Induction of secondary axis in hydra revisited:New insights into pattern formation

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 three species of hydra as well as
transgenic, GFP-expressing lines of hydra. We have found that pieces of hypostome and complete foot of a
donor hydra can induce a secondary axis all along (in upper, middle or lower parts of) the body column of a host
hydra, both within and across species with comparable rates. Thus, contrary to the available literature, our results
show that the host hypostome does not completely inhibit the induction of a secondary axis. The length of the
induced axis though is determined by the position of the graft. By using GFP-expressing lines of hydra we have
demonstrated that host ectodermal and endodermal cells actively contribute to the secondary axis. On
comparison, the hypostome was found to be a stronger and dominant Organizer than the foot. Foot grafting
experiments show a transient increase in the host length as well as the distance between the two Organizers. The
length becomes normal once the grafted foot reaches the budding zone. Our work brings out several new aspects
of the role of positional cues in pattern formation in hydra that can be now be explored at cellular and molecular levels.