RESPONSE TO INSULIN AND THE EXPRESSION PATTERN OF A GENE ENCODING AN INSULIN-RECEPTOR HOMOLOG SUGGEST A ROLE FOR AN INSULIN-LIKE MOLECULE IN REGULATING GROWTH AND PATTERNING IN HYDRA

A gene encoding a receptor protein-tyrosine kinase closely related to the vertebrate insulin receptor has been identified in the Cnidarian H ydra vulgaris. The gene is expressed in both epithelial layers of the adult polyp. A particularly high level of expression is seen in the ec toderm of the proximal portions of the tentacles and in a ring of ecto dermal cells at the border between the foot basal disk and body column . The expression pattern of the gene in asexual buds is dynamic; expre ssion is high throughout the newly emerging bud but the area of high e xpression becomes restricted to the apex as the bud lengthens. When th e bud begins hypostome and tentacle formation, a high level of express ion appears at the bases of the emerging tentacles. Finally, a ring of high expression appears just above the foot of the bud, completing th e pattern seen in the adult polyp. The presence of this receptor and i ts pattern of expression suggested that an endogenous molecule related to insulin plays a role in regulating cell division in the body colum n and in differentiation of the tentacle and foot cells in Hydra, with the switch between the two being determined by the level of the recep tor. Treatment of Hydra polyps with mammalian insulin caused an increa se in the number of ectodermal and endodermal cells undergoing DNA syn thesis.