Mox homeobox expression in muscle lineage of the gastropod Haliotis asinina: evidence for a conserved role in bilaterian myogenesis

New insights into mesoderm and endoderm development, and the nature of the onychophoran blastopore

BACKGROUND

Early during onychophoran development and prior to the formation of the germ band, a posterior tissue thickening forms the posterior pit. Anterior to this thickening forms a groove, the embryonic slit, that marks the anterior-posterior orientation of the developing embryo. This slit is by some authors considered the blastopore, and thus the origin of the endoderm, while others argue that the posterior pit represents the blastopore. This controversy is of evolutionary significance because if the slit represents the blastopore, then this would support the amphistomy hypothesis that suggests that a slit-like blastopore in the bilaterian ancestor evolved into protostomy and deuterostomy.

RESULTS

In this paper, we summarize our current knowledge about endoderm and mesoderm development in onychophorans and provide additional data on early endoderm- and mesoderm-determining marker genes such as Blimp, Mox, and the T-box genes.

CONCLUSION

We come to the conclusion that the endoderm of onychophorans forms prior to the development of the embryonic slit, and thus that the slit is not the primary origin of the endoderm. It is thus unlikely that the embryonic slit represents the blastopore. We suggest instead that the posterior pit indeed represents the lips of the blastopore, and that the embryonic slit (and surrounding tissue) represents a morphologically superficial archenteron-like structure. We conclude further that both endoderm and mesoderm development are under control of conserved gene regulatory networks, and that many of the features found in arthropods including the model Drosophila melanogaster are likely derived.

Early mesodermal development in the patellogastropod Lottia goshimai

Mesodermal development is essential to explore the interlineage variations in the development of spiralians. Compared with model mollusks such as Tritia and Crepidula, knowledge about the mesodermal development of other molluscan lineages is limited. Here, we investigated early mesodermal development in the patellogastropod Lottia goshimai, which shows equal cleavage and has a trochophore larva. The endomesoderm derived from the 4d blastomere, that is, the mesodermal bandlets, was situated dorsally and showed a characteristic morphology. Investigations of the potential mesodermal patterning genes revealed that twist1 and snail1 were expressed in a proportion of these endomesodermal tissues, while all of the five genes we investigated (twist1, twist2, snail1, snail2, and mox) were expressed in ventrally located ectomesodermal tissues. Relatively dynamic snail2 expression suggests additional roles in various internalization processes. By tracing snail2 expression in early gastrulae, the 3a²¹¹ and 3b²¹¹ blastomeres were suggested to be the precursors of the ectomesoderm, which elongated to become internalized before division. These results help to understand the variations in the mesodermal development of different spiralians and explore the different mechanisms by which ectomesodermal cells are internalized, which has important evolutionary implications.

Differentiation and Maturation of Muscle and Fat Cells in Cultivated Seafood: Lessons from Developmental Biology

Cultivated meat, also known as cultured or cell-based meat, is meat produced directly from cultured animal cells rather than from a whole animal. Cultivated meat and seafood have been proposed as a means of mitigating the substantial harms associated with current production methods, including damage to the environment, antibiotic resistance, food security challenges, poor animal welfare, and-in the case of seafood-overfishing and ecological damage associated with fishing and aquaculture. Because biomedical tissue engineering research, from which cultivated meat draws a great deal of inspiration, has thus far been conducted almost exclusively in mammals, cultivated seafood suffers from a lack of established protocols for producing complex tissues in vitro. At the same time, fish such as the zebrafish Danio rerio have been widely used as model organisms in developmental biology. Therefore, many of the mechanisms and signaling pathways involved in the formation of muscle, fat, and other relevant tissue are relatively well understood for this species. The same processes are understood to a lesser degree in aquatic invertebrates. This review discusses the differentiation and maturation of meat-relevant cell types in aquatic species and makes recommendations for future research aimed at recapitulating these processes to produce cultivated fish and shellfish.

