Origin of the trochophora larva

The gastropod Lottia peitaihoensis as a model to study the body patterning of trochophore larvae

The body patterning of trochophore larvae is important for understanding spiralian evolution and the origin of the bilateral body plan. However, considerable variations are observed among spiralian lineages, which have adopted varied strategies to develop trochophore larvae or even omit a trochophore stage. Some spiralians, such as patellogastropod mollusks, are suggested to exhibit ancestral traits by producing equal-cleaving fertilized eggs and possessing "typical" trochophore larvae. In recent years, we developed a potential model system using the patellogastropod Lottia peitaihoensis (= Lottia goshimai). Here, we introduce how the species were selected and establish sources and techniques, including gene knockdown, ectopic gene expression, and genome editing. Investigations on this species reveal essential aspects of trochophore body patterning, including organizer signaling, molecular and cellular processes connecting the various developmental functions of the organizer, the specification and behaviors of the endomesoderm and ectomesoderm, and the characteristic dorsoventral decoupling of Hox expression. These findings enrich the knowledge of trochophore body patterning and have important implications regarding the evolution of spiralians as well as bilateral body plans.

Postembryonic development and male paedomorphosis in Osedax (Siboglinidae, Annelida)

Most species of the bone-devouring marine annelid, Osedax, display distinct sexual dimorphism with macroscopic sedentary females rooted in bones and free-living microscopic dwarf males. The paedomorphic male resembles the non-feeding metatrochophore larva in size, presence of eight pairs of chaetae, and a head ciliation potentially representing a residual prototroch. The male development may thus uniquely reiterate and validate the theoretical heterochrony process "progenesis", which suggests that an accelerated sexual maturation and early arrest of somatic growth can lead to a miniaturized and paedomorphic adult. In this study, we describe the postembryonic larval and juvenile organogenesis of Osedax japonicus to test for a potential synchronous arrest of somatic growth during male development. Five postembryonic stages could be distinguished, resembling day one to five in the larval development at 10°C: (0D) first cleavage of fertilized eggs (embryos undergo unequal spiral cleavage), (1D) pre-trochophore, with apical organ, (2D) early trochophore, + prototroch, brain, circumesophageal connectives and subesophageal commissure, (3D) trochophore, + telotroch, four ventral nerves, (4D) early metatrochophore, + protonephridia, dorsal and terminal sensory organs, (5D) metatrochophore, + two ventral paratrochs, mid-ventral nerve, posterior trunk commissure, two dorsal nerves; competent for metamorphosis. The larval development largely mirrors that of other lecithotrophic annelid larvae but does not show continuous chaetogenesis or full gut development. Additionally, O. japonicus larvae exhibit an unpaired, mid-dorsal, sensory organ. Female individuals shed their larval traits during metamorphosis and continue organogenesis (including circulatory system) and extensive growth for 2-3 weeks before developing oocytes. In contrast, males develop sperm within a day of metamorphosis and display a synchronous metamorphic arrest in neural and muscular development, retaining a large portion of larval features post metamorphosis. Our findings hereby substantiate male miniaturization in Osedax to be the outcome of an early and synchronous offset of somatic development, fitting the theoretical process "progenesis". This may be the first compelling morpho-developmental exemplification of a progenetic origin of a microscopic body plan. The presented morphological staging system will further serve as a framework for future examination of molecular patterns and pathways determining Osedax development.

The Mediterranean mussel Mytilus galloprovincialis: a novel model for developmental studies in mollusks

A model organism in developmental biology is defined by its experimental amenability and by resources created for the model system by the scientific community. For the most powerful invertebrate models, the combination of both has already yielded a thorough understanding of developmental processes. However, the number of developmental model systems is still limited, and their phylogenetic distribution heavily biased. Members of one of the largest animal lineages, the Spiralia, for example, have long been neglected. In order to remedy this shortcoming, we have produced a detailed developmental transcriptome for the bivalve mollusk Mytilus galloprovincialis, and have expanded the list of experimental protocols available for this species. Our high-quality transcriptome allowed us to identify transcriptomic signatures of developmental progression and to perform a first comparison with another bivalve mollusk: the Pacific oyster Crassostrea gigas. To allow co-labelling studies, we optimized and combined protocols for immunohistochemistry and hybridization chain reaction to create high-resolution co-expression maps of developmental genes. The resources and protocols described here represent an enormous boost for the establishment of Mytilus galloprovincialis as an alternative model system in developmental biology.

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.

Single-Cell RNA Sequencing Atlas From a Bivalve Larva Enhances Classical Cell Lineage Studies

Ciliated trochophore-type larvae are widespread among protostome animals with spiral cleavage. The respective phyla are often united into the superclade Spiralia or Lophotrochozoa that includes, for example, mollusks, annelids, and platyhelminths. Mollusks (bivalves, gastropods, cephalopods, polyplacophorans, and their kin) in particular are known for their morphological innovations and lineage-specific plasticity of homologous characters (e.g., radula, shell, foot, neuromuscular systems), raising questions concerning the cell types and the molecular toolkit that underlie this variation. Here, we report on the gene expression profile of individual cells of the trochophore larva of the invasive freshwater bivalve Dreissena rostriformis as inferred from single cell RNA sequencing. We generated transcriptomes of 632 individual cells and identified seven transcriptionally distinct cell populations. Developmental trajectory analyses identify cell populations that, for example, share an ectodermal origin such as the nervous system, the shell field, and the prototroch. To annotate these cell populations, we examined ontology terms from the gene sets that characterize each individual cluster. These were compared to gene expression data previously reported from other lophotrochozoans. Genes expected to be specific to certain tissues, such as Hox1 (in the shell field), Caveolin (in prototrochal cells), or FoxJ (in other cillia-bearing cells) provide evidence that the recovered cell populations contribute to various distinct tissues and organs known from morphological studies. This dataset provides the first molecular atlas of gene expression underlying bivalve organogenesis and generates an important framework for future comparative studies into cell and tissue type development in Mollusca and Metazoa as a whole.

