ALIGNMENT OF CENTROSOMAL AND GROWTH AXES IS A LATE EVENT DURING POLARIZATION OF PELVETIA-COMPRESSA ZYGOTES

Zygotes and embryos of the fucoid brown alga Pelvetia compressa underg o a series of asymmetric cleavages. We are interested in the developme ntal role of these cleavages and the mechanism controlling their align ment. To assess the importance of division plane alignment, the orient ation of the first asymmetric division was altered and the effects on subsequent embryo elongation rates were analyzed. Although this divisi on is normally oriented transversely (90 degrees) to the growth axis, deviations up to 45 degrees had no significant effects on embryo elong ation. However, division planes that were parallel with the growth axi s (0-45 degrees) had drastic effects. Embryo elongation was severely i nhibited and the wall often bifurcated and avoided the rhizoid tip. Th e orientation of the division plane is determined by the position of t he centrosomes. We therefore investigated centrosomal position and fun ction during the first cell cycle within the three-dimensional context of the cell. We found that, after karyogamy, microtubule organization changed from a radially symmetric circumnuclear array into a bipolar centrosomal array. The reorganization coincided with the migration of the centrosomes around the nucleus. The centrosomes separated slowly a nd asynchronously until they reached opposite sides of the nuclear env elope. At this time the centrosomal axis, defined by the position of t he two centrosomes, was oriented randomly with respect to the cortical growth axis. The centrosomal axis then rotated into alignment paralle l with the growth axis late in the first cell cycle. These results ind icate that the growth axis and the centrosomal axis develop independen tly of each other and that the centrosomal axis does not align with th e growth axis until just prior to mitosis. (C) 1998 Academic Press.