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

Intrinsic reproductive isolation occurs when genetic differences between populations disrupt the development of hybrid organisms, preventing gene flow and enforcing speciation. ^1–4 While prior studies have examined the genetic origins of hybrid incompatibility, ^5–18 the effects of incompatible factors on development remain poorly understood. Here, we investigate the mechanistic basis of hybrid incompatibility in Caenorhabditis nematodes by capitalizing on the ability of C. brenneri females to produce embryos after mating with males from several other species. Contrary to expectations, hybrid incompatibility was evident immediately after fertilization, suggesting that post-fertilization barriers to hybridization originate from physical incompatibility between sperm and oocyte-derived factors rather than from zygotic transcription, which starts after the 4-cell stage. ^19–22 Sperm deliver chromatin, which expands to form a pronucleus, and a pair of centrioles, which form centrosomes that attach to the sperm-derived pronucleus and signal to establish the embryo’s anterior-posterior axis. ^23,24 In C. brenneri oocytes fertilized with C. elegans sperm, sperm pronuclear expansion was compromised, frequent centrosome detachment was observed, and cortical polarity was disrupted. Live imaging revealed that defective polar body extrusion contributes to defects in mitotic spindle morphology. C. brenneri oocytes fertilized with C. remanei or C. sp. 48 sperm showed similar defects, and their severity and frequency increased with phylogenetic distance. Defective expansion of the sperm-derived pronucleus and unreliable polar body extrusion immediately after fertilization generally underlie the inviability of hybrid embryos in this clade. These results indicate that physical mismatches between sperm and oocyte-derived structures may be a primary mechanism of hybrid incompatibility.