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

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Abstract
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.