WATER reveals heterochrony of molecular programs underlies developmental failure caused by minor spliceosome...

The final limb structure reflects coordinated deployment of molecular programs, defined not only by which genes are expressed but when they are activated and silenced across time. Existing omics analyses obscure the temporal unfolding of these programs and conflate program identity with deployment timing by assuming temporal equivalence between conditions. We developed WATER (Weighted Windowed Assignment of Temporal Expression of RNA), a framework that reconstructs temporal gene expression trajectories independently within each condition, enabling direct comparison of temporal program architecture between wild-type and perturbed systems. Applying WATER to U11-null mouse forelimb development revealed that minor spliceosome inhibition redistributes genes across inappropriate temporal trajectories. Minor spliceosome inhibition causes splicing defects in minor intron-containing genes such as the PRC2 component Eed, leading to reduced H3K27me3 deposition and chromatin-transcription divergence. Single-cell RNA sequencing revealed persistence of progenitor states, impaired chondrogenic progression, and p53-dependent apoptotic checkpoint activation. Orthogonal WATER analysis of Eed-knockout stem cells recapitulated key features of chromatin gating failure, including temporal redistribution of skeletal development programs and progenitor state persistence, confirming that Eed loss alone is sufficient to produce temporal program redistribution independently of other splicing defects. Trp53 ablation in U11-null limbs partially rescued distal limb structures without correcting the underlying splicing defects, establishing that checkpoint activation amplifies rather than initiates the timing disruption. The limb retains much of its molecular toolkit but executes it in the wrong order, demonstrating that developmental failure arises from mistimed deployment of intact molecular programs. Thus, temporal program architecture is a fundamental organizing principle of morphogenesis.