Maternal Huluwa regulates postfertilization microtubule array organization for asymmetrical transport of dorsal determinants in zebrafish.

The dorsal organizer, essential for vertebrate embryonic axis formation, is induced by microtubule-mediated transport of maternal determinants. Maternal Huluwa (Hwa) has been identified as an essential organizer inducer in zebrafish and frogs, functioning at midblastula stages to activate β-catenin signaling in the preorganizer. It remains unknown if maternal Hwa functions at or before fertilization. Here, we report that maternal Hwa protein is critical for organizing the vegetal parallel microtubule array immediately after fertilization in zebrafish. Hwa protein and mRNA are enriched at the vegetal pole and facilitate microtubule network formation, enabling asymmetrical transport of dorsal determinants. Loss of maternal Hwa disrupts this microtubule architecture and abrogates mRNA transport, revealing a self-reinforcing mechanism where Hwa regulates its own asymmetrical distribution. Hwa may regulate the vegetal microtubule network through other signaling pathways and microtubule-associated proteins. Our findings establish a dual-phase model of dorsal specification: Hwa initially governs symmetry breaking through postfertilization microtubule organization and later on activates β-catenin signaling at blastula stages. This work provides fundamental insights into how the key maternal factor regulates the organizer and body axis formation at different developmental stages.