Complementing aculiferan mitogenomics: comparative characterization of mitochondrial genomes of Solenogastres (Mollusca, Aplacophora).

Background: With the advances in high-throughput sequencing and bioinformatic pipelines, mitochondrial genomes have become increasingly popular for phylogenetic analyses across different clades of invertebrates. Despite the vast rise in available mitogenomic datasets of molluscs, one class of aplacophoran molluscs - Solenogastres (or Neomeniomorpha) - is still neglected.

Results: Here, we present six new mitochondrial genomes from five families of Solenogastres (Amphimeniidae, Gymnomeniidae, Proneomeniidae, Pruvotinidae, Simrothiellidae), including the first complete mitogenomes, thereby now representing three of the four traditional orders.

Microbial associations of shallow-water Mediterranean marine cave Solenogastres (Mollusca).

The first cave-dwelling Solenogastres-marine shell-less worm-like mollusks-were sampled from Mediterranean marine caves floor silt in the Marseille area. The mollusks were 1.5 mm in length, had a transparent body with shiny spicules and appear to represent a new species.

Phylogenomics of Aplacophora (Mollusca, Aculifera) and a solenogaster without a foot.

Recent molecular phylogenetic investigations strongly supported the placement of the shell-less, worm-shaped aplacophoran molluscs (Solenogastres and Caudofoveata) and chitons (Polyplacophora) in a clade called Aculifera, which is the sister taxon of all other molluscs. Thus, understanding the evolutionary history of aculiferan molluscs is important for understanding early molluscan evolution. In particular, fundamental questions about evolutionary relationships within Aplacophora have long been unanswered.

Molecular phylogeny of Caudofoveata (Mollusca) challenges traditional views.

The shell-less, worm-shaped Caudofoveata (=Chaetodermomorpha) is one of the least known groups of molluscs. The taxon consists of 141 recognized species found from intertidal environments to the deep-sea where they live burrowing in sediment. Evolutionary relationships of the group have been debated, but few studies based on morphological or molecular data have investigated the phylogeny of the group.

Expression of six3 and otx in Solenogastres (Mollusca) supports an ancestral role in bilaterian anterior-posterior axis patterning.

The homeodomain transcription factors six3 and otx are involved in patterning the anterior body and parts of the central nervous system (CNS) in bilaterians. Their similar expression patterns have been used as an argument for homology of heads, brains, segmentation, and ciliated larvae. We investigated the developmental expression of six3 and otx in the aplacophoran mollusk Wirenia argentea.

Abyssal fauna of the UK-1 polymetallic nodule exploration area, Clarion-Clipperton Zone, central Pacific Ocean: Mollusca.

We present the first DNA taxonomy publication on abyssal Mollusca from the Clarion-Clipperton Zone (CCZ), central Pacific ocean, using material collected as part of the Abyssal Baseline (ABYSSLINE) environmental survey cruise 'AB01' to the UK Seabed Resources Ltd (UKSRL) polymetallic-nodule exploration area 'UK-1' in the eastern CCZ. This is the third paper in a series to provide regional taxonomic data for a region that is undergoing intense deep-sea mineral exploration for high-grade polymetallic nodules. Taxonomic data are presented for 21 species from 42 records identified by a combination of morphological and genetic data, including molecular phylogenetic analyses.

Brain regionalization genes are co-opted into shell field patterning in Mollusca.

The 'brain regionalization genes' Six3/6, Otx, Pax2/5/8, Gbx, and Hox1 are expressed in a similar fashion in the deuterostome, ecdysozoan, and the cephalopod brain, questioning whether this holds also true for the remaining Mollusca. We investigated developmental Gbx-expression in representatives of both molluscan sister groups, the Aculifera and Conchifera. Gbx is expressed in the posterior central nervous system of an aculiferan polyplacophoran and solenogaster but not in a conchiferan bivalve suggesting that Gbx, together with Six3/6, Otx, Pax2/5/8, and Hox1, is involved in central nervous system regionalization as reported for other bilaterians.

Ancestral and novel roles of Pax family genes in mollusks.

