Gene transcription in mosaic embryos. I. The pattern of RNA synthesis in early development of the coot clam, mulinia lateralis.

Abstract

AbstractOf the many investigations reported in the past decade concerning the pattern and significance of RNA synthesis in early development, relatively few have involved mosaic embryos. These investigations have revealed a common pattern of RNA synthesis among embryos of anamniotic deuterostomes and a few have indicated that this pattern may not be shared by their protostome counterparts. The present study provides data for a thorough comparison of RNA metabolism in a mosaic embryo with that of regulative embryos.Oocytes and embryos of the coot clam, Mulinia lateralis, were pulse‐labeled with tritiated uridine and their RNA was extracted and analyzed by sucrose gradient centrifugation and MAK chromatography. Little or no RNA synthesis could be detected in oocytes before fertilization. During cleavage, newly synthesized RNA is primarily DNA‐like, heterogeneous in size and base composition and of lower average G + C content than ribosomal RNA. A prominent class of short‐lived RNA molecules sedimenting at 8–10s is also transcribed during cleavage and continues to be produced at least through gastrulation. Transfer RNA appears to be synthesized as early as mid‐ to late‐cleavage. Whereas ribosomal RNA synthesis was undetectable in these experiments prior to gastrulation, ribosomal and transfer RNA synthesis predominate in the larval stages. These results demonstrate that the temporal pattern of RNA synthesis common to embryos of the lower deuterostome phyla can be found in protostome embryos as well, and therefore that mosaic development need not entail a unique pattern of gene transcription.