Hydra, a fruitful model system for 270 years

The discovery of Hydra regeneration by Abraham Trembley in 1744 promoted much scientific curiosity thanks to his clever design of experimental strategies away from the natural environment. Since then, this little freshwater cnidarian polyp flourished as a potent and fruitful model system. Here, we review some general biological questions that benefitted from Hydra research, such as the nature of embryogenesis, neurogenesis, induction by organizers, sex reversal, symbiosis, aging, feeding behavior, light regulation, multipotency of somatic stem cells, temperature-induced cell death, neuronal transdifferentiation, to cite only a few. To understand how phenotypes arise, theoricists also chose Hydra to model patterning and morphogenetic events, providing helpful concepts such as reaction-diffusion, positional information, and autocatalysis combined with lateral inhibition. Indeed, throughout these past 270 years, scientists used transplantation and grafting experiments, together with tissue, cell and molecular labelings, as well as biochemical procedures, in order to establish the solid foundations of cell and developmental biology. Nowadays, thanks to transgenic, genomic and proteomic tools, Hydra remains a promising model for these fields, but also for addressing novel questions such as evolutionary mechanisms, maintenance of dynamic homeostasis, regulation of stemness, functions of autophagy, cell death, stress response, innate immunity, bioactive compounds in ecosystems, ecotoxicant sensing and science communication.     

The Hydra genome: insights, puzzles and opportunities for developmental biologists

The sequencing of a Hydra genome marked the beginning of a new era in the use of Hydra as a developmental model. Analysis of the genome sequence has led to a number of interesting findings, has required revisiting of previous work, and most importantly presents new opportunities for understanding the developmental biology of Hydra. This review will de-scribe the history of the Hydra genome project, a selection of results from it that are relevant to developmental biologists, and some future research opportunities provided by Hydra genomics.     

What Hydra can teach us about chemical ecology -how a simple, soft organism survives in a hostile aqueous environment

Hydra and its fellow cnidarians - sea anemones, corals and jellyfish - are simple, mostly sessile animals that depend on bioactive chemicals for survival. In this review, we briefly describe what is known about the chemical armament of Hydra, and detail future research directions where Hydra can help illuminate major questions in chemical ecology, pharmacology, developmental biology and evolution. Focusing on two groups of putative toxins from Hydra - phospholipase A2s and proteins containing ShK and zinc metalloprotease domains, we ask: how do different venom components act together during prey paralysis? How is a venom arsenal created and how does it evolve? How is the chemical arsenal delivered to its target? To what extent does a chemical and biotic coupling exist between an organism and its environment? We propose a model whereby in Hydra and other cnidarians, bioactive compounds are secreted both as localized point sources (nematocyte discharges) and across extensive body surfaces, likely combining to create complex "chemical landscapes". We speculate that these cnidarian-derived chemical landscapes may affect the surrounding community on scales from microns to, in the case of coral reefs, hundreds of kilometers.     

The Hydra model - a model for what?

The introductory personal remarks refer to my motivations for choosing research projects, and for moving from physics to molecular biology and then to development, with Hydra as a model system. Historically, Trembley's discovery of Hydra regeneration in 1744 was the beginning of developmental biology as we understand it, with passionate debates about preformation versus de novo generation, mechanisms versus organisms. In fact, seemingly conflicting bottom-up and top-down concepts are both required in combination to understand development. In modern terms, this means analysing the molecules involved, as well as searching for physical principles underlying development within systems of molecules, cells and tissues. During the last decade, molecular biology has provided surprising and impressive evidence that the same types of molecules and molecular systems are involved in pattern formation in a wide range of organisms, including coelenterates like Hydra, and thus appear to have been "invented" early in evolution. Likewise, the features of certain systems, especially those of developmental regulation, are found in many different organisms. This includes the generation of spatial structures by the interplay of self-enhancing activation and "lateral" inhibitory effects of wider range, which is a main topic of my essay. Hydra regeneration is a particularly clear model for the formation of defined patterns within initially near-uniform tissues. In conclusion, this essay emphasizes the analysis of development in terms of physical laws, including the application of mathematics, and insists that Hydra was, and will continue to be, a rewarding model for understanding general features of embryogenesis and regeneration.     

