Developmental Patterns of Cytoplasmic Transcript Prevalence in Sea Urchin Embryos

Several hundred two-cell stage, gastrula stage, and pluteus stage sea urchin embryo cDNA clones were screened with [^(32)P]cDNA transcribed from a developmental series of cytoplasmic and polysomal poly(A) RNAs. Transcript prevalence was estimated by reference to the reaction of a series of standard clones complementary to RNAs of known abundance. We describe the dominant pattern of prevalence change during development for abundant sequences, i.e., those present in the range 5 × 10^4–10^6 molecules/embryo. These transcripts are most often about equally abundant in egg and pluteus stage embryos, and are severalfold less prevalent at gastrula stage. Less than 10% of the sequences displaying this pattern undergo >10-fold changes in prevalence during development. A search for later embryo sequences not represented detectably in egg or cleavage stage embryo poly(A) RNA yielded a number of examples. These were confirmed by the RNA gel blot method. Sequences regulated in this manner all belonged to lower abundance classes, and are present at about 40 copies/cell or less at the gastrula stage