Background: Pax genes are transcription factors with significant roles in cell fate specification and tissue differentiation during animal ontogeny. Most information on their tempo-spatial mode of expression is available from well-studied model organisms where the Pax-subfamilies Pax2/5/8, Pax6, and Paxα/β are mainly involved in the development of the central nervous system (CNS), the eyes, and other sensory organs. In certain taxa, Pax2/5/8 seems to be additionally involved in the development of excretion organs.

Cell Proliferation Pattern and Twist Expression in an Aplacophoran Mollusk Argue Against Segmented Ancestry of Mollusca.

The study of aplacophoran mollusks (i.e., Solenogastres or Neomeniomorpha and Caudofoveata or Chaetodermomorpha) has traditionally been regarded as crucial for reconstructing the morphology of the last common ancestor of the Mollusca.

Phylogenomics of Lophotrochozoa with Consideration of Systematic Error.

Phylogenomic studies have improved understanding of deep metazoan phylogeny and show promise for resolving incongruences among analyses based on limited numbers of loci. One region of the animal tree that has been especially difficult to resolve, even with phylogenomic approaches, is relationships within Lophotrochozoa (the animal clade that includes molluscs, annelids, and flatworms among others). Lack of resolution in phylogenomic analyses could be due to insufficient phylogenetic signal, limitations in taxon and/or gene sampling, or systematic error.

Ancestral role of Pax2/5/8 in molluscan brain and multimodal sensory system development.

Background: Mollusks represent the largest lophotrochozoan phylum and exhibit highly diverse body plans. Previous studies have demonstrated that transcription factors such as Pax genes play important roles during their development. Accordingly, in ecdysozoan and vertebrate model organisms, orthologs of Pax2/5/8 are among others involved in the formation of the midbrain/hindbrain boundary, the auditory/geosensory organ systems, and the excretory system.

From complex to simple: myogenesis in an aplacophoran mollusk reveals key traits in aculiferan evolution.

Background: Recent studies suggest a bifurcation at the base of Mollusca, resulting in the primarily single-shelled Conchifera (Bivalvia, Gastropoda, Scaphopoda, Monoplacophora, Cephalopoda) and the spicule-bearing Aculifera (Polyplacophora, Neomeniomorpha, Chaetodermomorpha). A recent study revealed a complex larval musculature exclusively shared by Neomeniomorpha and Polyplacophora, supporting a close relationship of both taxa. However, the ontogenetic transition from the complex larval to the simple adult neomeniomorph musculature, which mainly consists of a three-layered body-wall musculature and serially iterated dorsoventral muscles, remains unknown.

The complete mitochondrial genome of Scutopus ventrolineatus (Mollusca: Chaetodermomorpha) supports the Aculifera hypothesis.

Background: With more than 100000 living species, mollusks are the second most diverse metazoan phylum. The current taxonomic classification of mollusks recognizes eight classes (Neomeniomorpha, Chaetodermomorpha, Polyplacophora, Monoplacophora, Cephalopoda, Gastropoda, Bivalvia, and Scaphopoda) that exhibit very distinct body plans. In the past, phylogenetic relationships among mollusk classes have been contentious due to the lack of indisputable morphological synapomorphies.

Development of the nervous system in Solenogastres (Mollusca) reveals putative ancestral spiralian features.

Background: The Solenogastres (or Neomeniomorpha) are a taxon of aplacophoran molluscs with contentious phylogenetic placement. Since available developmental data on non-conchiferan (that is, aculiferan) molluscs mainly stem from polyplacophorans, data on aplacophorans are needed to clarify evolutionary questions concerning the morphological features of the last common ancestor (LCA) of the Aculifera and the entire Mollusca. We therefore investigated the development of the nervous system in two solenogasters, Wirenia argentea and Gymnomenia pellucida, using immunocytochemistry and electron microscopy.

Aplacophoran mollusks evolved from ancestors with polyplacophoran-like features.