Transplantation analysis of developmental mechanisms in Hydra

Since the pioneering work of Ethel Browne (1909) who demonstrated for the first time the concept of organizer activity, i.e. the potency of an apical Hydra tissue to induce a secondary axis when transplanted onto a host, Hydra flourished as a fruitful model system for developmental studies. Over the next 60 years this efficient transplantation approach identified graded biological activities along the body column of Hydra named Head Acti-vation and Head Inhibition. These properties inspired theoretical modelers including Lewis Wolpert, Alfred Gierer and Hans Meinhardt to propose models for morphogenesis, respectively the positional information (1969) and reaction-diffusion (1972) models. In 1973, Tsutomu Sugiyama and Toshitaka Fujisawa initiated in Mishima a unique project to analyze the properties of Hydra strains with distinct morphological and developmental characters. To this end, they collected in several areas of Japan multiple Hydra strains that they subsequently characterized and crossed. They also established a lateral transplantation strategy that was much more powerful than the previous ones, as it combined quantitative measurements with cellular analyses thanks to the chimera procedures developed by Campbell and colleagues. In-deed this approach provided a paradigm to quantify in any morphological phenotype the Head Activation and Head Inhibition levels along the body column. In this article, I review the various strains identified by Sugiyama and colleagues, the principles and the main results deduced from the quantitative lateral transplantation strategy. In addition, I briefly discuss the relevance of this approach in the era of molecular biology.     

Hydra, a versatile model to study the homeostatic and developmental functions of cell death

In the freshwater cnidarian polyp Hydra, cell death takes place in multiple contexts. Indeed apoptosis occurs during oogenesis and spermatogenesis, during starvation, and in early head regenerating tips, promoting local compensatory proliferation at the boundary between heterografts. Apoptosis can also be induced upon exposure to pro-apoptotic agents (colchicine, wortmannin), upon heat-shock in the thermosensitive sf-1 mutant, and upon wounding. In all these contexts, the cells that undergo cell death belong predominantly to the interstitial cell lineage, whereas the epithelial cells, which are rather resistant to pro-apoptotic signals, engulf the apoptotic bodies. Beside this clear difference between the interstitial and the epithelial cell lineages, the different interstitial cell derivatives also show noticeable variations in their respective apoptotic sensitivity, with the precursor cells appearing as the most sensitive to pro-apoptotic signals. The apoptotic machinery has been well conserved across evolution. However, its specific role and regulation in each context are not known yet. Tools that help characterize apoptotic activity in Hydra have recently been developed. Among them, the aposensor Apoliner initially designed in Drosophila reliably measures wortmannin-induced apoptotic activity in a biochemical assay. Also, flow cytometry and TUNEL analyses help identify distinctive features between wortmannin-induced and heat-shock induced apoptosis in the sf-1 strain. Thanks to the live imaging tools already available, Hydra now offers a model system with which the functions of the apoptotic machinery to maintain long-term homeostasis, stem cell renewal, germ cell production, active developmental processes and non-self response can be deciphered.     

Emerging model systems in eco-evo-devo: the environmentally responsive spadefoot toad

Spadefoot toads have emerged as a model system for addressing fundamental questions in ecological and evolutionary developmental biology (eco-evo-devo). Their tadpoles produce a wide range of adaptive phenotypes in direct response to diverse environmental stimuli. Such phenotypic plasticity offers an excellent opportunity to examine how an organism's ecology affects its development as well as how an organism's development influences its ecology and evolution. By characterizing and understanding the interconnectedness between an organism's environment, its development responses, and its ecological interactions in natural populations, such research promises to clarify further the role of the environment in not only selecting among diverse phenotypes, but also creating such phenotypes in the first place.     

Hymyc1 downregulation promotes stem cell proliferation in Hydra vulgaris

Hydra is a unique model for studying the mechanisms underlying stem cell biology. The activity of the three stem cell lineages structuring its body constantly replenishes mature cells lost due to normal tissue turnover. By a poorly understood mechanism, stem cells are maintained through self-renewal while concomitantly producing differentiated progeny. In vertebrates, one of many genes that participate in regulating stem cell homeostasis is the protooncogene c-myc, which has been recently identified also in Hydra, and found expressed in the interstitial stem cell lineage. In the present paper, by developing a novel strategy of RNA interference-mediated gene silencing (RNAi) based on an enhanced uptake of small interfering RNAi (siRNA), we provide molecular and biological evidence for an unexpected function of the Hydra myc gene (Hymyc1) in the homeostasis of the interstitial stem cell lineage. We found that Hymyc1 inhibition impairs the balance between stem cell self renewal/differentiation, as shown by the accumulation of stem cell intermediate and terminal differentiation products in genetically interfered animals. The identical phenotype induced by the 10058-F4 inhibitor, a disruptor of c-Myc/Max dimerization, demonstrates the specificity of the RNAi approach. We show the kinetic and the reversible feature of Hymyc1 RNAi, together with the effects displayed on regenerating animals. Our results show the involvement of Hymyc1 in the control of interstitial stem cell dynamics, provide new clues to decipher the molecular control of the cell and tissue plasticity in Hydra, and also provide further insights into the complex myc network in higher organisms. The ability of Hydra cells to uptake double stranded RNA and to trigger a RNAi response lays the foundations of a comprehensive analysis of the RNAi response in Hydra allowing us to track back in the evolution and the origin of this process.     

