In early embryos of the fern Marsilea vestita, we have shown that transcription did not begin until the 8-cell stage, even though protein synthesis was taking place. As in animal species, the pre-existence of stored maternal mRNA has been suspected in embryonic cells. To investigate the presence of such long-lived mRNA we have used in situ hybridization method using [3H]polyuridylic acid, [3H]poly(U), as a probe.
Temporal and spatial changes in the distribution of poly(A)+ RNA were followed throughout all the different phases of embryogenesis. Poly(A)+ RNA was present in the cytoplasm of both unfertilized and fertilized eggs and early embryos, while the nucleus exhibited no, or a moderate, [3H]-poly(U) binding activity. New poly(A)+ RNA was detected in the nucleus only at the 16-cell stage, when cytological differentiation of the root apical cell became morphologically detectable.
After in situ hydridization with the labeled probes, the numerous silver grains detected autoradiographically over the mature sperm, indicate the presence of poly(A)+ RNA molecules associated with the male genome.
We discuss the possible role of poly(A)+ RNA of maternal origin in supporting early translation prior to synthesis of new mRNA. Additional studies are needed to elucidate the role of stored paternal mRNA.