Elsevier

Developmental Biology

Volume 194, Issue 2, 15 February 1998, Pages 246-256
Developmental Biology

Regular Article
Alignment of Centrosomal and Growth Axes Is a Late Event during Polarization ofPelvetia compressaZygotes

https://doi.org/10.1006/dbio.1997.8832Get rights and content
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Abstract

Zygotes and embryos of the fucoid brown algaPelvetia compressaundergo a series of asymmetric cleavages. We are interested in the developmental role of these cleavages and the mechanism controlling their alignment. To assess the importance of division plane alignment, the orientation of the first asymmetric division was altered and the effects on subsequent embryo elongation rates were analyzed. Although this division is normally oriented transversely (90°) to the growth axis, deviations up to 45° had no significant effects on embryo elongation. However, division planes that were parallel with the growth axis (0–45°) had drastic effects. Embryo elongation was severely inhibited and the wall often bifurcated and avoided the rhizoid tip. The orientation of the division plane is determined by the position of the centrosomes. We therefore investigated centrosomal position and function 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 and asynchronously until they reached opposite sides of the nuclear envelope. At this time the centrosomal axis, defined by the position of the two centrosomes, was oriented randomly with respect to the cortical growth axis. The centrosomal axis then rotated into alignment parallel with the growth axis late in the first cell cycle. These results indicate that the growth axis and the centrosomal axis develop independently of each other and that the centrosomal axis does not align with the growth axis until just prior to mitosis.

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To whom correspondence should be addressed at University of Utah, Department of Biology, 257 South 1400 East, Salt Lake City, UT 84112-0840. Fax: (801) 581-4668. E-mail:kropf@bioscience. utah.edu.