A mosaic of conserved and novel modes of gene expression and morphogenesis in mesoderm and muscle formation of a larval bivalve

The mesoderm gives rise to several key morphological features of bilaterian animals including endoskeletal elements and the musculature. A number of regulatory genes involved in mesoderm and/or muscle formation (e.g., Brachyury (Bra), even-skipped (eve), Mox, myosin II heavy chain (mhc)) have been identified chiefly from chordates and the ecdysozoans Drosophila and Caenorhabditis elegans, but data for non-model protostomes, especially those belonging to the ecdysozoan sister clade, Lophotrochozoa (e.g., flatworms, annelids, mollusks), are only beginning to emerge. Within the lophotrochozoans, Mollusca constitutes the most speciose and diverse phylum. Interestingly, however, information on the morphological and molecular underpinnings of key ontogenetic processes such as mesoderm formation and myogenesis remains scarce even for prominent molluscan sublineages such as the bivalves. Here, we investigated myogenesis and developmental expression of Bra, eve, Mox, and mhc in the quagga mussel Dreissena rostriformis, an invasive freshwater bivalve and an emerging model in invertebrate evodevo. We found that all four genes are expressed during mesoderm formation, but some show additional, individual sites of expression during ontogeny. While Mox and mhc are involved in early myogenesis, eve is also expressed in the embryonic shell field and Bra is additionally present in the foregut. Comparative analysis suggests that Mox has an ancestral role in mesoderm and possibly muscle formation in bilaterians, while Bra and eve are conserved regulators of mesoderm development of nephrozoans (protostomes and deuterostomes). The fully developed Dreissena veliger larva shows a highly complex muscular architecture, supporting a muscular ground pattern of autobranch bivalve larvae that includes at least a velum muscle ring, three or four pairs of velum retractors, one or two pairs of larval retractors, two pairs of foot retractors, a pedal plexus, possibly two pairs of mantle retractors, and the muscles of the pallial line, as well as an anterior and a posterior adductor. As is typical for their molluscan kin, remodelling and loss of prominent larval features such as the velum musculature and various retractor systems appear to be also common in bivalves.

Supplementary information

The online version contains supplementary material available at 10.1007/s13127-022-00569-5.

HES and Mox genes are expressed during early mesoderm formation in a mollusk with putative ancestral features

The mesoderm is considered the youngest of the three germ layers. Although its morphogenesis has been studied in some metazoans, the molecular components underlying this process remain obscure for numerous phyla including the highly diverse Mollusca. Here, expression of Hairy and enhancer of split (HES), Mox, and myosin heavy chain (MHC) was investigated in Acanthochitona fascicularis, a representative of Polyplacophora with putative ancestral molluscan features. While AfaMHC is expressed throughout myogenesis, AfaMox1 is only expressed during early stages of mesodermal band formation and in the ventrolateral muscle, an autapomorphy of the polyplacophoran trochophore. Comparing our findings to previously published data across Metazoa reveals Mox expression in the mesoderm in numerous bilaterians including gastropods, polychaetes, and brachiopods. It is also involved in myogenesis in molluscs, annelids, tunicates, and craniates, suggesting a dual role of Mox in mesoderm and muscle formation in the last common bilaterian ancestor. AfaHESC2 is expressed in the ectoderm of the polyplacophoran gastrula and later in the mesodermal bands and in putative neural tissue, whereas AfaHESC7 is expressed in the trochoblasts of the gastrula and during foregut formation. This confirms the high developmental variability of HES gene expression and demonstrates that Mox and HES genes are pleiotropic.

FGF signaling acts on different levels of mesoderm development within Spiralia

FGF signaling is involved in mesoderm induction in members of deuterostomes (e.g. tunicates, hemichordates), but not in flies and nematodes, in which it has a role in mesoderm patterning and migration. However, we need comparable studies in other protostome taxa in order to decipher whether this mesoderm-inducing function of FGF extends beyond the lineage of deuterostomes. Here, we investigated the role of FGF signaling in mesoderm development in three species of lophophorates, a clade within the protostome group Spiralia. Our gene expression analyses show that the mesodermal molecular patterning is conserved between brachiopods and phoronids, but the spatial and temporal recruitment of transcription factors differs significantly. Moreover, the use of the inhibitor SU5402 demonstrates that FGF signaling is involved in different steps of mesoderm development, as well as in morphogenetic movements of gastrulation and axial elongation. Our findings suggest that the mesoderm-inducing role of FGF extends beyond the group of deuterostomes.

