontology, and biogeochemistry is contributing to a better understanding of the current phyletic diversity. Unfortunately, the pattern of the metazoan tree of life still shows some important gaps in knowledge. It is the aim of this article to review some of the most important issues related to the inference of the metazoan tree, and point towards possible ways of solving certain obscure aspects in the history of animal evolution. A new hypothesis of the metazoan diversification during the Cambrian explosion is proposed by synthesizing ideas from phylogenetics, molecular evolution, paleontology, and developmental biology.

Parasites within the new phylogeny of eukaryotes

In the past few years, molecular phylogenetic and cladistic analyses of the interrelationships of the living phyla have resulted in a radical reorganization of eukaryote groups. This reorganization has significance for parasitologists, in that it places as sister taxa some of the more speciose and highly parasitic phyla (nematodes and insects), reorganizes what is now recognized as paraphyletic sets of 'wormy taxa' as the Aschelmintha, and draws numerous bridges between different realms (plants, fungi and animals). This review attempts to explore the role of parasites within the phylogeny of eukaryotes. Extant described parasitic organisms are less common among the eukaryotes than is commonly admitted in the literature.