Allele-Resolved Hybrid Embryos Reveal the Fates of Regulatory Divergence

Developmental programs can remain conserved despite extensive regulatory divergence, but how evolved regulatory differences are transmitted through embryonic lineages remains unclear. Here we generate a time resolved, allele resolved single-cell atlas of hybrid embryogenesis between Ciona intestinalis and Ciona savignyi, enabling regulatory differences accumulated between species to be followed across defined developmental lineages. We find that allelic differences are maintained or remodeled in lineage specific ways, with their outcomes associated with regulatory origin and allele specific chromatin accessibility. Across multiple tissues, allelic divergence increases along gene regulatory network (GRN) hierarchy from upstream regulators toward downstream regulators and effector genes. In the cardiopharyngeal lineage, Foxf illustrates how allelic dominance provides partial compensation for highly divergent regulatory sequences and thereby contributes to developmental system drift. Together, our results reveal that regulatory divergence is dynamically sorted during development according to lineage context and GRN hierarchy. This lineage resolved framework provides a developmental basis for understanding how extensive regulatory evolution can accumulate while conserved embryonic programs are maintained.