The cytaster, a colchicine-sensitive migration organelle of cleavage nuclei in an insect egg

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Time-lapse analyses of nuclear multiplication in the eggs of the gall midge Wachtliella persicariae L., documented in film D 1235 (available from the IWF, Göttingen), give evidence of a special migration organelle of cleavage nuclei. Each of these “migration cytasters” represents one greatly enlarged polar cytaster of the mitotic apparatus, which is connected to one nucleus. From the films it can be concluded that the astral rays temporarily adhere to peripheral egg structures and exert tractive forces toward the cytaster center. These forces combine and pull the accompanying daughter nucleus through the ooplasm after each mitosis. This “active” mode of migration, which is accompanied by extensive polarized transport of yolk particles toward the cytaster center, enables the energids (= cleavage nucleus and its associated island of cytoplasm) to move relative to the surrounding ooplasm. In addition, there is a “passive” mode of nuclear migration: The energids are moved by means of plasmic flows, thereby maintaining their position in relation to the surrounding ooplasm. Electron microscopic studies show solitary microtubules running radially toward the cytaster center. As a result of colchicine injection (1) the microtubules disintegrate, (2) the polarized transport of yolk particles cases, (3) the active nuclear migration stops and the nuclei are only passively moved by rhythmic ooplasmic flows. This inhibition of active nuclear migration gives further evidence that microtubules take an essential part in it. Control experiments with lumicolchicine show no effect on nuclear migration. Conversely, under the influence of cytochalasin B active nuclear migration is continued, while the ooplasmic flows are inhibited. Thus the mechanisms of active and passive nuclear migration can work independently of each other. The generation of tractive forces along the astral rays is discussed with respect to current models of spindle function.