Sperm entry without genome inheritance: incorporate or exclude – egg-intrinsic control of paternal genome fate in Cobitis fish

Abstract Fertilization normally ensures the union of maternal and paternal genomes. Yet, some vertebrates reproduce clonally while still requiring sperm to activate the egg, creating a paradox of sperm-dependent but genome-independent development. Here, we provide the first direct in vivo cytological evidence of an early cytoplasmic decision mechanism that distinguishes sperm incorporation from exclusion in gynogenetic vertebrates. Using time-resolved immunofluorescence and spinning-disk confocal microscopy in >1,300 embryos of the Cobitis species complex across sexual, hybrid, and heterogeneric crosses, we detected two reproducible post-fertilization outcomes. Some eggs incorporate the paternal genome and display sperm centrosome licensing, formation of a single microtubule aster, and coordinated chromatin remodelling, including acquisition of activation-associated acetylation marks. In contrast, other eggs exclude the paternal genome and show failure of centrosome licensing, loss of Aurora ABC recruitment, formation of multiple ectopic microtubule asters, and persistent paternal chromatin compaction lacking acetylation. Together, these results indicate that the paternal genome fate is determined by egg-intrinsic mechanisms linking centrosome function with chromatin remodelling.