Developmental signaling in Hydra: what does it take to build a "simple" animal?

Developmental processes in multicellular animals depend on an array of signal transduction pathways. Studies of model organisms have identified a number of such pathways and dissected them in detail. However, these model organisms are all bilaterians. Investigations of the roles of signal transduction pathways in the early-diverging metazoan Hydra have revealed that a number of the well-known developmental signaling pathways were already in place in the last common ancestor of Hydra and bilaterians. In addition to these shared pathways, it appears that developmental processes in Hydra make use of pathways involving a variety of peptides. Such pathways have not yet been identified as developmental regulators in more recently diverged animals. In this review I will summarize work to date on developmental signaling pathways in Hydra and discuss the future directions in which such work will need to proceed to realize the potential that lies in this simple animal.     

The hydroid Hydractinia: a versatile, informative cnidarian representative

The Cnidaria represent the most ancient eumetazoan phylum. Members of this group possess typical animal cells and tissues such as sensory cells, nerve cells, muscle cells and epithelia. Due to their unique phylogenetic position, cnidarians have traditionally been used as a reference group in various comparative studies. We propose the colonial marine hydroid, Hydractinia, as a convenient, versatile platform for basic and applied research in developmental biology, reproduction, immunology, environmental studies and more. In addition to being a typical cnidarian representative, Hydractinia offers many practical and theoretical advantages: studies that are feasible in Hydra like regeneration, pattern regulation, and cell renewal from stem cells, can be supplemented by genetic analyses and classical embryology in Hydractinia. Metamorphosis of the planula larva of Hydractinia can be used as a model for cell activation and communication and the presence of a genetically controlled allorecognition system makes it a suitable model for comparative immunology. Most importantly, Hydractinia may be manipulated at most aspects of its (short) life cycle. It has already been the subject of many studies in various disciplines, some of which are discussed in this essay.     

How to use Hydra as a model system to teach biology in the classroom

As scientists it is our duty to fight against obscurantism and loss of rational thinking if we want politicians and citizens to freely make the most intelligent choices for the future generations. With that aim, the scientific education and training of young students is an obvious and urgent necessity. We claim here that Hydra provides a highly versatile but cheap model organism to study biology at any age. Teachers of biology have the unenviable task of motivating young people, who with many other motivations that are quite valid, nevertheless must be guided along a path congruent with a 'syllabus' or a 'curriculum'. The biology of Hydra spans the history of biology as an experimental science from Trembley's first manipulations designed to determine if the green polyp he found was plant or animal to the dissection of the molecular cascades underpinning, regeneration, wound healing, stemness, aging and cancer. It is described here in terms designed to elicit its wider use in classrooms. Simple lessons are outlined in sufficient detail for beginners to enter the world of 'Hydra biology'. Protocols start with the simplest observations to experiments that have been pretested with students in the USA and in Europe. The lessons are practical and can be used to bring 'life', but also rational thinking into the study of life for the teachers of students from elementary school through early university.     

Stemness in Hydra - a current perspective

Hydra is a classic and simple model for pattern formation and regeneration research. More recently, it has also been promoted as a model to study ancestral stem cell biology. Three independent cell lineages form the body of the polyp and exhibit characteristics of stem cell systems. In order to define differences in stemness between the ectodermal and endodermal epitheliomuscular cell lineages and the interstitial cell lineage, we compare cellular properties and decision making. We argue that these three lineages are expected to show substantial variation in their stemness-related gene regulatory networks. Finally, we discuss Wnt signalling pathways and Myc oncoproteins, which are beginning to offer a perspective on how proliferation and differentiation might be regulated.     

Caenorhabditis evolution: if they all look alike, you aren't looking hard enough

Caenorhabditis elegans is widely known as a model organism for cell, molecular, developmental and neural biology, but it is also being used for evolutionary studies. A recent meeting of researchers in Portugal covered topics as diverse as phylogenetics, genetic mapping of quantitative and qualitative intraspecific variation, evolutionary developmental biology and population genetics. Here, we summarize the main findings of the meeting, which marks the formal birth of a research community dedicated to Caenorhabditis species evolution.     

