in 1909, several years before the famous `organizer’ experiments of spemann and mangold, ethel browne demonstrated induction of a secondary axis in hydra by grafting a hypostome. based on this and subsequent work, in the late sixties, lewis wolpert proposed the theory of morphogen gradients and positional information. we have studied secondary axis induction by hypostome and foot tissue using t...

In 1909, several years before the famous `Organizer’ experiments of Spemann and Mangold, Ethel Browne demonstrated induction of a secondary axis in hydra by grafting a hypostome. Based on this and subsequent work, in the late sixties, Lewis Wolpert proposed the theory of morphogen gradients and positional information. We have studied secondary axis induction by hypostome and foot tissue using t...

What is Hydra? Hydra is a member of the animal phylum Cnidaria. This phylum includes corals, sea anemones and jellyfish. While most cnidarians are marine, Hydra is unusual in that it lives exclusively in fresh water. Hydra was first described by Antoni van Leeuwenhoek (1632– 1723) in a letter he sent to the Royal Society on Christmas day, 1702. The letter included a red chalk drawing of Hydra o...

Hydra attenuata were rendered free of interstitial cells (I cells) and interstitial cell derivatives by colchicine treatment. These hydra were then cloned and cultivated for 18 months and their developmental capacities were studied. Some experimental hydra possessed a few (about 1% of the normal numbers) interstitial cells and retained this low level during prolonged culture and active growth w...

Aposymbiotic polyps of Hydra viridis were infected with 17 strains of in vitro cultured Chlorella sp. Larvae of Artemia fed with the chlorellae were used as an infecting vector. Of the 17 strains, seven formed stable symbioses and one formed a transient infection that disappeared within several weeks. Chlorellae of the nine other strains were cleared out of the infected hydra within 2-3 days. T...

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 proj...

Does Hydra have an associated microbiome? Yes, it does. Like the human gut, Hydra is home to an array of bacterial residents. The composition of the microbiome for a particular strain of Hydra seems to be quite stable. How the microbiome is maintained and what it contributes to the host are important questions that are starting to be addressed. Hydra is a much simpler model for studying host/mi...

Hydra represents a unique model system for the study of senescence, with the opportunity for the comparison of non-aging and induced senescence. Hydra maintains three stem cell lineages, used for continuous tissue morphogenesis and replacement. Recent work has elucidated the roles of the insulin/IGF-1 signaling target FoxO, of Myc proteins, and of PIWI proteins in Hydra stem cells. Under labora...

Hydra were known to react to light as early as 1744 when Trembley first described their movement towards a light source. This migration of the animal was then further elucidated by Wilson in 1891, who found that Hydra viridis and fusca are maximally sensitive to blue light. Haug ( ' 3 3 ) found that Hydra will react to a light stimulus by contracting first and then orienting; and Rushforth, Bur...

We consider the problem of determining minimal Horn formula size for a subclass of Horn formulas. A hydra formula is a Horn formula consisting of size 3 definite Horn clauses, specified by a set of bodies of size 2, and containing clauses formed by these bodies and all possible heads. A hydra formula can be specified by the undirected graph formed by the bodies occurring in the formula. Thus mi...