Observations on the Uptake of Tritiated Thymidine in the Pronuclei of Fertilized Sand Dollar Embryos

Following fertilization of the egg of the sand dollar Echinarachnius parma, tritiated thymidine (H(3)TDR) was taken up independently by the male and female pronuclei beginning within about 15 to 20 minutes, and the labeled pronuclei fused at about 30 to 40 minutes. At cleavage 90 minutes later the labeled nuclear material was distributed to both daughter cells. Unfertilized eggs and sperm exposed to H(3)TDR did not show nuclear localization of thymidine. DNA replication, thus, is initiated in the haploid pronuclei shortly after fertilization and prior to fusion. The major portion of DNA synthesis, as evidenced by thymidine uptake, appears to be during a 20 to 30 minute period after fertilization. Fertilization is associated with the activation of a mechanism which initiates early and independent replication of DNA in both the male and female pronuclei.