The pairing of nucleolar patterns in an epithelium as evidence for a conserved nuclear skeleton

The nucleoli in epidermal cells of fifth instar Calpodes and Manduca larvae undergo three cycles of unravelling into necklaces with condensation back to a dense particle or particles. Mitosis occurs before the first cycle and at the beginning of the third cycle. In spite of the formation and condensation of these nucleolar necklaces the nucleoli in the intercycle condition all have paired patterns. Adjacent pairs of nuclei have nucleoli that resemble one another in the number of their component particles, the shape and size of the particles and sometimes in their arrangement as mirror images. The paired patterns begin at mitosis and reform after necklace elongation and condensation. The patterns are presumed to reflect a nuclear skeleton that is paired from mitosis and conserved until the next mitosis. The nuclear pairing is related to the retention of mid bodies by sibling cells so that the epidermis is a collection of minimal two-cell syncytia.