Messenger RNA prevalence in sea urchin embryos measured with cloned cDNAs.

mRNA prevalence during sea urchin development was measured by treating cDNA clone colonies with labeled cDNAs transcribed from unfertilized egg and embryo poly(A)-RNAs. The number of cytoplasmic transcripts per embryo complementary to several clones was determined independently by titration with poly(A)-RNA in solution, and the amount of cDNA bound to these clones in colony hybridizations was shown to be proportional to the concentration of the respective poly(A)-RNAs in the embryo cytoplasm. At the gastrula stage, the most prevalent mRNA species occur in about 10(6) molecules per embryo. If all cells were equivalent, this would be a few hundred molecules per cell. By pluteus stage, the prevalence of some sequences has increased more than 10-fold. Most, though not all, sequences prevalent in later embryos are also present in the maternal RNA of the unfertilized egg. For most poly(A)-RNA sequences, the prevalence levels determined during oogenesis are maintained through the pluteus stage, whereas a minority of sequences display sharp stage-specific changes in representation during development.