Fusion of silkworm polar body nuclei

Polar bodies are produced during the development of oocytes. In many animals, polar bodies extrude from the egg and typically degenerate after the meiotic maturation of the oocyte. However, in insects, polar body nuclei remain within the egg and neither extrude nor degenerate during early embryogenesis. Previously, we reported that in the silkworm (Bombyx mori), the polar body nuclei differentiate into serosa cells. In this study, we observed the behavior of polar body nuclei during early embryogenesis using a tissue clearing method. Individuals with a reduced number of polar body nuclei were observed around 4-6 h after egg laying during incubation at 25℃, and after 6 h, the polar body nuclei were observed to have increased in number in some individuals. Analysis using serosa cell mutants (pe and re) revealed that most of the polar body-derived serosa cells had undergone fusion of the polar body nuclei. These findings indicate that in silkworms, polar body nuclei fuse approximately 4-6 h after egg laying, which corresponds to the cleavage stage, during which fertilized nuclei divide into several to dozens of nuclei. In some individuals, the fused polar body nuclei divide, increase in number, and differentiate into serosa cells.