Myogenesis of Siboglinum Fiordicum Sheds Light on Body Regionalisation in Beard Worms (Siboglinidae, Annelida)

BackgroundMany annelids, including well-studied species such as Platynereis, show similar structured segments along their body axis (homonomous segmentation). However, numerous annelid species diverge from this pattern and exhibit specialised segments or body regions (heteronomous segmentation). Recent phylogenomic studies and paleontological findings suggest that a heteronomous body architecture may represent an ancestral condition in Annelida. To better understand the segmentation within heteronomous species we describe the myogenesis and mesodermal delineation of segments in Siboglinum fiordicum during development. ResultsEmploying confocal and transmission electron microscopy we show that the somatic circular musculature lies inside the longitudinal musculature and is predominantly developed at the anterior end of the larva. The longitudinal musculature consists of four separate strands at the ventral, dorsal, and ventrolateral body sides. Posteriorly, the longitudinal strands form a continuous layer. Our application of transmission electron microscopy allows us to describe the developmental order of the non-muscular septa. The first septum to form is supported by thick bundles of longitudinal muscles and separates the body into an anterior and a posterior region. The second group of septa to develop further divides the posterior body region (opisthosoma) and is supported by developing circular muscles. At the late larval stage, a septum reinforced by circular muscles divides the anterior body region into a forepart and a trunk segment. The remaining septa and their circular muscles form one by one at the very posterior end of the opisthosoma. Functionally, the prominent ventrolateral longitudinal muscles in the larva are proposed to drive the search movements of the head, while the anterior circular muscles and the posterior continuous layers of longitudinal muscles support the burrowing behaviour of the larva.ConclusionsThe heteronomous Siboglinum lacks the strict anterior to posterior sequence of segment formation as it is found in the most studied annelid species. Instead, the first septum divides the body into two body regions, before segments are layed down in first the posterior opisthosoma and then in the anterior body, respectively. Similar pattern of segment formation is described for the heteronomous chaetopterid Chaetopterus variopedatus and may represent an ancestral segmentation process in Annelida.

Re-discovery and novel contributions to morphology and multigene phylogeny of Myxophyllum steenstrupi (Ciliophora: Pleuronematida), an obligate symbiont of terrestrial pulmonates

Myxophyllum steenstrupi is a symbiotic ciliate living in the body slime and mantle cavity of terrestrial pulmonates (Gastropoda: Pulmonata). In the present study, M. steenstrupi was re-discovered after almost 30 years and characterized using an integrative morpho-molecular approach for the first time. Myxophyllum is distinguished by a broadly ovate, about 140 × 115 μm-sized body, a nuclear apparatus typically composed of seven macronuclear nodules and a single micronucleus, a central contractile vacuole, a shallow oral cavity situated in the posterior body region and dense somatic ciliature with extensive thigmotactic field. According to the present phylogenetic analyses of two mitochondrial and five nuclear markers, M. steenstrupi is classified in the predominantly free-living order Pleuronematida (Oligohymenophorea: Scuticociliatia). This order also encompasses other taxa isolated from molluscs and traditionally classified along with Myxophyllum in the order Thigmotrichida. The proper classifications of Myxophyllum was hampered by the dramatic remodelling of its oral apparatus (reduction of the paroral membrane and adoral organelles, formation of vestibular kineties), a transformation that was likely induced by its firm association with terrestrial gastropods. The present study also documents that various ciliate lineages independently became commensals or parasites of various aquatic and terrestrial molluscs.

How zebrafish turn: analysis of pressure force dynamics and mechanical work

ABSTRACTWhereas many fishes swim steadily, zebrafish regularly exhibit unsteady burst-and-coast swimming, which is characterized by repeated sequences of turns followed by gliding periods. Such a behavior offers the opportunity to investigate the hypothesis that negative mechanical work occurs in posterior regions of the body during early phases of the turn near the time of maximal body curvature. Here, we used a modified particle image velocimetry (PIV) technique to obtain high-resolution flow fields around the zebrafish body during turns. Using detailed swimming kinematics coupled with body surface pressure computations, we estimated fluid–structure interaction forces and the pattern of forces and torques along the body during turning. We then calculated the mechanical work done by each body segment. We used estimated patterns of positive and negative work along the body to evaluate the hypothesis (based on fish midline kinematics) that the posterior body region would experience predominantly negative work. Between 10% and 20% of the total mechanical work was done by the fluid on the body (negative work), and negative work was concentrated in the anterior and middle areas of the body, not along the caudal region. Energetic costs of turning were calculated by considering the sum of positive and negative work and were compared with previous metabolic estimates of turning energetics in fishes. The analytical workflow presented here provides a rigorous way to quantify hydrodynamic mechanisms of fish locomotion and facilitates the understanding of how body kinematics generate locomotor forces in freely swimming fishes.

