Wnt9 directs zebrafish heart tube assembly via a combination of canonical and non-canonical pathway signaling.

During zebrafish heart formation, cardiac progenitor cells converge at the embryonic midline where they form the cardiac cone. Subsequently, this structure transforms into a heart tube. Only little is known about the molecular mechanisms that control these morphogenetic processes. Here, we use light sheet microscopy and combine genetic, molecular biological and pharmacological tools to show that the paralogous genes wnt9a/b are required for the assembly of the nascent heart tube. In wnt9a/b double mutants, cardiomyocyte progenitor cells are delayed in their convergence towards the embryonic midline, the formation of the heart cone is impaired and the transformation into an elongated heart tube fails. The same cardiac phenotype occurs when both canonical and non-canonical Wnt signaling pathways are simultaneously blocked by pharmacological inhibition. This demonstrates that Wnt9a/b and canonical and non-canonical Wnt signaling regulate the migration of cardiomyocyte progenitor cells and control the formation of the cardiac tube. This can be partly attributed to their regulation of the timing of cardiac progenitor cell differentiation. Our study demonstrates how these morphogens activate a combination of downstream pathways to direct cardiac morphogenesis.