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Bud initiation in a non-budding strain of hydra: role of interstitial cells.

A non-budding strain of Chlorohydra viridissima has been maintained by vegetative propagation and used to study factors which initiate budding. These hydra can be induced to bud by implanting small pieces of normal tissue into their columns. The cellular bases of the non-budding phenotype and the bud induction were investigated by means of grafting, radio-autography, and histological methods. No differences in the proportions or appearances of the cell types were observed between non-budding and normal animals. However, induced buds have higher proportions of interstitial cells and their derivatives (nerves and nematoblasts) than do normal buds. Many of these interstitial cells and derivatives origihate from cells in the grafted implant, as demonstrated by using 3H-thymidine labeled implanted tissue. Normal tissue from which interstitial cells have been previously removed will not induce buds in non-budding hydra. We conclude that bud induction is mediated by migratory interstitial cells, and that the non-budding strains of hydra owe their phenotype to a deficiency in those interstitial cell differentiation events which are necessary for bud initiation.