Loss of ectodermal competence for lateral line placode formation in the direct developing frog Eleutherodactylus coqui

In the direct-developing hog Eleutherodactylus coqui neuromasts and ganglia of the lateral line system never develop. We show here that this absence o f the lateral line system, which is evolutionarily derived in anurans, is d ue to very early changes in development. Ectodermal thickenings, which are typical of lateral line placodes, and from which neuromasts and ganglion ce lls of the lateral Line originate, never form in E. coqui, although other n eurogenic placodes are present. Moreover, although NeuroD is expressed in t he lateral line placodes of Xenopus laevis, corresponding expression sites are lacking in E. coqui. Heterospecific transplantation experiments show th at axolotl ectoderm can be induced to form lateral line placodes after tran splantation to E. coqui hosts but that E. coqui ectoderm does not form late ral line placodes on axolotl hosts. This suggests that the loss of the late ral line system in E. coqui is due to the specific loss of ectodermal compe tence to form lateral line placodes in response to inductive signals. Our r esults (1) indicate that the competence for lateral line placode formation is distinct and dissociable from the competence to form other neurogenic pl acodes and (2) support the idea that the lateral line system acts as a modu le in development and evolution. (C) 1999 Academic Press.