The complete cell lineage of C. elegans , mapped over four decades ago, was tractable because the animal is small, transparent, and lineage invariant 1 . Most animals are none of these, having orders of magnitude more cells, being opaque, and developing with substantial stochasticity 2 . Since 2016, genome editing-based lineage tracing has opened the door to dense cell lineage reconstruction in such organisms 3-8 , but delivering on that promise has proven technically challenging. Here we apply DNA Typewriter 9-11 , a prime editing-based recorder that writes stochastic symbolic insertions to an engineered genomic TAPE in strictly sequential order, to trace mouse development from zygote (E0) to late organogenesis (E13.5). We introduce constructs encoding a prime editor, engineered prime editing guide RNAs (epegRNAs), and Pol-III-driven circularized TAPE RNA (circTAPE) into wildtype zygotes by pronuclear injection (PNI), then assay embryos by single-nucleus transcriptional profiling (sci-RNA-seq3) 12 with paired circTAPE recovery. From one E13.5 embryo bearing ∼7 integrations of a constitutively expressed prime editor and 11 integrations of 6-unit circTAPE, we recover ∼1.75M single-nucleus transcriptomes and reconstruct a time-calibrated phylogeny of 1,340,794 annotated cells with parsimony-based node support. The first cell division is marked unequivocally, and although the resulting blastomeres contribute asymmetrically to the embryo proper, they are fate-neutral and serve as internal replicates that reproduce every finding. A modest cohort of pre-gastrulation founders dominates the embryo, with inequality exceeding neutral expectation within one to two cell cycles of founder allocation, yet these founders remain broadly multipotent; a second phase of clonal dominance arises in specific lineages during organogenesis. At the finest scale, sibling cells share cell type 9-fold in excess of chance, reaching 68- to 107-fold for cell types arising from spatially restricted founder pools, while the recent differentiations of organogenesis are legible in the heterotypic structure of terminal clades. From clade co-occurrence alone, we recover germ-layer organization and a dated hierarchy of cell-type couplings with branch points from E8.5 onwards. Finally, by integrating these data with our single-cell time-series of mouse development 13 , we impute transcriptional states and annotations for the majority of internal nodes and recover established state histories for diverse cell types. All data are made freely available, together with NextCell , an interactive browser for this annotated cellular phylogeny of mouse development from zygote to late organogenesis.
CRISPR Gene Editing