Pronuclear positioning and migration during fertilization in Pelvetia.

The position and migration of egg and sperm pronuclei were studied in the brown alga Pelvetia. The egg pronucleus was located near the center of the cell before and after fertilization and, unlike pronuclei in animal eggs, did not migrate. Inhibitor studies indicated that anchoring of the egg pronucleus in the cell center was dependent on microtubules and microfilaments. An extensive array of microtubules, many of which extended into the actin-rich egg cortex, was associated with the egg pronucleus. Migration of the sperm pronucleus was investigated quantitatively in both living and fixed zygotes. Migration occurred linearly at rates from 0.11 to 0.29 microns/min and was oriented directly toward the egg pronucleus in the cell center. Sperm penetration was inhibited by cytochalasin D, which disrupts F-actin function, whereas sperm pronuclear migration was sensitive to the microtubule-depolymerizing drug, nocodazole. Microtubules associated with the migrating sperm pronucleus formed a sperm trail that terminated at the egg cortex. As these were the only microtubules associated with the sperm at early stages of migration, we conclude that they provide the force for migration.