From apolar gastrula to polarized larva: Embryonic development of a marine hydroid, Dynamena pumila.

BACKGROUND: In almost all metazoans examined to this respect, the axial patterning system based on canonical Wnt (cWnt) signaling operates throughout the course of development. In most metazoans, gastrulation is polar, and embryos develop morphological landmarks of axial polarity, such as blastopore under control/regulation from cWnt signaling. However, in many cnidarian species, gastrulation is morphologically apolar. The question remains whether сWnt signaling providing the establishment of a body axis controls morphogenetic processes involved in apolar gastrulation. RESULTS: In this study, we focused on the embryonic development of Dynamena pumila, a cnidarian species with apolar gastrulation. We thoroughly described cell behavior, proliferation, and ultrastructure and examined axial patterning in the embryos of this species. We revealed that the first signs of morphological polarity appear only after the end of gastrulation, while molecular prepatterning of the embryo does exist during gastrulation. We have shown experimentally that in D. pumila, the direction of the oral-aboral axis is highly robust against perturbations in cWnt activity. CONCLUSIONS: Our results suggest that morphogenetic processes are uncoupled from molecular axial patterning during gastrulation in D. pumila. Investigation of D. pumila might significantly expand our understanding of the ways in which morphological polarization and axial molecular patterning are linked in Metazoa.

A cadherin switch marks germ layer formation in the diploblastic sea anemone Nematostella vectensis.

Morphogenesis is a shape-building process during development of multicellular organisms. During this process, the establishment and modulation of cell-cell contacts play an important role. Cadherins, the major cell adhesion molecules, form adherens junctions connecting epithelial cells. Numerous studies of Bilateria have shown that cadherins are associated with the regulation of cell differentiation, cell shape changes, cell migration and tissue morphogenesis. To date, the role of cadherins in non-bilaterians is unknown. Here, we study the expression and function of two paralogous classical cadherins, Cadherin 1 and Cadherin 3, in a diploblastic animal, the sea anemone Nematostella vectensis We show that a cadherin switch accompanies the formation of germ layers. Using specific antibodies, we show that both cadherins are localized to adherens junctions at apical and basal positions in ectoderm and endoderm. During gastrulation, partial epithelial-to-mesenchymal transition of endodermal cells is marked by stepwise downregulation of Cadherin 3 and upregulation of Cadherin 1. Knockdown experiments show that both cadherins are required for maintenance of tissue integrity and tissue morphogenesis. Thus, both sea anemones and bilaterians use independently duplicated cadherins combinatorially for tissue morphogenesis and germ layer differentiation.

сWnt signaling modulation results in a change of the colony architecture in a hydrozoan.

At the polyp stage, most hydrozoan cnidarians form highly elaborate colonies with a variety of branching patterns, which makes them excellent models for studying the evolutionary mechanisms of body plan diversification. At the same time, molecular mechanisms underlying the robust patterning of the architecturally complex hydrozoan colonies remain unexplored. Using non-model hydrozoan Dynamena pumila we showed that the key components of the Wnt/ß-catenin (cWnt) pathway (ß-catenin, TCF) and the cWnt-responsive gene, brachyury 2, are involved in specification and patterning of the developing colony shoots. Strikingly, pharmacological modulation of the cWnt pathway leads to radical modification of the monopodially branching colony of Dynamena which acquire branching patterns typical for colonies of other hydrozoan species. Our results suggest that modulation of the cWnt signaling is the driving force promoting the evolution of the vast variety of the body plans in hydrozoan colonies and offer an intriguing possibility that the involvement of the cWnt pathway in the regulation of branching morphogenesis might represent an ancestral feature predating the cnidarian-bilaterian split.

Epithelial folding in the morphogenesis of the colonial marine hydrozoan, Dynamena pumila.

Epithelial folding (EF) is a fundamental morphogenetic process that can be observed in the development of many organisms ranging from metazoans to green algae. Being early branching metazoans, cnidarians represent the best models to study evolutionarily conserved morphogenetic processes, including EF. Hydrozoa is the most evolutionary advanced group of the phylum Cnidaria. All colonial hydrozoans grow continuously, changing the shape of their colonies and spreading over the substrate with the help of elongating stolons. Owing to high diversity of colony architecture, they are ideal objects for comparative and evolutionary morphology. In the hydrozoan Dynamena pumila, the growth of the colony proceeds via a variety of morphogenetic processes. Our work is focused on the formation of the anchoring disc of the stolon, which is accompanied by inward-folding morphogenesis of the ectodermal layer. Successive stages of anchoring disc development were described with light, confocal transmission electron microscopy. We have shown that EF in Dynamena is associated with accumulation of F-actin in the constricting apical domains of forming bottle cells located at the bottom of the emerging fold. In addition, the nuclei of these cells are displaced to the basal domains. Taken together, these features may indicate that EF in Dynamena proceeds as an active invagination, although this process has never been described in the development of hydrozoans. Apparently, development of the anchoring disc can be viewed as a reliable and versatile model system for studying the cell-shape-change-driven epithelial sheet morphogenesis, which can be easily observed and analysed.

Embryonic development of thecate hydrozoan Gonothyraea loveni (Allman, 1859).

Progress of Evo-Devo requires broad phylogenetic sampling providing the data for comparative analysis as well as new objects suitable for experimental investigation. Representatives of the early-branching animal phylum Cnidaria and particularly hydrozoans draw great attention due to the high diversity of embryonic and post-embryonic development and life-cycles in general. Most detailed studies on embryonic development in hydrozoans were performed on the species shedding their gametes with subsequent embryo development in the water column. Widely distributed thecate hydrozoan Gonothyraea loveni broods its embryos within reduced medusae attached to the colony until development of a free-swimming metamorphosis competent planula-larva. In the current essay we present a detailed description of G. loveni embryonic development based on in vivo observations, histology, immuno-cytochemistry, and electron microscopy. Starting from early cleavage, the embryo becomes a morula without any sign of blastocoele. Gastrulation proceeds as mixed delamination and ends with parenchymula formation. The first morphological sign of primary body axis appears only in the beginning of parenchymula-preplanula transition. In mature metamorphosis competent planula only the cells of the oral two-thirds of endoderm retain proliferative activity resulting in accumulation of great number of i-cells and nematoblasts, which can be used during metamorphosis accompanied with essential reorganization of larval tissues. G. loveni demonstrates the diversity as well as evolutionary plasticity of hydrozoans development: in brooding hydrozoans embryonic and larval development is highly embryonized in comparison with the spawning species with free-swimming embryos.

Morphomechanical programming of morphogenesis in cnidarian embryos.

The factors governing the pattern formation process in the early morphogenesis of a marine colonial hydroid, Dynamena pumila, have been studied. Two different types of morphogenesis have been distinguished. Morphogenesis of the first type goes on via changes in cell shape and cell axis orientation, while morphogenesis of the second type is based upon the active coordinated cell movements associated with cell rearrangements. It was shown that morphogenesis of both types can be considered as cascades in which any event is a consequence of the previous one. The spatial structure of each developmental stage contains information about the direction and the initial conditions of further morphogenesis. So, an "epigenetic program" of morphogenesis gradually originates in the course of development and provides the stable reproduction of spatial structures. It is reasonable to consider the activity of epigenetic factors guiding Dynamena morphogenesis (geometry/topology of an embryo, heterogeneity of an embryo spatial structure, configuration of the field of mechanical stresses of the embryo surface) as "morphomechanical programming" of morphogenesis.