Astral Microtubule Crosslinking by Feo Safeguards Uniform Nuclear Distribution In The Drosophila Syncytium

The early insect embryo develops as multinucleated cell distributing genomes uniformly to the cell cortex. Mechanistic insight for nuclear positioning beyond cytoskeletal requirements is missing to date. Contemporary hypotheses propose actomyosin driven cytoplasmic movement transporting nuclei, or repulsion of neighbor nuclei driven by microtubule motors. Here, we show that microtubule crosslinking by Feo and Klp3A is essential for nuclear distribution and internuclear distance maintenance in Drosophila. RNAi knockdown in the germline causes irregular, less dense nuclear delivery to the embryo cortex and smaller distribution in ex vivo embryo explants. A minimal internuclear distance is maintained in explants from control embryos but not from Feo depleted embryos, following micromanipulation assisted repositioning. A dominant-negative Feo protein abolishes nuclear separation in embryo explants while the full-length protein rescues the genetic knockdown. We conclude that antiparallel microtubule overlap crosslinking by Feo and Klp3A generates a length-regulated mechanical link between neighboring microtubule asters. Enabled by a novel experimental approach, our study illuminates an essential process of embryonic multicellularity.