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. Since their proposed close relatives, the Polyplacophora, show a distinct seriality in certain organ systems, the aplacophorans are also in the focus of attention with regard to the question of a potential segmented ancestry of mollusks. To contribute to this question, we investigated cell proliferation patterns and the expression of the twist ortholog during larval development in solenogasters. In advanced to late larvae, during the outgrowth of the trunk, a pair of longitudinal bands of proliferating cells is found subepithelially in a lateral to ventrolateral position. These bands elongate during subsequent development as the trunk grows longer. Likewise, expression of twist occurs in two laterally positioned, subepithelial longitudinal stripes in advanced larvae. Both, the pattern of proliferating cells and the expression domain of twist demonstrate the existence of extensive and long‐lived mesodermal bands in a worm‐shaped aculiferan, a situation which is similar to annelids but in stark contrast to conchiferans, where the mesodermal bands are usually rudimentary and ephemeral. Yet, in contrast to annelids, neither the bands of proliferating cells nor the twist expression domain show a separation into distinct serial subunits, which clearly argues against a segmented ancestry of mollusks. Furthermore, the lack of twist expression during the development of the ventromedian muscle argues against homology of a ventromedian longitudinal muscle in protostomes with the notochord of chordates.

An Overview of Hox Genes in Lophotrochozoa: Evolution and Functionality

Hox genes are regulators of animal embryonic development. Changes in the number and sequence of Hox genes as well as in their expression patterns have been related to the evolution of the body plan. Lophotrochozoa is a clade of Protostomia characterized by several phyla which show a wide morphological diversity. Despite that the works summarized in this review emphasize the fragmentary nature of the data available regarding the presence and expression of Hox genes, they also offer interesting insight into the evolution of the Hox cluster and the role played by Hox genes in several phyla. However, the number of genes involved in the cluster of the lophotrochozoan ancestor is still a question of debate. The data presented here suggest that at least nine genes were present while two other genes, Lox4 and Post-2, may either have been present in the ancestor or may have arisen as a result of duplication in the Brachiopoda-Mollusca-Annelida lineage. Spatial and temporal collinearity is a feature of Hox gene expression which was probably present in the ancestor of deuterostomes and protostomes. However, in Lophotrochozoa, it has been detected in only a few species belonging to Annelida and Mollusca.

Mesodermal gene expression during the embryonic and larval development of the articulate brachiopod Terebratalia transversa

Background

Brachiopods undergo radial cleavage, which is distinct from the stereotyped development of closely related spiralian taxa. The mesoderm has been inferred to derive from the archenteron walls following gastrulation, and the primary mesoderm derivative in the larva is a complex musculature. To investigate the specification and differentiation of the mesoderm in the articulate brachiopod Terebratalia transversa, we have identified orthologs of genes involved in mesoderm development in other taxa and investigated their spatial and temporal expression during the embryonic and larval development of T. transversa.

Results

Orthologs of 17 developmental regulatory genes with roles in the development of the mesoderm in other bilaterian animals were found to be expressed in the developing mesoderm of T. transversa. Five genes, Tt.twist, Tt.GATA456, Tt.dachshund, Tt.mPrx, and Tt.NK1, were found to have expression throughout the archenteron wall at the radial gastrula stage, shortly after the initiation of gastrulation. Three additional genes, Tt.Pax1/9, Tt.MyoD, and Tt.Six1/2, showed expression at this stage in only a portion of the archenteron wall. Tt.eya, Tt.FoxC, Tt.FoxF, Tt.Mox, Tt.paraxis, Tt.Limpet, and Tt.Mef2 all showed initial mesodermal expression during later gastrula or early larval stages. At the late larval stage, Tt.dachshund, Tt.Limpet, and Tt.Mef2 showed expression in nearly all mesoderm cells, while all other genes were localized to specific regions of the mesoderm. Tt.FoxD and Tt.noggin both showed expression in the ventral mesoderm at the larval stages, with gastrula expression patterns in the archenteron roof and blastopore lip, respectively.

Conclusions

Expression analyses support conserved roles for developmental regulators in the specification and differentiation of the mesoderm during the development of T. transversa. Expression of multiple mesodermal factors in the archenteron wall during gastrulation supports previous morphological observations that this region gives rise to larval mesoderm. Localized expression domains during gastrulation and larval development evidence early regionalization of the mesoderm and provide a basis for hypotheses regarding the molecular regulation underlying the complex system of musculature observed in the larva.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0004-8) contains supplementary material, which is available to authorized users.

