Hybrid incompatibility emerges at the one-cell stage in interspecies Caenorhabditis embryos

Intrinsic reproductive isolation occurs when genetic divergence between populations disrupts hybrid development, preventing gene flow and reinforcing speciation.^1,2,3,4 Molecular mechanisms explaining a few dozen cases of hybrid incompatibility have been uncovered in animals,⁵ including mismatches in zygotic gene regulation,^{6,7,8,9,10,11} symbiont-driven incompatibilities,¹² nucleoporin mismatches affecting nuclear-cytoplasmic transport,¹³ and divergence in centromeric or heterochromatic regions and their regulatory proteins that lead to the inability of the oocyte cytoplasm to segregate sperm-derived chromosomes.^{14,15,16,17,18,19} Expanding mechanistic work to more diverse taxa is important for elucidating broader patterns of hybrid incompatibility. Here, we investigate hybrid incompatibility in Caenorhabditis elegans group nematodes. Within this group, most species pairs do not mate, and hybrids typically die during embryogenesis in those that do.^{20,21,22,23,24,25,26} Although individual embryos within a cross arrest at variable time points,^27,28 we show that incompatibilities typically originate between fertilization and the 4-cell stage, prior to the onset of zygotic transcription.^29,30,31,32 In Caenorhabditis, like most animals,^33,34 sperm deliver chromatin and centrioles into the oocyte.^35,36,37,38 After oocyte meiosis, the sperm chromatin acquires a nuclear envelope, and centrioles initiate centrosome formation.^39,40,41 Centrosomes remain tethered to the sperm pronucleus, which positions them near the cortex to establish anterior-posterior polarity.^42,43 We identify two key processes that are destabilized in hybrids: (1) oocyte control of sperm-derived pronuclear expansion and (2) polar body formation. When sperm pronuclear expansion is delayed, centrosomes detach, leading to defects in polarity establishment. Hybrid embryos typically experience one or more failures of early developmental events that accumulate and eventually kill them.