Uncoupling heart cell specification and migration in the simple chordate Ciona intestinalis.

The bHLH transcription factor Mesp has an essential but ambiguous role in early chordate heart development. Here, we employ the genetic and morphological simplicity of the basal chordate Ciona intestinalis to elucidate Mesp regulation and function. Characterization of a minimal cardiac enhancer for the Ciona Mesp gene demonstrated direct activation by the T-box transcription factor Tbx6c. The Mesp enhancer was fused to GFP, permitting high-resolution visualization of heart cells as they migrate and divide. The enhancer was also used to drive targeted expression of an activator form of Mesp, which induces heart formation without migration. We discuss the implications of Tbx6-Mesp interactions for the evolution of cardiac mesoderm in invertebrates and vertebrates.