Alterations in ambient salinity and pH lead to modulation of developmental gene expression in Microhyla ornata (Duméril and Bibron) and Xenopus laevis (Daudin).

Naturally fertilized Microhyla ornata and Xenopus laevis embryos at dorsal lip of blastopore stage were exposed to 0.3, and 0.6% sodium chloride for high salinity treatment and dilute hydrochloric acid and sodium hydroxide for treatment with low and high pH, respectively. After treatment for different durations, embryos were studied morphologically and using in situ hybridization with selected genes important for normal development and for coping with environmental stress. Altered salinity and pH caused defects in axis formation and neural tube closure, delay in hatching, abnormal swimming of tadpoles and even developmental arrest. This was accompanied by significant decrease in the expression of selected development-regulating genes like goosecoid (required for gastrulation movements), brachyury (mesodermal marker gene), noggin (involved in neural induction), NCAM (required for neural cell adhesion) and MyoD (essential for muscle development), and considerable increase in the transcription of stress response genes hsp30 and hsp70. Altering the expression of embryonic genes could be one of the mechanisms through which environmental factors influence development of amphibian embryos.

Evo-devo: Hydra raises its Noggin.

Noggin, along with other secreted bone morphogenetic protein (BMP) inhibitors, plays a crucial role in neural induction and neural tube patterning as well as in somitogenesis, cardiac morphogenesis and formation of the skeleton in vertebrates. The BMP signalling pathway is one of the seven fundamental pathways that drive embryonic development and pattern formation in animals. Understanding its evolutionary origin and role in pattern formation is, therefore, important to evolutionary developmental biology (evo-devo).We have studied the evolutionary origin of BMP–Noggin antagonism in hydra, which is a powerful diploblastic model to study evolution of pattern-forming mechanisms because of the unusual cellular dynamics during its pattern formation and its remarkable ability to regenerate. We cloned and characterized the noggin gene from hydra and found it to exhibit considerable similarity with its orthologues at the amino acid level. Microinjection of hydra Noggin mRNA led to duplication of the dorsoventral axis in Xenopus embryos, demonstrating its functional conservation across the taxa. Our data, along with those of others, indicate that the evolutionarily conserved antagonism between BMP and its inhibitors predates bilateral divergence. This article reviews the various roles of Noggin in different organisms and some of our recent work on hydra Noggin in the context of evolution of developmental signalling pathways.

Genome-wide screening reveals the emergence and divergence of RTK homologues in basal Metazoan Hydra magnipapillata.

Receptor tyrosine kinases (RTKs) are key components of cell–cell signalling required for growth and development of multicellular organisms. It is therefore likely that the divergence of RTKs and associated components played a significant role in the evolution of multicellular organisms. We have carried out the present study in hydra, a diploblast, to investigate the divergence of RTKs after parazoa and before emergence of triploblast phyla. The domain-based screening using Hidden Markov Models (HMMs) for RTKs in Genomescan predicted gene models of the Hydra magnipapillata genome resulted in identification of 15 RTKs. These RTKs have been classified into eight families based on domain architecture and homology. Only 5 of these RTKs have been previously reported and a few of these have been partially characterized. A phylogeny-based analysis of these predicted RTKs revealed that seven subtype duplications occurred between ‘parazoan–eumetazoan split’ and ‘diploblast–triploblast split’ in animal phyla. These results suggest that most of the RTKs evolved before the radiata–bilateria divergence during animal evolution.

Modulation of cell surface architecture in gastrulating chick embryo in response to altered fibroblast growth factor (FGF) signaling.

Gastrulation is a fundamental process that results in formation of the three germ layers in an embryo. It involves highly coordinated cell migration. Cell to cell communication through cell surface and the surrounding molecular environment governs cell migration. In the present work, cell surface features, which are indicative of the migratory status of a cell, of an early gastrulating chick embryo were studied using scanning electron microscopy. The distinct ultrastructural features of cells located in the various regions of the epiblast are described. Differences in the surface features of cells from distinct embryonic regions indicate differences in their migratory capacities. Further, the dynamic nature of these cell surface features by their response to altered fibroblast growth factor (FGF) signaling, experimentally created by using either excess FGF or inhibition of FGF signaling are demonstrated.

Ultraviolet irradiation initiates ectopic foot formation in regenerating hydra and promotes budding.

We have studied the effects of ultraviolet-C (UVC) and Ultraviolet-B (UVB) on growth and pattern formation in Pelmatohydra oligactis. UVC brings about a significant increase in budding in intact hydra while UVB does not exhibit such an effect. Excessive budding could be a response for survival at wavelengths that damage biological tissues. If the head or base piece of a bisected hydra is irradiated and recombined with the unirradiated missing part, regeneration proceeds normally indicating that exposure of a body part with either an intact head or foot to UVC does not influence pattern formation. Most significantly, in the middle piece, but not in the head or the base piece of a trisected hydra, UVC leads to initiation of ectopic feet formation in almost one third of the cases. Thus, UV irradiation interferes with pattern formation in regenerating hydra, possibly by changing positional values, and promotes budding in intact hydra. This is the first report on induction of ectopic feet formation by UV in regenerating hydra and opens up the possibility of using UV irradiation as a tool to understand pattern formation in the enigmatic hydra.

Mesoderm induction in amphibians and chick.

Induction is a process in which the developmental pathway of one cell is controlled by signals emitted from another. Mesoderm induction is the first inductive interaction in the Xenopus enbryo and probably occurs in all vertebrates. It is a very important event as it is implicated in the regulation of morphogenesis. Nieuwkoop first demonstrated the importance of vegetal endoderm in inducing the mesoderm. Slack and co-workers incorporated the information obtained from experimental embryology in a "three signal" model for mesoderm induction in amphibians (signals arising from ventral vegetal hemisphere, dorsal vegetal hemisphere and the organizer). More recent research has resulted in the detection of mesoderm inducing factors which are members of FGF and TGF--β families. Activin, a member of the TGF-ß family, has been shown to induce differential gene expression and cell differentiation in a concentrationdependent manner giving credence to the theory of morphogen gradients. Study of mesoderm induction in the chick embryo is much more difficult due to several reasons. Novel experimental approaches, however, have been used which point to the role of activin and FGF in chick mesoderm induction. The demonstration of mesoderm inducing activity of activin and FGF in other groups of vertebrates, particularly the chick embryo brings out the possibility of a universal mechanism of mesoderm induction being operative in all the vertebrates.

Immunoneutralization of insulin partially modifies dorsoventral patterning in developing frog embryos.

Determination of anteroposterior and dorsoventral axes is an important early event in the development of vertebrates involving extensive cellular interactions including inductive events. Recently we showed that insulin plays an essential role in prepancreatic development of the frog Microhyla ornata. In the present study we have investigated the effects of immunoneutralization of endogenous insulin on the process of pattern formation. Treatment of neurulating embryos with antiserum to insulin caused abnormal pattern formation. The defects included loss of normal architecture of the neural tube, reduction in the size of the neural tube and, most conspicuously, rotation of the dorsoventral axis of the neural tube, notochord and adjoining mesodermal elements. The effects could be alleviated partially by pretreatment of embryos with exogenous insulin. This supports our belief that insulin plays an important role in induction and pattern formation of the amphibian nervous system. In addition, using 2-deoxy-α-D-glucose, an inhibitor of glucose metabolism, it is shown that the stimulatory effects of exogenous insulin on developing frog embryos are, at least partially, through the glucose metabolism pathway.