Roles of ectodermal and endodermal epithelial cells in hydra morphogenesis: Construction of chimeric strains

A method is described for constructing chimeric hydra that consist of ectoderm from one strain of hydra and endoderm from a different strain. A ring of vitally stained tissue (ectoderm and endoderm) is grafted into an unmarked polyp. Differential movement of the ectoderm and endoderm eventually results in chimeric regions which are visible as marked ectoderm overlying unmarked endoderm and unmarked ectoderm overlying marked endoderm. These chimeric regions are then excised and each is allowed to regenerate, forming a whole chimeric animal. Isolated chimeric regions smaller than 0.5 mm in length were grafted into nitrogen mustard treated animals as hosts for in vivo culturing. Reciprocal chimeras between the 40L strain of H. magnipapillata (40L) and H. attenuata (Ha) were used to illustrate the effectiveness of the method for constructing chimeras. These two strains are different in body morphology, size, and color. The [40L ectoderm/Ha endoderm] chimera has 40L shape but is the size and color of Ha. The [Ha ectoderm/40L endoderm] chimera resembles Ha in body shape but is the size and color of 40L. The stability of these traits indicates that the original parental lineage of the epithelial cells is maintained in the chimeric hydra.