Competition-based head versus foot decision in chimeric hydras.

The decision head versus foot in a regenerating fragment of Hydra has been proposed to result from long-range competition for resources such as head-specific precursor cells and hormonal factors, with the winning end of the body column forming the head and the losing end forming the foot. The present study presents new experimental support for this hypothesis. Chimeras prepared from two strains of Hydra magnipapillata with high and low capacity for head regeneration reveal that the low capacity of reg-16 to regenerate a head resides in a low ability to recruit head-promoting resources. When confronted with competing wt105 tissue taken from the same body region, reg-16 tissue is caused to form feet while wt105 is enabled to from enlarged heads which sometimes split into two. Triplet chimeras prepared with labeled donor animals and two competing unlabeled recipients indicate that head-forming wt105 tissue incorporates migrating cells more effectively then head-forming reg-16 tissue.