DYNAMIC MICROTUBULES AT THE VEGETAL CORTEX PREDICT THE ZEBRAFISH EMBRYONIC AXIS

Abstract

In the zebrafish embryo, maternal dorsal determinants are localized to the vegetal pole and are transported via microtubules after fertilization. However, microtubule dynamics and their contribution to the transport of determinants are poorly understood. Using transgenic reporters that mark the microtubule cytoskeleton and live-imaging, I have identified three populations of microtubules at the vegetal cortex: a parallel array, perpendicular bundles and a non-directional meshwork. The parallel array resides at one side of the vegetal cortex before the first cell division and marks the future embryonic dorsal side. Formation of the parallel array is independent of fertilization but dependent on Ca2+ transients in the embryo. The perpendicular bundles align with the animal-vegetal axis of the embryo. These two microtubule populations are very transient and disappear at about 30 minutes post fertilization. The non-directional meshwork is dynamic. Its organization changes periodically at intervals of about 20 minutes. I also found evidence supporting cortical rotation-like movement in early zebrafish embryos. Disruption of the parallel array affects movement of vegetal cortical granules and nuclear localization of ß-catenin to dorsal nuclei. Thus microtubules are highly dynamic during early zebrafish development and can contribute to localization of dorsal determinants via directed transport and cortical rotation-like movement.