Ultrastructure and development of the rhabdomeric eyes in Lineus viridis (Heteronemertea, Nemertea)

Nemerteans are undoubtedly members of the Spiralia, although their phylogenetic relationships are still a matter of debate. The apparently acoelomate organization suggests a relationship with the platyhelminths, whereas the blood-vascular system has been interpreted as an equivalent to coelomic cavities of annelids, indicating a close relation between annelids and nemerteans. Like other spiralians, most nemertean species are known to have one or several pairs of rhabdomeric and subepidermally situated eyes when adult. The development of these eyes as well as the mode in which the eyes are multiplied is as yet unknown. This is the first attempt to investigate eye formation in a nemertean. In the heteronemertean Lineus viridis (Müller, 1774) the everse rhabdomeric eyes are located deeply underneath the epidermis and consist of a few pigment cells that form a cup-like structure with interdigitating processes that contain numerous pigment granules. In hatchlings, the optical cavity contains processes of 12 sensory cells, each bearing a single cilium and various microvilli. The perikarya of these cells are located distally from the pigment cup. During further development the number of cells increases. Eye development starts with a small anlage situated underneath the epidermis, irrespective of whether this is the first eye or any additional one. The anlage consists of five unpigmented cells and three dendritic processes, each bearing apical microvilli and a single cilium. There is no evidence for an epidermal origin of the eyes. In L. viridis eye formation resembles that described in platyhelminths in which eyes also develop as cerebral derivatives. Although this result has the potential to influence the discussion on the position of Nemertea, the data have to be interpreted with care, since development of L. viridis is derived within the Nemertea.     

What cell lineages tell us about the evolution of spiralia remains to be seen

Abstract.— Cell-lineage trees may contain information about spiralian phylogeny, as proposed by Guralnick and Lind-berg (2001). Here we discuss this possibility further and conclude that the cell-division pattern must be known in greater detail and the coding methods refined before a possible phylogenetic signal can be identified.     

RECONNECTING CELL AND ANIMAL LINEAGES: WHAT DO CELL LINEAGES TELL US ABOUT THE EVOLUTION AND DEVELOPMENT OF SPIRALIA?

Cell lineage studies in the clade Eutrochozoa, and especially the Spiralia, remains a rich and relatively untapped source for understanding broad evolutionary developmental problems; including (1) the utility of cell timing formation for phylogenetic hypotheses; (2) the evolution of cell timing changes and its relation to heterochronic patterns; (3) stereotypy or lack thereof in rates of change of cell growth during evolution and its relation to both evolutionary history and current usage; and (4) how mosaic cleavage timing variation may be expected to differ from other groups. A compilation of available cell timing information was made from previous studies where each division was explicitly followed and the total number of cells followed was greater than 24. From that compilation, we performed a series of heuristic and quantitative analyses, including a phylogenetic analysis using cell timing data as characters and analyses of timing variation across all taxa. Our results show that: (1) cell lineage data reconstructs a phylogenetic hypothesis that has similarities, especially among the Mollusca. to the patterns found in morphological and molecular analyses; (2) the mesentoblast (4d) is a unique cell compared to other cell in that it speeds up and slows down relative to other cells in taxa with both unequal and equal cell sizes; (3) some cells that form in the same quartet at the same point in the cell lineage hierarchy have much lower variations than analogous other cells, arguing for architectural constraint or stabilizing selection acting on those cells; and (4) although variation in cell timing generally increases during development, timing of formation of progeny cells in the first quartet has lower variation than the parent cells, arguing that some regulation-like behavior might be present.     

Immunocytochemistry of the neuromuscular systems of Loxosomella vivipara and L. parguerensis (Entoprocta: Loxosomatidae)

