CHARACTERISTICS OF PRONUCLEAR MIGRATION IN BEROE-OVATA

In the large eggs (similar to 1 mm) of the ctenophore Beroe ovata, fem ale pronuclei migrate long distances to join stationary male pronuclei in the peripheral cytoplasm that surrounds the yolky interior. We hav e investigated the mechanism of nuclear migration using time lapse vid eo recording, automated image analysis, visualization of microtubules by immunofluorescence and rhodamine-tubulin injection, and electron mi croscopy. Female pronuclei migrated at average speeds of 0.2 mu m/sec, and were found to show periodic oscillations in velocity. Alternating phases of acceleration and deceleration occurred with an average peri odicity of 235 seconds covering distances of 47 mu m (about 3 times th e nuclear diameter). Migration velocities and velocity oscillations we re similar in fertilized and unfertilized eggs; however, changes in mi gration direction were much more frequent in unfertilized eggs. Charac teristic deformations of the pronuclear membrane and occasional rotati on of the nuclear contents were observed during migration. Inhibitor s tudies indicated that microtubules are required for nuclear migration. In fertilized eggs the top of the nucleus was found to move through t he dense layer of aligned sperm aster microtubules. The frequent chang es in direction of pronuclear migration in unfertilized eggs reflect t he random organization of the microtubule layer in the absence of sper m derived centrosomes. Densely packed endoplasmic reticulum was found intermeshed with sperm aster microtubules and connected extensively wi th the nuclear membrane during migration. Most nuclear pores were grou ped in an infolding of the nuclear membrane. We suggest that in fertil ized eggs the female pronucleus is transported to the minus ends of sp erm aster microtubules using motor molecules attached either to the ou ter nuclear membrane and/or to the network of connecting ER. (C) 1994 Wiley-Liss, Inc.