Mitochondria are the powerhouses of eukaryotic cells and their positioning contributes to fertilization and early developmental processes. We report that sperm fusion triggers Ca2+ oscillations and mitochondrial movement toward fused sperm (mitochondrial chemotaxis) in mouse eggs. Mitochondria functioned in Ca2+ storage and were colocalized with endoplasmic reticulum (ER) during Ca2+ oscillations. Mitochondria then moved toward the fused sperm. Sperm extracts lacking nuclei induced Ca2+ oscillations, but did not promote mitochondrial chemotaxis. Our results suggest that sperm fusion motivates Ca2+ oscillation-independent mitochondrial chemotaxis. This phenomenon indicates that egg mitochondria interact with sperm materials, presumably nuclear substances, and their network tethers egg and sperm nuclei at the early stage of zygote formation.
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