Meeting Abstract
37.4 Sunday, Jan. 5 08:45 The Starfish Oocyte is an Excitable Medium VON DASSOW, G.*; SU, K. C.; BEMENT, W. M.; University of Oregon; Massachusetts Institute of Technology; University of Wisconsin, Madison dassow@uoregon.edu
Most animal oocytes undergo two extreme-asymmetrical divisions during meiosis, creating the egg and two polar bodies. In oocytes of the starfish Patiria miniata and several other species, meiotic cytokinesis is preceded by a wave of Rho activity which sweeps from vegetal to animal, culminating in formation of a ring around the emerging polar body. Close inspection shows that the Rho wave is actually composed of smaller, transient wavelets. We found that overexpression of Ect2, the principle cytokinetic activator of Rho, progressively amplified Rho activation: at moderate levels Ect2 enhanced and prolonged wavelet activity associated with meiotic telophase; at higher levels, Ect2 converted the normal cortical behavior into a cataleptic state in which repeating trains of Rho waves, followed closely by actin assembly waves, traversed the entire oocyte surface. Experiments with enucleation, cyclin mutants, and roscovitine suggested that cortical excitability is strongly damped by, but dependent on, MPF activity. Meanwhile, application of actin poisons showed that actin assembly antagonizes Rho activation; local application of latrunculin elicited immediate Rho activation and a change in wavelet wavelength. Rho wavelets are also present during cytokinesis in the large blastomeres of early starfish embryos, in the zygotes of sand dollars, and in large cells of early Xenopus embryos. We suggest that excitability reflects the operation of a feedback-regulated amplifier of cortical contractility. This amplifier may be an adaptation to facilitate cytokinesis in large cells in which the key furrow-inducing signal from the mitotic apparatus is faint and poorly confined. Such adaptations, if they exist, are sure to be important factors in the evolution of egg size and developmental mode.
