Inhomogeneous distribution of egg RNA sequences in the early embryo

Abstract

The sequence complexity of total RNA from separated blastomeres of 16-cell sea urchin embryos was assayed by RNA excess hybridization using a single-copy DNA probe (+egg DNA) for those egg RNA sequences that are transcribed from single-copy DNA. RNA from whole 16-cell embryos, from gastrulae, and from pooled mesomeres and macromeres reacted with +egg DNA to 90% of the extent of the egg RNA/+egg DNA reaction. Total RNA from micromeres, however, reacted only to 67–80% of the extent of hybridization shown by 16-cell embryo RNA, gastrula RNA or meso-macromere RNA with +egg DNA. The same results were obtained with RNA from blastomeres or whole embryos developing under a transcription block imposed by actinomycin D. Hybridization of blastomere RNAs to a single-copy DNA probe not homologous to egg RNA sequences (−egg DNA) showed, as do the data on complexity of RNA from embryos grown in actinomycin D, that participation of newly synthesized or nuclear RNA was quantitatively insignificant in setting saturation levels observed in the blastomere RNA/+egg DNA hybridizations. These data therefore indicate that at the 16-cell stage and probably even earlier, there is a large asymmetry of informational RNA sequences between the micromeres and other parts of the embryo. They suggest strongly that the asymmetry resides in RNA which was present in the egg before fertilization-that is, in “maternal” RNA.