Nuclear and cytoplasmic DNA synthesis during early embryogenesis of Paracentrotus lividus

Abstract

An electron microscope autoradiographic study of DNA synthesis in the nuclei and cytoplasm during early embryogenesis of Paracentrotus lividus is presented. Within 15–30 min after fertilization both the male and female pronuclei of monospermic embryos commence the incorporation of thymidine-3H, an indication that the pronuclei “S” phases occur prior to karyogamy. All differentiated male pronuclei in polyspermic embryos synthesize DNA simultaneously. After Tris-MgSO4 DNase and trichloroacetic acid treatments, radioactivity is lost from pronuclei. Tritiated thymidine is also incorporated into the mitochondria and yolk platelets of fertilized sea urchin embryos. The synthesis of DNA in mitochondria begins after fertilization; the phase of mitochondrial DNA synthesis appears to be independent of pronuclei “S” phases. Structures resembling bacteria and containing bacterial-type nucleoids are present in the ooplasm of sea urchin embryos. These structures account for some cytoplasmic incorporation of thymidine-3H and are presumed to be endosymbionts. These results suggest that ooplasmic activation at fertilization stimulates synthesis of DNA in male and female pronuclei and in mitochondria and yolk platelets during early development of sea urchin embryos.