Discovery of genes expressed in Hydra embryogenesis

Hydra's remarkable capacity to regenerate, to proliferate asexually by budding, and to form a pattern de novo from aggregates allows studying complex cellular and molecular processes typical for embryonic development. The underlying assumption is that patterning in adult hydra tissue relies on factors and genes which are active also during early embryogenesis. Previously, we reported that in Hydra the timing of expression of conserved regulatory genes, known to be involved in adult patterning, differs greatly in adults and embryos (Fr?bius, A.C., Genikhovich, G., Kürn, U., Anton-Erxleben, F. and Bosch, T.C.G., 2003. Expression of developmental genes during early embryogenesis of Hydra. Dev. Genes Evol. 213, 445-455). Here, we describe an unbiased screening strategy to identify genes that are relevant to Hydra vulgaris embryogenesis. The approach yielded two sets of differentially expressed genes: one set was expressed exclusively or nearly exclusively in the embryos, while the second set was upregulated in embryos in comparison to adult polyps. Many of the genes identified in hydra embryos had no matches in the database. Among the conserved genes upregulated in embryos is the Hydra orthologue of Embryonic Ectoderm Development (HyEED). The expression pattern of HyEED in developing embryos suggests that interstitial stem cells in Hydra originate in the endoderm. Importantly, the observations uncover previously unknown differences in genes expressed by embryos and polyps and indicate that not only the timing of expression of developmental genes but also the genetic context is different in Hydra embryos compared to adults.     

Model systems in stem cell biology

Stem cell scientists and ethicists have focused intently on questions relevant to the developmental stage and developmental capacities of stem cells. Comparably less attention has been paid to an equally important set of questions about the nature of stem cells, their common characteristics, their non-negligible differences and their possible developmental species specificity. Answers to these questions are essential to the project of justly inferring anything about human stem cell biology from studies in non-human model systems--and so to the possibility of eventually developing human therapies based on stem cell biology. After introducing and discussing these questions, I conclude with a brief discussion of the creation of novel model systems in stem cell biology: human-to-animal embryonic chimeras. Such novel model systems may help to overcome obstacles to extrapolation, but they are also scientifically and ethically contentious.     

Four-dimensional imaging of cytoskeletal dynamics in Xenopus oocytes and eggs

The Xenopus laevis (African clawed frog) system has long been popular for studies of both developmental and cell biology, based on a variety of its intrinsic features including the large size of Xenopus oocytes, eggs, and embryos, and the relative ease of manipulation. Unfortunately, the large size has also been considered a serious impediment for high-resolution light microscopy, as has the heavy pigmentation. However, the recent development and exploitation of 4D imaging approaches, and the fact that much of what is of most interest to cell and developmental biologists takes place near the cell surface, indicates that such concerns are no longer valid. Consequently, the Xenopus system in many respects is now as good as other model systems considered to be ideal for microscopy-based studies. Here, 4D imaging and its recent applications to cytoskeletal imaging in Xenopus oocytes and eggs are discussed.     

Value of the Hydra model system for studying symbiosis

Green Hydra is used as a classical example for explaining symbiosis in schools as well as an excellent research model. Indeed the cosmopolitan green Hydra (Hydra viridissima) provides a potent experimental framework to investigate the symbiotic relationships between a complex eumetazoan organism and a unicellular photoautotrophic green algae named Chlorella. Chlorella populates a single somatic cell type, the gastrodermal myoepithelial cells (also named digestive cells) and the oocyte at the time of sexual reproduction. This symbiotic relationship is stable, well-determined and provides biological advantages to the algal symbionts, but also to green Hydra over the related non-symbiotic Hydra i.e. brown hydra. These advantages likely result from the bidirectional flow of metabolites between the host and the symbiont. Moreover genetic flow through horizontal gene transfer might also participate in the establishment of these selective advantages. However, these relationships between the host and the symbionts may be more complex. Thus, Jolley and Smith showed that the reproductive rate of the algae increases dramatically outside of Hydra cells, although this endosymbiont isolation is debated. Recently it became possible to keep different species of endosymbionts isolated from green Hydra in stable and permanent cultures and compare them to free-living Chlorella species. Future studies testing metabolic relationships and genetic flow should help elucidate the mechanisms that support the maintenance of symbiosis in a eumetazoan species.     

GFP expression in Hydra: lessons from the particle gun

The cnidarian Hydra is an important model organism to study pattern formation and tem cell differentiation. In the past, however, it has been difficult to study gene function in Hydra because the animals have hot been accessible to gene transfection studies, we have now developed a method to transiently express GFP-tagged proteins in Hydra using a green fluorescent protein (GFP) expression plasmid under the control of the Hydra actin promoter and a particle gun to introduce it into Hydra cell nuclei. We achieve strong transient GFP expression in a small but reproducible number of epithelial and interstitial cells. Implications for the use of this method to carry out single cell assays with GFP-tagged Hydra proteins are discussed.