PATTERNS OF DISTAL-LESS GENE-EXPRESSION AND INDUCTIVE INTERACTIONS INTHE HEAD OF THE DIRECT-DEVELOPING FROG ELEUTHERODACTYLUS-COQUI

The direct developing frog Eleutherodactylus coqui exhibits radical ch anges in its embryogenesis. A frog-like head forms directly with no ap pearance of a cement gland or several jaw cartilages characteristic of tadpoles, and limbs appear early in development. The numerous differe nces in the embryogenesis of E. coqui provide an opportunity to examin e developmental causes for the evolutionary shift from biphasic to dir ect development. We have cloned DNA fragments corresponding to four E. coqui genes related to the Drosophila distal-less gene D11. While the expression patterns of the distal-less genes are generally conserved, there are some spatiotemporal differences when embryos of E. coqui ar e compared to those of Xenopus laevis. The changes in gene expression are correlated with the embryonic changes in head structures including craniofacial cartilages and in particular, the cement gland. We have then examined inductive interactions involved in cement gland formatio n by interspecific transplants and recombinants. E. coqui embryos can generate signaling that culminates in cement gland formation, but E. c oqui ectoderm appears to be incapable of a cement gland response. Thes e results show here that inductive interactions in the anterior region of the E. coqui embryo have been modified during the evolution of dir ect development, and that changes in the competence of the E. coqui ec toderm may be responsible for the loss of certain tadpole-specific str uctures, such as cement gland. (C) 1996 Academic Press, Inc.