The evolutionary history of Brachyury genes in Hydrozoa involves duplications, divergence, and neofunctionalization.

Brachyury, a member of T-box gene family, is widely known for its major role in mesoderm specification in bilaterians. It is also present in non-bilaterian metazoans, such as cnidarians, where it acts as a component of an axial patterning system. In this study, we present a phylogenetic analysis of Brachyury genes within phylum Cnidaria, investigate differential expression and address a functional framework of Brachyury paralogs in hydrozoan Dynamena pumila. Our analysis indicates two duplication events of Brachyury within the cnidarian lineage. The first duplication likely appeared in the medusozoan ancestor, resulting in two copies in medusozoans, while the second duplication arose in the hydrozoan ancestor, resulting in three copies in hydrozoans. Brachyury1 and 2 display a conservative expression pattern marking the oral pole of the body axis in D. pumila. On the contrary, Brachyury3 expression was detected in scattered presumably nerve cells of the D. pumila larva. Pharmacological modulations indicated that Brachyury3 is not under regulation of cWnt signaling in contrast to the other two Brachyury genes. Divergence in expression patterns and regulation suggest neofunctionalization of Brachyury3 in hydrozoans.

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.

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.

Local and global dynamics in collective movements of embryonic cells.

Several important morphogenetic processes belong to the category of collective cell movements (CCM), by which we mean coordinated rearrangements of many neighboring cells. The causes of the dynamic order established during CCM are still unclear. We performed statistical studies of rates and angular orientations of cell rearrangements in two kinds of embryonic tissues, which we categorized as "committed" (in the sense of being capable of autonomous CCM) as opposed to "naïve" tissues, which are those that require external forces in order to exhibit full scale CCM. In addition, we distinguished two types of cell rearrangements: first, those in which mutual cell-cell shifts characterizing the local dynamics (LD); and, second, those which moved in reference to common external coordinates (global dynamics, GD). We observed that in most cases LD rates deviated from normal distributions and do so by creating excesses of extensively converging and moderately diverging cells. In contrast, GD was characterized by nearly random behavior of slowly moving cells, combined with increased angular focusing of the fast cells trajectories as well as bimodal distribution of cell rates. When committed tissues were opposed by external mechanical forces, then they tended to preserve the inherent CCM patterns. On the other hand, the naïve ones reacted by creating two orthogonal cells flows, one of these coinciding with the force direction. We consider CCM as a self-organizing process based on feedbacks between converging and diverging cell shifts, which is able to focus the trajectories imposed by external forces.

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.

Expression of serotonergic system components during early Xenopus embryogenesis.

Despite abundant research studies on the physiological and biochemical nature of embryonic neurotransmitter function, little is known about the molecular genetic mechanisms involved. The expression of the main components of the serotonergic system during early Xenopus embryogenesis was investigated using RT-PCR, real time PCR and in situ hybridization. Transcripts encoding the serotonin receptors HTR2C and HTR7, as well as the vesicular monoamine transporter VMAT2, the serotonin transporter (SERT) and the serotonin synthesis enzymes tryptophan hydroxylase (TPH2) and aromatic amino acid decarboxylase (AAAD) were found to be expressed during the cleavage division stages, whereas the degradation enzyme monoamine oxidase A (MAOA) was absent. The main components of the serotonergic system were found to be expressed during the earliest stages of embryonic development. The embryonic transmitter mechanism, its complexity, and its variability among various species are discussed.

Active reinforcement of externally imposed folding in amphibians embryonic tissues.

Although the folding of epithelial layers is one of the most common morphogenetic events, the underlying mechanisms of this process are still poorly understood. We aimed to determine whether an artificial bending of an embryonic cell sheet, which normally remains flat, is reinforced and stabilized by intrinsic cell transformations. We observed both reinforcement and stabilization in double explants of blastocoel roof tissue from Xenopus early gastrula embryos. The reinforcement of artificial bending occurred over the course of a few hours and was driven by the gradual apical constriction and radial elongation of previously compressed cells situated at the bending arch of the concave layer of explant. Apical constriction was associated with actomyosin contraction and endocytosis-mediated engulfing of the apical cell membranes. Cooperative apical constrictions of the concave layer of cells produced a tensile force that extended over the entire surface of the explant and correlated with apical contraction of the concave side cells. In the explants taken from the anterior regions of the embryo, this reinforcement was more stable and the bending better expressed than in those taken from suprablastoporal areas. The morphogenetic role of cell responses to the bending force is discussed.