Pattern regulation properties of a Hydra strain which produces additional heads along the body axis.

The multiheaded one (mh-1) strain, isolated from inbred crossings of wild type Hydra magnipapillata, develops additional heads along the body axis. This strain reproduces asexually by budding like the wild type (wt) does. We found that young polyps have a wt-like shape and display wt-like properties. When they grow in size and before they produce extra heads along the body axis, the tissue between the head and the budding zone changes its property: in this region, where later on the extra heads preferentially form, foot regeneration is significantly delayed while head regeneration remains unaffected. Further, following various transplantations additional heads form under conditions under which the wild type did not. The observed changes in pattern control and regulation indicate a two-step process of pattern formation. Morphogenetic signalling is suggested to cause the positional value to increase slowly in the form of patches and preferentially in the region between the head and the budding zone. This increase causes an altered morphogenetic signalling, which is eventually responsible for additional head formation.