Primordial germ cell migration in the yellowtail kingfish (Seriola lalandi) and identification of stromal cell-derived factor 1

Primordial germ cells (PGCs) are progenitors of the germ cell lineage, giving rise to either spermatogonia or oogonia after the completion of gonadal differentiation. Currently, there is little information on the mechanism of PGCs migration leading to the formation of the primordial gonad in perciform fish. Yellowtail kingfish (Seriola lalandi) (YTK) (order Perciforms) inhabit tropical and temperate waters in the southern hemisphere. Fundamental details into the molecular basis of larval development in this species can be easily studied in Australia, as they are commercially cultured and readily available. In this study, histological analysis of YTK larvae revealed critical time points for the migration of PGCs to the genital ridge, resulting in the subsequent development of the primordial gonad. In YTK larvae at 3, 5, 7 and 10 days post hatch (DPH), PGCs were not yet enclosed by somatic cells, indicating the primordial gonad had not yet started to form. While at 15, 18 and 20 DPH PGCs had already settled at the genital ridge and started to become enclosed by somatic cells indicating the primordial gonad had started to develop. A higher number of PGCs were observed in the larvae at 15 and 18 DPH indicating PGCs proliferation, which corresponds with them becoming enclosed by the somatic cells. Directional migration of PGCs toward the genital ridge is a critical event in the subsequent development of a gonad. In zebrafish, mouse and chicken, stromal-cell derived factor (SDF1) signalling is one of the key molecules for PGC migration. We subsequently isolated from YTK the SDF1 (Slal-SDF1) gene, which encodes for a 98-residue precursor protein with a signal peptide at the N-terminus. There is spatial conservation between fish species of four cysteine residues at positions C9, C11, C34 and C49, expected to form disulphide bonds and stabilize the SDF structure. In YTK, Slal-SDF1 gene expression analyses shows that this gene is expressed in larvae from 1 to 22 DPH and demonstrates distinct spatial localisation in the larvae at 7 DPH. These results provide a platform for further studies into the molecular machinery of PGC migration in yellowtail kingfish, as well as other perciform fish species.

Molecular analysis of two FMRFamide-encoding transcripts expressed during the development of the tropical abalone Haliotis asinina

FMRFamide-related peptides (FaRPs) are involved in numerous neural functions across the animal kingdom and serve as important models for understanding the evolution of neuropeptides. Gastropod molluscs have proved to be particularly useful foci for such studies, but the developmental expression of FaRPs and the evolution of specific transcripts for different peptides are unclear within the molluscs. Here we show that FaRPs are encoded by two transcripts that appear to be splice variants of a single gene in the abalone, Haliotis asinina, which represents the basal vetigastropods. Has-FMRF1 comprises 1,438 nucleotides and encodes a precursor protein of 329 amino acids that can potentially produce two copies of FLRFamide, one copy each of TLAGDSFLRFamide, QFYRIamide, SDPDLDDVIRASLLAYSLDDSPNN, and SVATAPVEAKAVEAGNKDIE, and 13 copies of FMRFamide. The second 1,241-nucleotide transcript, Has-FMRF2, encodes a 206-amino acid precursor protein with single copies of FLRFamide and FMRFamide along with such extended forms as NFGEPFLRFamide, FDSYEDKALRFamide, and NGWLHFamide, in addition to SDPGEDMLKSILLRGAPSNNGLQY and DTUDETTUNDNAHSRQ. Both transcripts are present early in life and are expressed in different but overlapping patterns within the developing larval nervous system. Mass spectrometry and immunocytochemistry demonstrate that FaRPs are cleaved from larger precursors and localize to the developing nervous system. Our results confirm previous evidence that FaRPs are expressed early and potentially play many roles during molluscan development and suggest that the last common ancestor to living gastropods used alternative splicing of an FMRFamide gene to generate a diversity of FaRPs in spatially restricted patterns in the nervous system.

Identifying the germline in an equally cleaving mollusc: Vasa and Nanos expression during embryonic and larval development of the vetigastropod Haliotis asinina

Members of the Vasa and Nanos gene families are important for the specification and development of the germline in diverse animals. Here, we determine spatial and temporal expression of Vasa and Nanos to investigate germline development in the vetigastropod Haliotis asinina. This is the first time these genes have been examined in an equally cleaving lophotrochozoan species. We find that HasVasa and HasNanos have largely overlapping, but not identical, expression patterns during embryonic and larval development, with both being maternally expressed and localized to the micromere cell lineages during cleavage. As embryonic development continues, HasVasa and HasNanos become progressively more enriched in the dorsal quadrant of the embryo. By the trochophore stage, both HasVasa and HasNanos are expressed in the putative mesodermal bands of the larva. This differs from the unequally cleaving gastropod Illyanasa obsoleta, in which IoVasa and IoNanos expression is detectable only in the early embryo and not during gastrulation and larval development. Our results suggest that the H. asinina germline arises from the 4d cell lineage and that primordial germ cells (PGCs) are not specified exclusively by maternally inherited determinants (preformation). As such, we infer that inductive signals (epigenesis) play an important role in specifying PGCs in H. asinina. We hypothesize that HasVasa is expressed in a population of undifferentiated multipotent cells, from which the PGCs are segregated later during development.

