A key process during vertebrate heart development is the migration of bilateral populations of myocardial precursors towards
the midline to form the primitive heart tube. In zebrafish, signaling mediated by sphingosine-1-phosphate (S1P) and its cognate
G protein-coupled receptor (S1pr2/Mil) is essential for myocardial migration, but the underlying mechanisms remain undefined.
Here, we show that suppression of Gα13 signaling disrupts myocardial migration, leading to the formation of two bilaterally located hearts (cardia bifida). Genetic
studies indicate that Gα13 acts downstream of S1pr2 to regulate myocardial migration through a RhoGEF-dependent pathway. Furthermore, disrupting any
component of the S1pr2/Gα13/RhoGEF pathway impairs endoderm convergence during segmentation, and the endodermal defects correlate with the extent of
cardia bifida. Moreover, endoderm transplantation reveals that the presence of wild-type anterior endodermal cells in Gα13-deficient embryos is sufficient to rescue the endoderm convergence defect and cardia bifida, and, conversely, that the presence
of anterior endodermal cells defective for S1pr2 or Gα13 in wild-type embryos causes such defects. Thus, S1pr2/Gα13 signaling probably acts in the endoderm to regulate myocardial migration. In support of this notion, cardiac-specific expression
of Gα13 fails to rescue cardia bifida in the context of global Gα13 inhibition. Our data demonstrate for the first time that the Gα13/RhoGEF-dependent pathway functions downstream of S1pr2 to regulate convergent movement of the endoderm, an event that is
crucial for coordinating myocardial migration.