Molluscan dorsal-ventral patterning relying on BMP2/4 and Chordin provides insights into spiralian development and evolution

Although a conserved mechanism relying on BMP2/4 and Chordin is suggested for animal dorsal–ventral (DV) patterning, this mechanism has not been reported in spiralians, one of the three major clades of bilaterians. Studies on limited spiralian representatives have suggested markedly diverse DV patterning mechanisms, a considerable number of which no longer deploy BMP signaling. Here, we showed that BMP2/4 and Chordin regulate DV patterning in the mollusk Lottia goshimai, which was predicted in spiralians but not previously reported. In the context of the diverse reports in spiralians, it conversely represents a relatively unusual case. We showed that BMP2/4 and Chordin coordinate to mediate signaling from the D-quadrant organizer to induce the DV axis, and Chordin relays the symmetry-breaking information from the organizer. Further investigations on L. goshimai embryos with impaired DV patterning suggested roles of BMP signaling in regulating the behavior of the blastopore and the organization of the nervous system. These findings provide insights into the evolution of animal DV patterning and the unique development mode of spiralians driven by the D-quadrant organizer.

Diversity in the Development of the Neuromuscular System of Nemertean Larvae (Nemertea, Spiralia)

In studies on the development of nervous systems and musculature, fluorescent labeling of neuroactive substances and filamentous actin (f-actin) of muscle cells and the subsequent analysis with confocal laser scanning microscopy (CLSM), has led to a broad comparative data set for the majority of the clades of the superphylum Spiralia. However, a number of clades remain understudied, which results in gaps in our knowledge that drastically hamper the formulation of broad-scale hypotheses on the evolutionary developmental biology (EvoDevo) of the structures in question. Regarding comparative data on the development of the peptidergic nervous system and the musculature of species belonging to the spiralian clade Nemertea (ribbon worms), such considerable knowledge gaps are manifest. This paper presents first findings on fluorescent labeling of the FMRFamide-like component of the nervous system and contributes additional data on the muscle development in the presently still underrepresented larvae of palaeo- and hoplonemertean species. Whereas the architecture of the FMRFamide-like nervous system is comparably uniform between the studied representatives, the formation of the musculature differs considerably, exhibiting developmental modes yet undescribed for any spiralian species. The presented results fill a significant gap in the spiralian EvoDevo data set and thus allow for further elaboration of hypotheses on the ancestral pattern of the musculature and a prominent component of the nervous system in Nemertea. However, with respect to the variety observed, it is expected that the true diversity of the developmental pathways is still to be discovered when more detailed data on other nemertean species will be available.

Multiple origins of feeding head larvae by the Early Cambrian

In many animals the head develops early, most of the body axis later. A larva composed mostly of the developing front end therefore can attain mobility and feeding earlier in development. Fossils, functional morphology, and inferred homologies indicate that feeding head larvae existed by the Early Cambrian in members of three major clades of animals: ecdysozoans, lophotrochozoans, and deuterostomes. Some of these early larval feeding mechanisms were also those of juveniles and adults (the lophophore of brachiopod larvae and possibly the ciliary band of the dipleurula of hemichordates and echinoderms); some were derived from structures that previously had other functions (appendages of the nauplius). Trochophores that swim with a preoral band of cilia, the prototroch, originated before divergence of annelids and molluscs, but evidence of larval growth and thus a prototrochal role in feeding is lacking for molluscs until the Ordovician. Feeding larvae that definitely originated much later, as in insects, teleost fish, and amphibians, develop all or nearly all of what will become the adult body axis before they begin feeding. On present evidence, head larvae, including feeding head larvae, evolved multiple times early in the evolution of bilaterian animals and never since.

Evolutionary transcriptomics of metazoan biphasic life cycle supports a single intercalation origin of metazoan larvae

The transient larva-bearing biphasic life cycle is the hallmark of many metazoan phyla, but how metazoan larvae originated remains a major enigma in animal evolution. There are two hypotheses for larval origin. The 'larva-first' hypothesis suggests that the first metazoans were similar to extant larvae, with later evolution of the adult-added biphasic life cycle; the 'adult-first' hypothesis suggests that the first metazoans were adult forms, with the biphasic life cycle arising later via larval intercalation. Here, we investigate the evolutionary origin of primary larvae by conducting ontogenetic transcriptome profiling for Mollusca-the largest marine phylum characterized by a trochophore larval stage and highly variable adult forms. We reveal that trochophore larvae exhibit rapid transcriptome evolution with extraordinary incorporation of novel genes (potentially contributing to adult shell evolution), and that cell signalling/communication genes (for example, caveolin and innexin) are probably crucial for larval evolution. Transcriptome age analysis of eight metazoan species reveals the wide presence of young larval transcriptomes in both trochozoans and other major metazoan lineages, therefore arguing against the prevailing larva-first hypothesis. Our findings support an adult-first evolutionary scenario with a single metazoan larval intercalation, and suggest that the first appearance of proto-larva probably occurred after the divergence of direct-developing Ctenophora from a metazoan ancestor.

Evolution of the bilaterian mouth and anus

It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.