Ektaphelenchoides shiroodensis n. sp. (Rhabditida: Ektaphelenchinae) a new member of the genus

Summary Ektaphelenchoides shiroodensis n. sp. is described and illustrated based upon morphological, morphometric and molecular data. It was recovered from the bark samples of a dead alder tree (Alnus sp.) from countryside around Shirood city, Mazandaran province, in the north of Iran. The new species is characterised by 768-985 μm long females, its lip region separated from the body contour by constriction, lateral field with three barely visible lines, forming two weak bands, 26-29 μm long stylet with wide lumen without conophore and basal swellings, excretory pore always at the level with median bulb, post-vulval uterine sac (PUS) 43-76 μm long, elongate conoid posterior body region ending to a long filiform part with pointed tip, males common with dorsally convex conical tail ending to a short narrower region with pointed tip, seven precloacal + cloacal + caudal papillae and arcuate spicules with wide condylus, blunt rostrum and simple end. Based upon the similar posterior body region (‘tail’) and comparable PUS length, the new species was compared to three known species, viz., E. attenuata, E. hunti and E. pini. Comparisons with the aforementioned species and similar species under the genus Seinura are discussed. The phylogenetic affinities of the new species, based upon two partial small and large subunit ribosomal DNA (SSU and LSU rDNA D2-D3) sequences, are discussed. Furthermore, several ektaphelenchid and seinurid species previously described from Iran were recovered in the present study, sequenced for their aforementioned genomic markers, and included in the phylogenetic analyses.

Morphology of Anomalocaris canadensis from the Burgess Shale

Recent description of the oral cone of Anomalocaris canadensis from the Burgess Shale (Cambrian Series 3, Stage 5) highlighted significant differences from published accounts of this iconic species, and prompts a new evaluation of its morphology as a whole. All known specimens of A. canadensis, including previously unpublished material, were examined with the aim of providing a cohesive morphological description of this stem lineage arthropod. In contrast to previous descriptions, the dorsal surface of the head is shown to be covered by a small, oval carapace in close association with paired stalked eyes, and the ventral surface bears only the triradial oral cone, with no evidence of a hypostome or an anterior sclerite. The frontal appendages reveal new details of the arthrodial membranes and a narrower cross-section in dorsal view than previously reconstructed. The posterior body region reveals a complex suite of digestive, respiratory, and locomotory characters that include a differentiated foregut and hindgut, a midgut with paired glands, gill-like setal blades, and evidence of muscle bundles and struts that presumably supported the swimming movement of the body flaps. The tail fan includes a central blade in addition to the previously described three pairs of lateral blades. Some of these structures have not been identified in other anomalocaridids, making Anomalocaris critical for understanding the functional morphology of the group as a whole and corroborating its arthropod affinities.

Mutations Affecting Symmetrical Migration of Distal Tip Cells in Caenorhabditis elegans

The rotational symmetry of the Caenorhabditis elegans gonad arms is generated by the symmetrical migration of two distal tip cells (DTCs), located on the anterior and posterior ends of the gonad primordium. Mutations that cause asymmetrical migration of the two DTCs were isolated. All seven mutations were recessive and assigned to six different complementation groups. vab-3(k121) and vab-3(k143) affected anterior DTC migration more frequently than posterior, although null mutants showed no bias. The other five mutations, mig-14(k124), mig-17(k113), mig-18(k140), mig-19(k142), and mig-20(k148), affected posterior DTC migration more frequently than anterior. These observations imply that the migration of each DTC is regulated differently. mig-14 and mig-19 also affected the migration of other cells in the posterior body region. Four distinct types of DTC migration abnormalities were defined on the basis of the mutant phenotypes. vab-3; mig-14 double mutants exhibited the types of DTC migration defects seen for vab-3 single mutants. Combination of mig-17 and mig-18 or mig-19, which are characterized by the same types of posterior DTC migration defects, exhibited strong enhancement of anterior DTC migration defects, suggesting that they affect the same or parallel pathways regulating anterior DTC migration.

