The endoderm cell trajectory of urochordate Styela clava reveals the dual developmental origin and evolution of digestive tract

The digestive system exhibits extensive diversity in developmental mechanisms and morphology across metazoans, yet the evolutionary origins underlying its organ differentiation remain unclear. Here, single-cell RNA sequencing was employed to investigate endodermal cell lineage specification during metamorphosis in the urochordate Styela clava, a newly established model for chordate evolution. By profiling 26,099 cells across five stages, we identified 21 major cell clusters and reconstructed the endodermal differentiation trajectories. Our analysis reveals two larval endodermal progenitor populations with distinct differentiation potentials. Pseudotime and RNA velocity analyses indicate that these progenitors give rise to stomach and intestinal lineages, respectively. Cross-species comparisons reveal putative homologous relationships between ascidian endodermal lineages and mouse definitive and visceral endodermal lineages, suggesting dual origins of digestive tract in chordates. We also identified conserved TGF-{beta} and FGF regulatory programs in digestive organ patterning and highlight earlier fate restriction of stomach and intestinal progenitors in ascidians compared to vertebrates. These findings provide insights into how chordate digestive organs evolved from ancestral endodermal patterning programs.