THE ROLE OF METABOLISM AND CALCIUM IN THE CONTROL OF MITOSIS AND OOPLASMIC MOVEMENTS IN INSECT EGGS: A WORKING HYPOTHESIS

Patterns of mitosis and ooplasmic movements in the plasmodial phase of insect embryogenesis and their supramolecular basis are reviewed. Evidence is provided for both the relative independence and the precise correlation of the nuclear cycle and various cycling movements of the ooplasm. I suggest that the timing of these cycles is controlled by a metabolic cycle. The latter may act via a cyclic rise and fall either of the level of free calcium or of the sensitivity of contractile cytoplasmic proteins to a constant level of free calcium. Thus mitotic patterns may reflect metabolic patterns, which in turn may reflect the distribution and activity of mitochondria and may also be related to egg size and shape by a gradient of surface-to-volume ratios. The total number of cycles may depend on a limiting cytoplasmic factor, which together with the number of nuclei in a given cycle defines the nucleo-cytoplasmic ratio. I also propose that both natural and experimental activation of insect eggs trigger the metabolic cycle either directly, by supplying oxygen or water, or indirectly, via a calcium pulse. Possible molecular mechanisms of control are discussed and applied to mitosis and ooplasmic movements. A brief discussion of various cell cycle models in light of data from insect embryogenesis is included.