RNA Synthesis and the Ribosomal Cistrons in Early Molluscan Development

Abstract

A review of RNA synthesis patterns in molluscs points to two conclusions. First, there is no common pattern which could be characteristic of “mosaic” development. Second, among bivalves the pattern of gene expression in early development is the same as in sea urchin embryos. DNA-like RNA predominates during cleavage, including a prominent, short-lived species (about 9S) which enters light polysomes. Ribosomal RNA (rRNA) synthesis cannot be detected prior to gastrulation. New gene transcripts appear and are required for development beyond gastrulation. Among gastropods, there is variation regarding the need for embryonic gene expression for gastrulation and subsequent differentiation, and the time of onset of rRNA synthesis. The synthesis of poly(A) + RNA has been demonstrated in snail veligers. The principle limiting factor in rRNA synthesis during cleavage of molluscan and other embryos is probably the length of interphase. The time of activation of rRNA synthesis beyond this stage may be influenced by the magnitude of the rRNA stockpile. The oocytes of two species of dam with similar requirements for rRNA and with equal time available for oogenesis differ in the degree of amplification of the rRNA cistrons, suggesting that they may also differ in the redundancy or rate of transcription of those cistrons.