title: Cell cycles in embryos of the silkworm, Bombyx mori: G2-arrest at diapause stage.
abstract: In the silkworm, Bombyx mori, diapause occurs at a specific embryonic stage, i.e. after formation of the germ band with cephalic lobes and telson and sequential mesoderm segmentation. As long as the eggs are incubated at 25° C, cell divisions and morphological development of the embryos cease. To examine changes in percentage of embryonic cells in the G1, S and G2phases during embryogenesis, nuclear fractions were isolated from embryos, stained with propidium iodide and then subjected to flow cytometric analysis. The percentages of embryonic cells in G1, S and G2were 10, 35 and 55%, respectively, at the stage of formation of cephalic lobes, whilst 98% of cells were in G2at diapause stage. After termination of diapause by acclimation at 5° C or by a combination of chilling and HCl, cell division resumed in the embryos. During this period, the cells rapidly entered S phase through G1from G2, suggesting that their G1phase was short. In eggs in which diapause was averted by HCl-treatment after incubation at 25° C for 20 h after oviposition, embryonic development proceeded continuously for 9.5 days at 25° C until hatching. Along with this development, the G1fraction increased to levels of about 90%. These results indicate that embryonic cells are arrested in G2at diapause and suggest that, concomitant with further embryonic development, cell cycles become slower in proportion to an increasing length of G1. Finally, most of the cells may be arrested in G1, while there is only a small fraction of cells continuously cycling.
authors.affiliation: Laboratory of Silkworm Genetics and Pathology, Faculty of Textile Science and Technology, Shinshu University, 386, Tokida, Ueda, Japan.
authors.forename: Masao
authors.initials: M
authors.lastname: Nakagaki
authors.affiliation: Laboratory of Silkworm Genetics and Pathology, Faculty of Textile Science and Technology, Shinshu University, 386, Tokida, Ueda, Japan.
authors.forename: Ryuzo
authors.initials: R
authors.lastname: Takei
authors.affiliation: Laboratory of Silkworm Genetics and Pathology, Faculty of Textile Science and Technology, Shinshu University, 386, Tokida, Ueda, Japan.
authors.forename: Eiichi
authors.initials: E
authors.lastname: Nagashima
authors.affiliation: Laboratory of Sericultural Science, Faculty of Agriculture, Nagoya University, 464-01, Chikusa, Nagoya, Japan.
authors.forename: Toshinobu
authors.initials: T
authors.lastname: Yaginuma
keywords: Bombyx mori; Cell cycle; Diapause; Embryogenesis; Insect