A role for biliverdin IX in dorsal axis development of Xenopus laevis embryos

The determinants of Xenopus laevis embryos that act before their first cell division are mandatory for the formation of mRNas required to establish the dorsal axis. Although their chemical identities are unknown, a number of their properties have long been recognized. One of the determinants is present in the cytoplasm and is sensitive to UV light. Thus, exposing stage 1 embryos to either standard 254-nm or, as shown here, to 366-nm UV light during the 0.3–0.4 time fraction of their first cycle inactivates the cytoplasmic determinant. As a consequence, both types of irradiated embryos fail to express dorsal markers, e.g., goosecoid and chordin, without affecting formation of ventral markers, e.g., Vent-1. The developmental outcome is dorsal axis-deficient morphology. We report here that biliverdin IX , a normal constituent of cytoplasmic yolk platelets, is photo-transformed by irradiation with either 254or 366-nm UV light and that the transformation triggers the dorsal axis deficiency. When the 254or 366-nm UV-irradiated embryos, fated to dorsal axis deficiency, are incubated solely with M amounts of biliverdin, they recover and form the axis. In contrast, incubation with either in vitro photo-transformed biliverdin or biliverdin IX dimethyl ester does not induce recovery. The results define an approach to produce dorsal axisdeficient embryos by photo-transforming its biliverdin by irradiation with 366-nm UV light and identify an unsuspected role for biliverdin IX in X. laevis embryogenesis.