Arrest of intravitelline mitoses in Drosophila embryos by u.v. irradiation of the egg surface

The intravitelline mitosis in Drosophila was arrested at the anaphase within the span of a single cell cycle after irradiation with 300 nm u.v. Embryos at and before the 8-nucleus stage were influenced by the u.v. only when irradiated anteriorly, while at and after the 16-nucleus stage, embryos are sensitive to either anterior or posterior irradiation. In embryos anteriorly irradiated at or before the 8-nucleus stage all nuclei in the embryo were prevented from performing mitosis. When irradiated at or after the 16-nucleus stage, inhibition of the intravitelline mitosis is limited to the nuclei in approximately anterior-half region of embryos in anterior irradiation, and to those inapproximately posterior-half region in posterior irradiation, resulting in incomplete blastoderm formation. Sites sensitive to 300 nm u.v. are postulated to be present in the peripheral cytoplasmic region of the embryo and not in the nucleus, because the half-attenuation thickness of 300 nm u.v. light for the Drosophila egg cytoplasm is 3 µm and nuclei are at least 50 µm away from the periphery at the stage of irradiation. In addition lateral irradiation of a portion of an egg where there is no nucleus underneath was also effective in arresting division of nuclei in the same egg. It is suggested that the effects of 300nm u.v. may not be conveyed to the nuclei from the periphery by simple diffusion of a substance, and a hypothesis is proposed for the involvement of cytoskeletal elements associated with the u.v. sensitive sites on the surface to the control mechanism of the intravitelline mitosis of the Drosophila embryo.