Developmental patterns in spiralian embryos

At least five animal phyla exhibit spiralian development, which is characterized by striking similarities in the geometry of the early cleavage pattern and the fate map of the blastula, along with similarities in larval morphology. Recent advances in reconstructing the phylogeny of spiralians and their relatives suggest that the common ancestor of a large clade of protostome phyla known as the Lophotrochozoa had spiralian development. In this minireview, I describe characteristics of spiralian development and some recent insights into its mechanisms and evolution.

Major muscle systems in the larval caenogastropod, Ilyanassa obsoleta, display different patterns of development

This study describes the anatomical and developmental aspects of muscular development from the early embryo to competent larval stage in the gastropod Ilyanassa obsoleta. Staining of F-actin revealed differential spatial and temporal patterns of several muscles. In particular, two major muscles, the larval retractor and pedal retractor muscles originate independently and display distinct developmental patterns similar to observations in other gastropod species. Additionally, together with the larval retractor muscle, the accessory larval muscle developed in the embryo at the trochophore stage. Therefore, both these muscles develop prior to ontogenetic torsion. The pedal retractor muscle marked the most abundant growth in the mid veliger stage. Also during the middle stage, the metapodial retractor muscle and opercular retractor muscle grew concurrently with development of the foot. We show evidence that juvenile muscles, such as the buccal mass muscle and siphon muscle develop initially during the late veliger stage. Collectively, these findings substantiate that larval myogenesis involves a complex sequence of events that appear evolutionary conserved within the gastropods, and set the stage for future studies using this model species to address issues concerning the evolution and eventual fates of larval musculature in molluscs.

The role of MAPK signaling in patterning and establishing axial symmetry in the gastropod Haliotis asinina

Gastropods are members of the Spiralia, a diverse group of invertebrates that share a common early developmental program, which includes spiral cleavage and a larval trochophore stage. The spiral cleavage program results in the division of the embryo into four quadrants. Specification of the dorsal (D) quadrant is intimately linked with body plan organization and in equally cleaving gastropods occurs when one of the vegetal macromeres makes contact with overlying micromeres and receives an inductive signal that activates a MAPK signaling cascade. Following the induction of the 3D macromere, the embryo begins to gastrulate and assumes a bilateral cleavage pattern. Here we inhibit MAPK activation in 3D with U0126 and examine its effect on the formation and patterning of the trochophore, using a suite of territory-specific markers. The head (pretrochal) region appears to maintain quadri-radial symmetry in U0126-treated embryos, supporting a role for MAPK signaling in 3D in establishing dorsoventral polarity in this region. Posterior (posttrochal) structures - larval musculature, shell and foot--fail to develop in MAPK inhibited trochophores. Inhibition of 3D specification by an alternative method--monensin treatment--yields similar abnormal trochophores. However, genes that are normally expressed in the ectodermal structures (shell and foot) are detected in U0126- and monensin-perturbed larvae in patterns that suggest that this region has latent dorsoventral polarity that is manifested even in the absence of D quadrant specification.

A tolloid homologue from the Pacific oyster Crassostrea gigas

The genes governing mesoderm specification have been extensively studied in vertebrates, arthropods and nematodes. The latter two phyla belong to the Ecdysozoan clade but little is understood of the role that these genes might play in the development of the other major protostomal clade, the Lophotrochozoa. As part of a wider project to analyze the functions associated with transforming growth factor beta superfamily members in Lophotrochozoa, we have cloned a gene encoding a tolloid homologue from the bivalve mollusc Crassostrea gigas. Tolloid is a key developmental protein that regulates the activity of bone morphogenetic proteins (BMPs). We have determined the intron-exon structure of the gene encoding C. gigas tolloid and have compared it with those of homologous genes from both protostomes and deuterostomes. In order to analyze the functionality of oyster tolloid the zebrafish embryo has been employed as a reporter organism and we show that over-expression of this protein results in the ventralization of zebrafish embryos at 24h post fertilization. The expression of the C. gigas tolloid gene during embryonic and larval development as well as in adult tissues is also explored.

