Evidence for the appearance of novel gene products during amphibian blastulation.

Genetic information begins to be transcribed during early cleavage in amphibian embryos,"1 2 and the tempo of informational RNA synthesis increases gradually through cleavage and into the early blastula stage. Previous studies from this laboratory' 4 have shown that at this point a remarkable, near embryo-wide acceleration of informational RNA synthesis occurs, resulting during the mid-tolate blastular period in at least a 20-fold increase in the rate of synthesis of heterogeneously sedimenting, DNA-like RNA. Two possibilities present themselves with respect to the significance for the embryo of this first large-scale increase in the level of embryo genome function: either the gene products synthesized in the mid-to-late blastular period are novel, perhaps constituting a precociously transcribed program for gastrular differentiation; or they are at least in part already represented in the enormous stockpiles of informational RNA which our earlier studies have shown to be stored in the embryo from the lampbrush stage of oogenesis.5 In the latter case, pregastrular RNA synthesis is to be regarded more as an amplification by the embryo of the patterns of gene activity laid down originally during o6genesis, and indications to this effect now exist in the sea urchin literature. Thus Glisin et al.6 have shown that virtually all the species of RNA synthesized in the mesenchyme blastula stage are already represented in the informational RNA stored in the unfertilized egg. The generality of homology between the informational RNA stockpile inherited from o6genesis and the informational RNA's synthesized in early embryogenesis is evidently a question of considerable significance in understanding the early function of the embryo genomes, and an object of the present study is to assay this homology in an organism only distantly related to the echinoderms. The experiments which come closest to examining this question in amphibian material are those of Denis,7 who demonstrated (in Xenopus) that there was no homology between the RNA population in synthesis during mid-to-late gastrulation and the RNA's already present in the embryo between late cleavage and late blastulation. In total informational content the latter RNA population consists in large part of RNA's inherited from odgenesis, but it is evident that gastrular gene activity cannot be considered a pr-ior-i to be representative of blastular gene activity, and that this experiment therefore leaves the novelty of blastular transcription unexplored. Previous to the present report, then, the earliest qualitatively new pattern of gene activity in amphibian embryos for which evidence exists7 is in the mid-to-late gastrular period, i.e., in a period following rather than preceding the manifest appearance of new, differentiated cell types. Methods.-Preparation of nucleic acids: Eggs of Xenopus laetis were labeled with p12_ phosphate by injection into the gravid female as described by Brown and Littna.1 P2RNA from late blastulae (stage 9 of Nieuwkoop and Faber8), unlabeled RNA from early blastulae (stage 7), late blastulae (stage 9), and gastrulae (stage 10), as well as unlabeled