A southpaw joins the roster: the role of the zebrafish nodal-related gene southpaw in cardiac LR asymmetry.

The establishment of the left-right (LR) asymmetry of the zebrafish heart involves at least two discrete events: cardiac jogging and cardiac looping. The nodal-related gene southpaw is required for both aspects of heart LR asymmetry as well as for LR patterning of other visceral organs. Reductions in southpaw activity abolish the normal LR biases of jogging and looping and uncouple the normal correlation between jogging and looping polarities. These loss-of-function experiments also reveal that southpaw is required for organizing at least the initial phase of diencephalic LR asymmetry and suggest that heart and brain LR patterning may be coordinated by southpaw signaling originating within the anterior, left lateral plate mesoderm.