Neuronal cell migration in C. elegans: regulation of Hox gene expression and cell position

In C. elegans, the Hox gene mab-5, which specifies the fates of cells in the posterior body region, has been shown to direct the migrations of certain cells within its domain of function. mab-5 expression switches on in the neuroblast QL as it migrates into the posterior body region. mab-5 activity is then required for the descendants of QL to migrate to posterior rather than anterior positions. What information activates Hox gene expression during this cell migration? How are these cells subsequently guided to their final positions? We address these questions by describing four genes, egl-20, mig-14, mig-1 and lin-17, that are required to activate expression of mab-5 during migration of the QL neuroblast. We find that two of these genes, egl-20 and mig-14, also act in a mab-5-independent way to determine the final stopping points of the migrating Q descendants. The Q descendants do not migrate toward any obvious physical targets in wild-type or mutant animals. Therefore, these genes appear to be part of a system that positions the migrating Q descendants along the anteroposterior axis.

Studies on the biology of the life-cycle of Trichuris suis Schrank, 1788

The biology of the life-cycle of Trichuris suis of the pig is described and discussed together with aspects of the hosi-parasite relationship. Development of the infective L. 1 larval stage within the egg was shown to be influenced by temperature. Following ingestion of infective ova by the pig, all subsequent larval development to the adult stage occurred in the mucosa of the caecum and colon. Eggs hatched in the distal region of the small intestine and throughout the large intestine. Larvae then penetrated the caecum and colonic mucosa via the crypts of Lieberkühn, where they entered the cells lining the crypts. The ensuing histotrophic phase lasted 13 days, during which time a gradual larval migration occurred from the deeper regions of the lamina propria to areas immediately beneath the surface mucosal epithelium. Luminal development was initially seen on day 16 when the posterior extremities of larvae were protruded into the gut lumen; from day 20 the entire posterior body region was exposed, while only the filamentous anterior region of the parasite remained embedded in the mucosal surface. The prepatent period varied from 41 to 47 days. Four moults were observed during development within the host and these occurred on days 10, 16, 20, 32 and 37 (to form the L. 2, L. 3, L. 4 and L. 5 or adult stage respectively). It is suggested that although T. suis may not be an important pathogen in its own right, its activities in the caecal and colonic mucosa may cause sufficient damage to enable secondary pathogenic invaders to become established.

The reactions to carcinogens in the axolotl (Ambystoma mexicanum) in relation to the ‘regeneration field control’ hypothesis

About 200 axolotls, between 5 and 21 months old, were treated in the body wall with carcinogens or control substances, by subcutaneous or intradermal injection, or by subcutaneous implantation. In response to an injection of dibenzanthracene in olive oil an initial reaction appeared within 13 days. This consisted of an epidermal proliferation and a subcutaneous infiltration of macrophages. The epidermis returned to normal after several weeks, but the subcutaneous response took 6 months to disappear. The initial reaction appeared to be, at least partially, a wound healing response; its regression could not have been due to regeneration field control as it occurred in the posterior body region. Following the disappearance of the initial reaction a secondary reaction of papilloma-like outgrowths arose between 7 and 20 months after injection. Of the axolotls surviving for a sufficient length of time, outgrowths arose in 14 out of 54 animals injected with dibenzanthracene and in 18 out of 57 sites injected with methylcholanthrene. These outgrowths had some features in common with the papillomata of mice, but none have yet progressed to carcinomata. In a group of axolotls injected in three different regions the frequency of outgrowths varied according to the sites; however, this was thought to be due to differences in the difficulty of injecting the three regions. Two tumours, a sarcoma and a hepatoma, arose in the course of these experiments, 3 and 2 years respectively after carcinogen treatment. From this it is suggested tentatively that tumour induction by polycyclic hydrocarbons in the axolotl may require a long latent period and involve a low tumour incidence; however, it is possible that at least one of the tumours was not induced by the treatment.