The accumulation of prominent tadpole mRNAs occurs at the beginning of neurulation in Xenopus laevis embryos.

Cloned cDNA probes have been used to measure the sizes and titers of transcripts in total RNA preparations during early development in Xenopus laevis. Of more than 20 different sequences derived from abundant and moderately abundant RNA which were present in full-grown oocytes and persisted during early development, the transcript sizes of all but 3 of these sequences were invariant. Two transcripts were of a higher molecular weight in oocytes than in embryos, but their titers in oocytes were less than 5% their titers in embryos and thus these larger maternal transcripts do not significantly contribute to embryonic, polysomal mRNA. The oocyte transcripts and the embryonic transcripts of one of these sequences are transcribed from different though cross-hybridizing genes. Cellular titers of a number of RNA sequences have also been studied and show that increases in the cellular titers of several poly(A)+RNA species are the result of de novo transcription and not simply polyadenylation. A number of sequences abundant in tadpole RNA but absent or very rare in eggs have also been examined. All of these sequences first appear in development in substantial titers in the late gastrula or early neurula, 12-15 hr after fertilization. Many other sequences already present in eggs which persist during development show an increase in titer 12-15 hr after fertilization. These data suggest that this late gastrula transcriptional event may be a major transition of gene expression that accompanies the cellular differentiation and morphogenesis that begin at this developmental time.