Hau-Pax3/7A is an early marker of leech mesoderm involved in segmental morphogenesis, nephridial development, and body cavity formation

Two genes of the Pax III subfamily, Hau-Pax3/7A and -Pax3/7B, were identified from the leech Helobdella, and the expression and function of Hau-Pax3/7A in development are described. Leech embryos undergo spiral cleavage, then produce a set of teloblastic stem cells that generate segmented mesoderm and ectoderm. Hau-Pax3/7A is present as a maternal transcript in both ectodermal and mesodermal progenitors, but this pool of early RNA disappears and is replaced by a pattern of zygotic transcription restricted to the blast cell progeny of the mesodermal M teloblasts. Each mesodermal blast cell clone goes through multiple phases of Hau-Pax3/7A expression, the last of which is associated with the organogenesis of the nephridia and other segment-specific structures. Morpholino-mediated knockdown of Hau-Pax3/7A expression causes the mesodermal blast cell clones to undergo irregular patterns of morphogenesis that disrupt the segmental organization of the germinal plate, and interferes with both the specification and morphological differentiation of the mesodermal nephridia. Knockdown of Hau-Pax3/7A in the mesoderm can also lead to abnormalities in the formation of the dorsal cavities, possibly through indirect effects of this germ layer on neighboring tissues. This is the first report of broad mesodermal Pax III expression outside of chordates, and raises the possibility that such expression may be a primitive trait inherited from the last common ancestor of the bilaterian superphyla.

Dorsoventral patterning in hemichordates: insights into early chordate evolution

We have compared the dorsoventral development of hemichordates and chordates to deduce the organization of their common ancestor, and hence to identify the evolutionary modifications of the chordate body axis after the lineages split. In the hemichordate embryo, genes encoding bone morphogenetic proteins (Bmp) 2/4 and 5/8, as well as several genes for modulators of Bmp activity, are expressed in a thin stripe of ectoderm on one midline, historically called "dorsal." On the opposite midline, the genes encoding Chordin and Anti-dorsalizing morphogenetic protein (Admp) are expressed. Thus, we find a Bmp-Chordin developmental axis preceding and underlying the anatomical dorsoventral axis of hemichordates, adding to the evidence from Drosophila and chordates that this axis may be at least as ancient as the first bilateral animals. Numerous genes encoding transcription factors and signaling ligands are expressed in the three germ layers of hemichordate embryos in distinct dorsoventral domains, such as pox neuro, pituitary homeobox, distalless, and tbx2/3 on the Bmp side and netrin, mnx, mox, and single-minded on the Chordin-Admp side. When we expose the embryo to excess Bmp protein, or when we deplete endogenous Bmp by small interfering RNA injections, these expression domains expand or contract, reflecting their activation or repression by Bmp, and the embryos develop as dorsalized or ventralized limit forms. Dorsoventral patterning is independent of anterior/posterior patterning, as in Drosophila but not chordates. Unlike both chordates and Drosophila, neural gene expression in hemichordates is not repressed by high Bmp levels, consistent with their development of a diffuse rather than centralized nervous system. We suggest that the common ancestor of hemichordates and chordates did not use its Bmp-Chordin axis to segregate epidermal and neural ectoderm but to pattern many other dorsoventral aspects of the germ layers, including neural cell fates within a diffuse nervous system. Accordingly, centralization was added in the chordate line by neural-epidermal segregation, mediated by the pre-existing Bmp-Chordin axis. Finally, since hemichordates develop the mouth on the non-Bmp side, like arthropods but opposite to chordates, the mouth and Bmp-Chordin axis may have rearranged in the chordate line, one relative to the other.