Little detailed information exists on the anatomy of the nervous system and the musculature of Entoprocta. Herein we describe the distribution of the neurotransmitters RFamide and serotonin as well as the myo-anatomy of adults and asexually produced budding stages of the solitary entoproct species Loxosomella vivipara and L. parguerensis using immunocytochemistry and epifluorescence as well as confocal microscopy. The development of the RFamidergic and serotonergic nervous system starts in early budding stages. In the adults, RFamide is present in the bilateral symmetric cerebral ganglion, a pair of oral nerves that innervate two pairs of nerve cell clusters in the heel of the foot, a pair of aboral nerves, the paired lateral nerves, the calyx nerves, the atrial ring nerve, the tentacle nerves, the stomach nerves, and the rectal nerves. Serotonin is only found in the cerebral ganglion, the oral nerves, and in the tentacle nerves. Some differences in the distribution of both neurotransmitters were found between L. vivipara and L. parguerensis and are most obvious in the differing number of large serotonergic perikarya associated with the oral nerves. Nerves arising from the cerebral ganglion and running in a ventral direction have not been described for Entoprocta before, and the homology of these to the ventral nerve cords of other Spiralia is considered possible. The body musculature of both Loxosomella species comprises longitudinal and diagonal muscles in the foot, the stalk, and the calyx. We found several circular muscles in the calyx. The stalk and parts of the foot and the calyx are surrounded by a fine outer layer of ring muscles. In addition to the congruent details regarding the myo-anatomy of both species, species-specific muscle structures could be revealed. The comparison of our data with recent findings of the myo-anatomy of two Loxosoma species indicates that longitudinal and diagonal body muscles, atrial ring muscles, tentacle muscles, esophageal and rectal ring muscles, as well as intestinal and anal sphincters are probably part of the ancestral entoproct muscle bauplan.     

Fine structure of the pharyngeal apparatus of the pelagosphera larva in Phascolosoma agassizii (Sipuncula) and its phylogenetic significance

Sipuncula is a small taxon of worm-like marine organisms of still uncertain phylogenetic position. Sipunculans are characterized by an unsegmented body composed of a trunk into which the anterior part, the introvert, can be withdrawn. The group has been placed at various positions within Metazoa; currently, it is either seen as sister group of a clade comprising Mollusca and Annelida or as sister to each of these. An in-group position in either Mollusca or Annelida has usually been precluded till now due to the lack of so-called annelid or molluscan “key-characters” such as segmentation and chaetae or the radula. In the development of certain taxa the trochophore stage is followed by a planktonic larva, the pelagosphera, which might exhibit phylogenetically important structures. Among these is the buccal organ, which has been considered homologous either to the ventral pharyngeal organ present in many sedentary polychaetes or to the radular apparatus of molluscs. In the present paper, the ventral pharynx of the pelagosphera larva of Phascolosoma agassizii is investigated by transmission electron microscopy. The pharynx comprises dorsolateral ciliary folds, a muscle bulb formed by transverse muscle fibres with large intercellular spaces, and an investing muscle. A tongue-like organ is lacking. These results show great structural correspondences to the ventral pharynx of polychaetes, especially to that of the flabelligerid Diplocirrus longisetosus. In contrast, there are no signs of structural similarities to the corresponding structures of molluscs. Thus evidence increases that Sipuncula are closely related to annelids; moreover, an in-group position of Sipuncula within Annelida, as suggested by recent molecular studies, is not precluded by the present data. Instead these studies find additional support. Hence the lack of segmentation and chitinous chaetae in Sipuncula would be a secondary rather than a primary situation, as has recently been shown for Echiura and Pogonophora.     

Trochophore concepts: ciliary bands and the evolution of larvae in spiralian Metazoa

‘Trochophore’ is a term used in a strict sense for larvae having an opposed-band method of feeding, involving a prototroch and metatroch. Other ciliary bands such as a telotroch and neurotroch may be present. The trochophore has been proposed to represent the ancestral larval form for a group of metazoan phyla (including all members of the Spiralia). The name trochophore is also often applied to larvae that do not conform to the above definition. A cladistic analysis of spiralian taxa (with special reference to polychaete annelids), based on a suite of adult and larval characters, is used to assess several hypotheses: (1) that the trochophore (in a strict sense) is a plesiomorphic form for the Spiralia; (2) that die stricdy defined trochophore is plesiomorphic for members of the Spiralia such as the Polychaeta. The homology of each of the various separate ciliary bands of spiralian larvae, and features such as the apical tuft and protonephridia is also assessed. The results favour the conclusion that the trochophore, if defined as a feeding larval form using opposed bands, should not be regarded as an ancestral (= plesiomorphic) type for the Spiralia, or any other large taxon such as the Polychaeta or Mollusca. The evidence suggests that the various ciliary bands have differing evolutionary histories, and only the Echiura (possibly an annelid group) has members with the classical trochophore. The trochophore is re-defined as a larval form with a prototroch. This broad definition covers a wide variety of larvae, and matches the current usage more accurately than the restricted term. Features such as the neurotroch, telotroch and opposed-band feeding show convergence and reversals. The nature of the metatroch requires further investigation. The presence of a prototroch (and hence trochophore larvae) is used to identify an apomorphy-based taxon, Trochozoa, that includes the first ancestor to have evolved a prototroch and all its descendants. This minimally includes the Annelida [sensu lato), Echiura, Entoprocta, Mollusca and Sipuncula and is a less inclusive taxon than the Spiralia.     

