The amphibian egg is axially symmetric before fertilization: its contents are polarized along one dimension, the animal-vegetal axis. The earliest step in the establishment of the embryo's bilaterally symmetric body plan, dorsal-ventral axis specification, occurs after fertilization, but before the onset of zygotic gene expression. Thus, all the components necessary for this patterning event are already present at fertilization, and become asymmetrically localized or modified during dorsal- ventral axis specification. This project proposes to identify such components, determine the mechanisms of their localization, and assess their roles in dorsal-ventral patterning in the frog Xenopus laevis. Two cDNA libraries enriched for either dorsal or ventral transcripts in embryos were recently constructed. Several clones representing regionally enriched maternal transcripts have been isolated. Digoxigenin labelled antisense RNA probes are presently being used for in situ hybridization analysis to learn the details of localization of the corresponding RNAs during oogenesis and early development. The spatial, temporal, and quantitative changes in expressions of specific RNAs will be compared with the final morphology of embryos subjected to centrifugation, UV irradiation, and over expression of injected maternal mRNAs. These analyses will be used to ascertain which transcripts are involved in the initiation of early pattern events. By providing new information on the relationships between the spatial localization of some maternal transcripts relative to the regulation of specific zygotic transcripts, these experiments will lead to a better understanding of the relationship between egg organization and embryonic patterning.
