Transcriptional divergence of the zebrafish sox17 lineage begins during gastrulation

The endoderm is specified at the onset of gastrulation and subsequently undergoes extensive migration before forming an epithelial sheet that gives rise to multiple organs, including the gut and respiratory tracts. Although the gene regulatory network underlying endoderm specification and the later processes that regionalize the gut are increasingly well understood, comparatively little is known about the intervening developmental events. Using single cell transcriptomics, we profiled the zebrafish sox17 lineage, comprising endoderm and dorsal forerunner cells, throughout and immediately after gastrulation. We found that dorsal forerunner cells remain transcriptionally homogeneous while undergoing coordinated temporal changes, associated with ciliogenesis and epithelial organization, during assembly of Kupffer's vesicle. In contrast, endoderm cells transition from a migratory to an epithelial transcriptional state while progressively acquiring distinct regional identities. These findings indicate that endoderm regionalization emerges within the context of a broadly shared transcriptional program associated with migration and epithelialization.