Reconstructing the phylogeny of the Sipuncula

Sipunculans are marine spiralian worms with possible close affinities to the Mollusca or Annelida. Currently 147 species, 17 genera, 6 families, 4 orders and 2 classes are recognized. In this paper we review sipunculan morphology, anatomy, paleontological data and historical affiliations. We have conducted cladistic analyses for two data sets to elucidate the phylogenetic relationships among sipunculan species. We first analyzed the relationships among the 45 species of Phascolosomatidea with representatives of the Sipunculidea as outgroups, using 35 morphological characters. The resulting consensus tree has low resolution and branch support is low for most branches. The second analysis was based on DNA sequence data from two nuclear ribosomal genes (18S rRNA and 28S rRNA) and one nuclear protein-coding gene, histone H3. Outgroups were chosen among representative spiralians. In a third analysis, the molecular data were combined with the morphological data. Data were analyzed using parsimony as the optimality criterion and branch support evaluated with jackknifing and Bremer support values. Branch support for outgroup relationships is low but the monophyly of the Sipuncula is well supported. Within Sipuncula, the monophyly of the two major groups, Phascolosomatidea and Sipunculidea is not confirmed. Of the currently recognized families, only Themistidae appears monophyletic. The Aspidosiphonidae, Phascolosomatidae and Golfingiidae would be monophyletic with some adjustments in their definition. The Sipunculidae is clearly polyphyletic, with Sipunculus nudus as the sister group to the remaining Sipuncula, Siphonosoma cumanense the sister group to a clade containing Siphonosoma vastumand the Phascolosomatidea, and Phascolopsis gouldi grouping within the Golfingiiformes, as suggested previously by some authors. Of the genera with multiple representatives, only Phascolosoma and Themiste are monophyletic as currently defined. We are aiming to expand our current dataset with more species in our molecular database and more detailed morphological studies.     

Larval development of Myzostoma cirriferum (Myzostomida)

The larval development of Myzostoma cirriferum is described by means of SEM, TEM, and cLSM. It is similar to that of other myzostomids and includes three stages: the protrochophore, the trochophore, and the metatrochophore. The protrochophore is a ball-shaped larva present in culture from 18-48 h after egg laying. It has no internal organs and its body is made of three cell types: covering cells and ciliated cells that are external and surrounded by a cuticle, and resting cells that fill the blastocoel. The trochophore is a pear-shaped larva that develops 20-72 h after egg laying; the body includes the same three cell types as the previous stage. The metatrochophore is a pear-shaped larva that develops between 40 h and 14 days and is characterized by the presence of two bundles of four chaetae. When fully developed, the metatrochophore has a digestive system (made of a pharynx, an esophagus, and a blind digestive pouch), two pairs of protonephridia, and a nervous system composed of a supraesophageal ganglion, circumesophageal connectives, and dorsal and ventral nerves. Metamorphosis generally occurs 7 days after egg laying. At that time, the metatrochophore loses its chaetae and becomes pleated ventrally. This ultrastructural analysis suggests that chaetae and the five ventral longitudinal nerve cords of M. cirriferum metatrochophores are homologous structures to those observed in some polychaete trochophores. Coupled with recent phylogenetic analyses, where the Myzostomida are placed outside the Annelida, homologies between myzostomid and polychaete larvae support the view that a trochophore appeared early during the spiralian evolution.