Altered Morphogenesis and Its Effects on Gene Activity in Xenopus laevis Embryos

During many weeks of development, the giant amphibian oocytes accumulate large amounts of RNA and protein (for review, see Davidson 1976; Smith and Richter 1985). In Xenopus these components are sufficient to support normal embryonic development for several hours after fertilization, since 13 cell divisions take place without any measurable transcription. The proteins inherited from the oocyte are distributed among the arising embryonic cells and are supplemented by new synthesis on polysomes using maternal mRNAs. About 7 hours after fertilization, during a period called the midblastula transition (MBT), the zygotic genome of Xenopus embryos is activated for the first time (Brown and Littna 1964; Bachvarova and Davidson 1966; Shiokawa et al. 1981; Newport and Kirschner 1982a,b). At this time, one set of genes is expressed whose members have not been transcribed during oogenesis and whose products are therefore not represented in the maternal RNA population (Sargent and Dawid 1983). Expression...