Although the liver and ventral pancreas are thought to arise from a common multipotent progenitor pool, it is unclear whether
these progenitors of the hepatopancreas system are specified by a common genetic mechanism. Efforts to determine the role
of Hnf1b and Wnt signaling in this crucial process have been confounded by a combination of factors, including a narrow time
frame for hepatopancreas specification, functional redundancy among Wnt ligands, and pleiotropic defects caused by either
severe loss of Wnt signaling or Hnf1b function. Using a novel hypomorphic hnf1ba zebrafish mutant that exhibits pancreas hypoplasia, as observed in HNF1B monogenic diabetes, we show that hnf1ba plays essential roles in regulating β-cell number and pancreas specification, distinct from its function in regulating pancreas
size and liver specification, respectively. By combining Hnf1ba partial loss of function with conditional loss of Wnt signaling,
we uncover a crucial developmental window when these pathways synergize to specify the entire ventrally derived hepatopancreas
progenitor population. Furthermore, our in vivo genetic studies demonstrate that hnf1ba generates a permissive domain for Wnt signaling activity in the foregut endoderm. Collectively, our findings provide a new
model for HNF1B function, yield insight into pancreas and β-cell development, and suggest a new mechanism for hepatopancreatic
specification.
