Recent molecular phylogenetic analyses suggest a close relationship between two worm-shaped phyla, the nonsegmented Sipuncula (peanut worms) and the segmented Annelida (e.g., earthworms and polychaetes) [1-5]. The striking differences in their bodyplans are exemplified by the annelids' paired, ladder-like ventral nervous system, which contains segmentally arranged ganglia, and the sipunculans' single ventral nerve cord (VNC), which is devoid of any segmental structures [6, 7]. Investigating central nervous system (CNS) formation with serotonin and FMRFamide labeling in a representative sipunculan, Phascolosoma agassizii, we found that neurogenesis initially follows a segmental pattern similar to that of annelids. Starting out with paired FMRFamidergic and serotonergic axons, four pairs of associated serotonergic perikarya and interconnecting commissures form one after another in an anterior-posterior progression. In late-stage larvae, the two serotonergic axons of the VNCs fuse, the commissures disappear, and one additional pair of perikarya is formed. These cells (ten in total) migrate toward one another, eventually forming two clusters of five cells each. These neural-remodeling processes result in the single nonmetameric CNS of the adult sipunculan. Our data confirm the segmental ancestry of Sipuncula and render Phascolosoma a textbook example for the Haeckelian hypothesis of ontogenetic recapitulation of the evolutionary history of a species [8].

BackgroundStudies on the development of the nervous system and the musculature of invertebrates have become more sophisticated and numerous within the last decade and have proven to provide new insights into the evolutionary history of organisms. In order to provide new morphogenetic data on opisthobranch gastropods we investigated the neuromuscular development in the nudibranch Aeolidiella stephanieae Valdéz, 2005 using immunocytochemistry as well as F-actin labelling in conjunction with confocal laser scanning microscopy (cLSM).ResultsThe ontogenetic development of Aeolidiella stephanieae can be subdivided into 8 stages, each recognisable by characteristic morphological and behavioural features as well as specific characters of the nervous system and the muscular system, respectively. The larval nervous system of A. stephanieae includes an apical organ, developing central ganglia, and peripheral neurons associated with the velum, foot and posterior, visceral part of the larva. The first serotonergic and FMRFamidergic neural structures appear in the apical organ that exhibits an array of three sensory, flask-shaped and two non-sensory, round neurons, which altogether disappear prior to metamorphosis. The postmetamorphic central nervous system (CNS) becomes concentrated, and the rhinophoral ganglia develop together with the anlage of the future rhinophores whereas oral tentacle ganglia are not found. The myogenesis in A. stephanieae begins with the larval retractor muscle followed by the accessory larval retractor muscle, the velar or prototroch muscles and the pedal retractors that all together degenerate during metamorphosis, and the adult muscle complex forms de novo.ConclusionsAeolidiella stephanieae comprises features of the larval and postmetamorphic nervous as well as muscular system that represent the ground plan of the Mollusca or even the Trochozoa (e. g. presence of the prototrochal or velar muscle ring). On the one hand, A. stephanieae shows some features shared by all nudibranchs like the postmetamorphic condensation of the CNS, the possession of rhinophoral ganglia and the lack of oral tentacle ganglia as well as the de novo formation of the adult muscle complex. On the other hand, the structure and arrangement of the serotonergic apical organ is similar to other caenogastropod and opisthobranch gastropods supporting their sister group relationship.

BackgroundOweniids are marine tubeworms burrowing in muddy sediments that in current phylogenies form an early branching lineage within Annelida. Little is known about their general morphology, in particular the nervous system. Here we provide an immunocytochemical investigation of the nervous system of Galathowenia oculata in order to discuss putative ancestral neuronal features in Oweniidae.ResultsAdult Galathowenia oculata have neither a supraesophageal ganglion nor ganglia associated with the ventral nerve cord. Instead, there is a dorsal brain commissure in the head collar that is engulfed by a cellular cortex. Accordingly, we herein term this neural structure “medullary brain commissure”. The anterior margin of the head collar exhibits numerous neurites that emerge from the brain commissure. The dorsolateral folds are innervated by the ventrolateral neurite bundles extending from the circumesophageal connectives. In the anterior uniramous and biramous segments immunoreactive somata are distributed evenly along the ventral nerve cord and arranged metamerically in the posterior-most short segments. One dorsal and two pairs of lateral neurite bundles extend longitudinally along the body. Numerous serially arranged circular neurite bundles were labeled in anteriormost long segments. Metameric arrangement of the circular neurite bundles stained against FMRFamide and acetylated α-tubulin is revealed in posterior short segments. For the first time immunoreactive somata arranged in clusters are reported within the pygidium in oweniids.ConclusionsDue to the lack of head appendages and a sedentary mode of life, G. oculata exhibits a single dorsal commissure (versus a brain with four commissures in most annelids). A “medullary brain commissure” is known so far only in Oweniidae and Echiura. Lack of ganglia and metamery in the ventral nerve cord of the anteriormost segments might be the result of the elongation of these segments. In the short posterior segments the metamery of immunoreactive somata and circular neurite bundles is conserved. We hypothesize that the unpaired ventral nerve cord in adult oweniids might be a result of an initially paired ventral nerve cord that fuses during development, a condition not uncommon within Annelida.

