Microtubule distribution and function in early Pelvetia development

Abstract

ABSTRACT We have used immunofluorescence microscopy to examine the distribution of microtubules (Mts) during the first two cell cycles in embryos of the brown alga, Pelvetia. Prior to germination of the zygote at 12 h post-fertilization, Mts radiated from the circumnuclear region into the peripheral cytoplasm. After rhizoid emergence, Mts resolved into two perinuclear microtubule organizing centers (MTOCs). The axis defined by the pair of MTOCs was oriented transverse to the growth axis, with Mts extending from each MTOC into the rhizoid. The axis defined by the MTOCs then reoriented by 90 degrees, and aligned with the growth axis. The first mitotic spindle formed between these MTOCs. The division plane bisected the spindle, giving rise to rhizoid and thallus cells with distinct developmental potentials. During the second cell cycle, the axis defined by MTOCs in the rhizoid cell again reoriented from an orthogonal to an axial alignment with respect to the growth axis. MTOC reorientation did not occur in the thallus cell, and the division planes in the rhizoid and thallus cells were orthogonal to one another. Zygotes treated with amiprophos methyl (APM) or taxol established an axis and initiated rhizoid outgrowth. However, treated zygotes ceased growing soon after germination and failed to divide. Cytochalasin D, which prevents establishment of the developmental axis, interfered with the proper orientation of the spindle. From these results we conclude that(1)Mts are not required for establishment of the rhizoid-thallus axis or rhizoid germination;(2)an F-actin-dependent process, probably establishment of a developmental axis, is required for rotation of the axis defined by MTOCs; and (3) the alignment of perinuclear MTOCs dictates the orientation of spindle and subsequent division planes, and thereby controls cell lineage.