Direct development is an evolutionarily derived life history mode in vertebrates and invertebrates in which the free living, aquatic larval stage is lost. This project examines the pattern and mechanisms of cranial development in direct developing anurans, and how these differ from those in metamorphosing taxa. Experimental and descriptive methods include immunohistochemistry (to localize the embryonic distribution of type II collagen and visualize the initial skeletal pattern); scanning electron microscopy (to document neural crest morphology and migration); heterospecific transplantation of premigratory cranial neural crest and cranial ectoderm (to examine inductive tissue interactions that underlie the development, and loss, of cranial tissues); in vivo administration of thyroid hormone and TH- blocking agents; TH radioimmunoassay (to measure the contribution of TH from maternal and embryonic sources); and conventional histology. Analyses will focus on Eleutherodactylus coqui (Leptodactylidae), whose ontogeny is among the most derived of any direct developing anuran with respect to the ancestral metamorphic pattern of cranial development. Reference data concerning the ancestral pattern will be obtained from several phylogenetically diverse species,including Bombina orientalis (Discoglossidae), Xenopus laevis (Pipidae), and Rana pipiens (Ranidae). Results will contribute to our understanding of the basis of direct development in amphibians and how this important life history mode has evolved. They also will be used to address a series of issues in developmental and systematic biology, concerning the relation between development and morphological evolution. These include the evolutionary implication of mechanisms of cranial pattern formation; the importance of ontogenetic repatterning vs. heterochrony; the role of inductive tissue interactions; and the hormonal mediation of embryonic development. finally, they will allow comparison of direct development in amphibians with that in other groups with complex life cycles.