BackgroundMollusks display a striking morphological disparity, including, among others, worm-like animals (the aplacophorans), snails and slugs, bivalves, and cephalopods. This phenotypic diversity renders them ideal for studies into animal evolution. Despite being one of the most species-rich phyla, molecular and in silico studies concerning specific key developmental gene families are still scarce, thus hampering deeper insights into the molecular machinery that governs the development and evolution of the various molluscan class-level taxa.ResultsNext-generation sequencing was used to retrieve transcriptomes of representatives of seven out of the eight recent class-level taxa of mollusks. Similarity searches, phylogenetic inferences, and a detailed manual curation were used to identify and confirm the orthology of numerous molluscan Hox and ParaHox genes, which resulted in a comprehensive catalog that highlights the evolution of these genes in Mollusca and other metazoans. The identification of a specific molluscan motif in the Hox paralog group 5 and a lophotrochozoan ParaHox motif in the Gsx gene is described. Functional analyses using KEGG and GO tools enabled a detailed description of key developmental genes expressed in important pathways such as Hedgehog, Wnt, and Notch during development of the respective species. The KEGG analysis revealed Wnt8, Wnt11, and Wnt16 as Wnt genes hitherto not reported for mollusks, thereby enlarging the known Wnt complement of the phylum. In addition, novel Hedgehog (Hh)-related genes were identified in the gastropod Lottia cf. kogamogai, demonstrating a more complex gene content in this species than in other mollusks.ConclusionsThe use of de novo transcriptome assembly and well-designed in silico protocols proved to be a robust approach for surveying and mining large sequence data in a wide range of non-model mollusks. The data presented herein constitute only a small fraction of the information retrieved from the analysed molluscan transcriptomes, which can be promptly employed in the identification of novel genes and gene families, phylogenetic inferences, and other studies using molecular tools. As such, our study provides an important framework for understanding some of the underlying molecular mechanisms involved in molluscan body plan diversification and hints towards functions of key developmental genes in molluscan morphogenesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-3080-9) contains supplementary material, which is available to authorized users.

BackgroundThe shipworm Lyrodus pedicellatus is a wood-boring bivalve with an unusual vermiform body. Although its larvae are brooded, they retain the general appearance of a typical bivalve veliger-type larva. Here, we describe myogenesis of L. pedicellatus revealed by filamentous actin labelling and discuss the data in a comparative framework in order to test for homologous structures that might be part of the bivalve (larval) muscular ground pattern.ResultsFive major muscle systems were identified: a velum retractor, foot retractor, larval retractor, a distinct mantle musculature and an adductor system. For a short period of larval life, an additional ventral larval retractor is present. Early in development, a velum muscle ring and an oral velum musculature emerge. In late stages the lateral and dorsal mantle musculature, paired finger-shaped muscles, an accessory adductor and a pedal plexus are formed. Similar to other bivalve larvae, L. pedicellatus exhibits three velum retractor muscles, but in contrast to other species, one of them disappears in early stages of L. pedicellatus. The remaining two velum retractors are considerably remodelled during late larval development and are most likely incorporated into the elaborate mantle musculature of the adult.ConclusionsTo our knowledge, this is the first account of any larval retractor system that might contribute to the adult bodyplan of a (conchiferan) mollusk. A comparative analysis shows that a pedal plexus, adductors, a larval velum ring, velum retractors and a ventral larval retractor are commonly found among bivalve larvae, and thus most likely belong to the ground pattern of the bivalve larval musculature.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-014-0090-9) contains supplementary material, which is available to authorized users.