Structural and functional evidence for a singular repertoire of BMP receptor signal transducing proteins in the lophotrochozoan Crassostrea gigas suggests a shared ancestral BMP/activin pathway

The transforming growth factor beta (TGF-beta) superfamily includes bone morphogenetic proteins, activins and TGF-betasensu stricto (s.s). These ligands, which transduce their signal through a heteromeric complex of type I and type II receptors, have been shown to play a key role in numerous biological processes including early embryonic development in both deuterostomes and ecdyzozoans. Lophochotrozoans, the third major group of bilaterian animals, have remained in the background of the molecular survey of metazoan development. We report the cloning and functional study of the central part of the BMP pathway machinery in the bivalve mollusc Crassostrea gigas (Cg-BMPR1 type I receptor and Cg-TGFbetasfR2 type II receptor), showing an unusual functional mode of signal transduction for this superfamily. The use of the zebrafish embryo as a reporter organism revealed that Cg-BMPR1, Cg-TGFbetasfR2, Cg-ALR I, an activin Type I receptor or their dominant negative acting truncated forms, when overexpressed during gastrulation, resulted in a range of phenotypes displaying severe disturbance of anterioposterior patterning, due to strong modulations of ventrolateral mesoderm patterning. The results suggest that Cg-BMPR1, and to a certain degree Cg-TGFbetasfR2 proteins, function in C. gigas in a similar way to their zebrafish orthologues. Finally, based on phylogenetic analyses, we propose an evolutionary model within the complete TGF-beta superfamily. Thus, evidence provided by this study argues for a possible conserved endomesoderm/ectomesoderm inductive mechanism in spiralians through an ancestral BMP/activin pathway in which the singular, promiscuous and probably unique Cg-TGFbetasfR2 would be the shared type II receptor interface for both BMP and activin ligands.

Presence of Proboscipedia and Caudal Gene Homologues in a Bivalve Mollusc

Homeobox genes encode a family of transcription factors that have essential roles in regulating the development of eukaryotes. Although they have been extensively studied in different phyla, relatively little is known about homeoboxcontaining genes and their function in molluscs. In this study, we used a polymerase chain reaction to investigate homeobox genes in the bivalve mollusc Pecten maximus. Four different homeobox sequences were identified; two were homologues of the non-Hox cluster gene caudal and the two remaining sequences had a significant homology to the ANT-C gene Proboscipedia. These sequences represent the first cad and pb homologues isolated from a member of the class Bivalvia, phylum Mollusca.

Expression of Pax258 in the gastropod statocyst: insights into the antiquity of metazoan geosensory organs

Most animals have sensory systems that allow them to balance and orient relative to the pull of gravity. Structures responsible for these functions range from very simple statocysts found in many aquatic invertebrates to the complex inner ear of mammals. Previous studies suggest that the specialized mechanosensory structures responsible for balance in vertebrates and insects may be homologous based on the requirement and expression of group II Pax genes (i.e., Pax-2/5/8 genes). Here we report the expression of a Pax-258 gene in the statocysts and other chemosensory and mechanosensory cells during the development of the gastropod mollusk Haliotis asinina, a member of the Lophotrochozoa. Based on the phylogenetic distribution of geosensory systems and the consistent expression of Pax-258 in the cells that form these systems, we propose that Pax-258, along with POU-III and -IV genes, has an ancient and conserved role in the formation of structures responsible for balance and geotaxis in eumetazoans.

Expression of anterior Hox genes during larval development of the gastropod Haliotis asinina

We report the spatial expression patterns of five anterior Hox genes during larval development of the gastropod mollusc Haliotis asinina, an unsegmented spiralian lophotrochozoan. Molecular alignments and phylogenetic analysis indicate that these genes are homologues of Drosophila HOM-C genes labial, proboscipedia, zen, Deformed, and Sex combs reduced; the abalone genes are named Has-Hox1, -Hox2, -Hox3, -Hox4, and -Hox5. Has-Hox transcripts are first detected in the free-swimming trochophore larval stage and restricted to the posttrochal ectoderm. Has-Hox2, -Hox3, and -Hox4 are expressed in bilaterally symmetrical and overlapping patterns in presumptive neuroectodermal cells on the ventral side of the trochophore. Has-Hox1 expression is restricted to a ring of cells on the dorsoposterior surface, corresponding to the outer mantle edge where new larval shell is being synthesized. There appears to be little change in the expression domains of these Has-Hox genes in pre- and posttorsional veliger larvae, with expression maintained in ectodermal and neuroectodermal tissues. Has-Hox2, -Hox3, -Hox4, and-Hox5 appear to be expressed in a colinear manner in the ganglia and connectives in the twisted nervous system. This pattern is not evident in older larvae. Has-Hox1 and-Hox4 are expressed in the margin of the mantle in the posttorsional veliger, suggesting that Hox genes play a role in gastropod shell formation.