Mollusca is an animal phylum with vast morphological diversity and includes worm-shaped aplacophorans, snails, bivalves, and the complex cephalopods. The interrelationships of these class-level taxa are still contentious, but recent phylogenomic analyses suggest a dichotomy at the base of Mollusca, resulting in a monophyletic Aculifera (comprising the shell-less, sclerite-bearing aplacophorans and the eight-shelled polyplacophorans) and Conchifera (all other, primarily univalved groups). The Aculifera concept has recently gained support via description of the fossil Kulindroplax, which shows both aplacophoran- and polyplacophoran-like features and suggests that the aplacophorans originated from a shelled ancestor, but the overall morphology of the last common aculiferan ancestor remains obscure.

Phylogenomics reveals deep molluscan relationships.

Evolutionary relationships among the eight major lineages of Mollusca have remained unresolved despite their diversity and importance. Previous investigations of molluscan phylogeny, based primarily on nuclear ribosomal gene sequences or morphological data, have been unsuccessful at elucidating these relationships. Recently, phylogenomic studies using dozens to hundreds of genes have greatly improved our understanding of deep animal relationships.

Comparative neuroanatomy suggests repeated reduction of neuroarchitectural complexity in Annelida.

Background: Paired mushroom bodies, an unpaired central complex, and bilaterally arranged clusters of olfactory glomeruli are among the most distinctive components of arthropod neuroarchitecture. Mushroom body neuropils, unpaired midline neuropils, and olfactory glomeruli also occur in the brains of some polychaete annelids, showing varying degrees of morphological similarity to their arthropod counterparts. Attempts to elucidate the evolutionary origin of these neuropils and to deduce an ancestral ground pattern of annelid cerebral complexity are impeded by the incomplete knowledge of annelid phylogeny and by a lack of comparative neuroanatomical data for this group.

Fast evolving 18S rRNA sequences from Solenogastres (Mollusca) resist standard PCR amplification and give new insights into mollusk substitution rate heterogeneity.

Background: The 18S rRNA gene is one of the most important molecular markers, used in diverse applications such as molecular phylogenetic analyses and biodiversity screening. The Mollusca is the second largest phylum within the animal kingdom and mollusks show an outstanding high diversity in body plans and ecological adaptations. Although an enormous amount of 18S data is available for higher mollusks, data on some early branching lineages are still limited.

Of tests, trochs, shells, and spicules: Development of the basal mollusk Wirenia argentea (Solenogastres) and its bearing on the evolution of trochozoan larval key features.

Background: The phylogenetic status of the aplacophoran mollusk taxon Solenogastres (Neomeniomorpha) is controversially discussed. Some authors propose the clade to represent the most basal branch within Mollusca, while others claim aplacophoran mollusks (Solenogastres and Caudofoveata) to be derived. Larval characters are central in these discussions, specifically the larval test (calymma, apical cap), the ontogeny of the epidermal scleritome, and the proposed absence of larval protonephridia.

Structure and evolution of the pharynx simplex in acoel flatworms (Acoela).

The homology of pharynges within the mostly pharynx-less Acoela has been a matter of discussion for decades and even the basic question of whether a pharynx is a primitive trait within the Acoela and homologous to the pharynx of platyhelminth turbellarians is open. By using fluorescence staining of musculature, as well as conventional histological techniques and transmission electron microscopy, the present study sets focus on the mouth and pharynx (where present) of seven species of Acoela within Paratomellidae, Solenofilomorphidae, Hofsteniidae, Proporidae, and Convolutidae, as well as one species of Nemertodermatida and Catenulida, respectively. It is shown that among the investigated families of acoels there is a great variability in muscle systems associated with the mouth and pharynx and that pharynx histology and ultrastructural characters are widely diverse.

Morphology and ultrastructure of the pharynx in Solenofilomorphidae (Acoela).

The homology of pharynges within the mostly pharynx-less Acoela has been a matter of discussion for decades. Here, we analyze the pharynges of three members of the Solenofilomorphidae, Myopea sp. and two species of the genus Solenofilomorpha, by means of light and transmission electron microscopy.

Foregut glands of Solenogastres (mollusca): anatomy and revised terminology.

In the molluscan class Solenogastres, different types of foregut glands vary in number, structure, and location within the foregut. The present article describes their anatomy and cytology and intends to clarify their confused terminology. Pharyngeal glands, esophageal glands, and the more complex dorsal and ventrolateral foregut glands can be distinguished.