Building the Heart: The multichambered heart of birds and mammals develops through addition of second heart field (SHF)–derived precursor cells to a primary heart tube. Stolfi et al. (p. 565 ) show that, in the simple chordate Ciona intestinalis , the heart and atrial siphon muscle (ASM) precursors arise from common progenitors following asymmetric cell divisions and that the transcription factor COE (Collier/Olf1/EBF) is involved in this fate choice. The ASM precursors express molecular markers of the vertebrate pharyngeal mesoderm that gives rise to the SHF and lower jaw muscles, suggesting that the origins of both can be traced back to the last common ancestor of tunicates and vertebrates.

Comprehensive gene networks in Ciona intestinalis embryos provide a foundation for characterizing complex developmental processes, such as the initial phases of chordate heart development. The basic helix–loop–helix regulatory gene Ci-Mesp is required for activation of cardiac transcription factors. Evidence is presented that Ci-Ets1/2, a transcriptional effector of receptor tyrosine kinase (RTK) signaling, acts downstream from Mesp to establish the heart field. Asymmetric activation of Ets1/2, possibly through localized expression of FGF9, drives heart specification within this field. During gastrulation, Ets1/2 is expressed in a group of four cells descended from two Mesp-expressing founder cells (the B7.5 cells). After gastrulation, these cells divide asymmetrically; the smaller rostral daughters exhibit RTK activation (phosphorylation of ERK) and form the heart lineage while the larger caudal daughters form the anterior tail muscle lineage. Inhibition of RTK signaling prevents heart specification. Targeted inhibition of Ets1/2 activity or FGF receptor function also blocks heart specification. Conversely, application of FGF or targeted expression of constitutively active Ets1/2 (EtsVp16) cause both rostral and caudal B7.5 lineages to form heart cells. This expansion produces an unexpected phenotype: transformation of a single-compartment heart into a functional multicompartment organ. We discuss these results with regard to the development and evolution of the multichambered vertebrate heart.

Neuronal precursor cells in the tunicate Ciona intestinalis are shown to delaminate and undergo directed cell migration along either side of the neural tube before differentiating into bipolar neurons, suggesting that vertebrate neural-crest-derived sensory neurons have much deeper evolutionary roots. Many of the features that make vertebrates distinctive derive from an embryonic tissue called the neural crest, which starts life on the borders of the developing neural plate and migrates through the body. How did such a distinctive embryonic tissue evolve? Until recently no trace of it was known in the invertebrates closest to vertebrates such as tunicates and amphioxus. However, signs of neural plate border cells expressing cognates of vertebrate-specific neural-crest-related genes have been found. Here Lionel Christiaen and colleagues take the research a step further by showing how neuronal cells in the 'tadpole' larva of the tunicate Ciona intestinalis (the sea squirt) share features with neural-crest-derived spinal ganglia neurons in vertebrates. The precursors of these cells derive from caudal neural plate border cells and delaminate and undergo directed cell migration along either side of the neural tube before differentiating into axons, suggesting that migration — previously thought to be unique to vertebrate neural crests — has much deeper evolutionary roots. The neural crest is an evolutionary novelty that fostered the emergence of vertebrate anatomical innovations such as the cranium and jaws1. During embryonic development, multipotent neural crest cells are specified at the lateral borders of the neural plate before delaminating, migrating and differentiating into various cell types. In invertebrate chordates (cephalochordates and tunicates), neural plate border cells express conserved factors such as Msx, Snail and Pax3/7 and generate melanin-containing pigment cells2,3,4, a derivative of the neural crest in vertebrates. However, invertebrate neural plate border cells have not been shown to generate homologues of other neural crest derivatives. Thus, proposed models of neural crest evolution postulate vertebrate-specific elaborations on an ancestral neural plate border program, through acquisition of migratory capabilities and the potential to generate several cell types5,6,7. Here we show that a particular neuronal cell type in the tadpole larva of the tunicate Ciona intestinalis, the bipolar tail neuron, shares a set of features with neural-crest-derived spinal ganglia neurons in vertebrates. Bipolar tail neuron precursors derive from caudal neural plate border cells, delaminate and migrate along the paraxial mesoderm on either side of the neural tube, eventually differentiating into afferent neurons that form synaptic contacts with both epidermal sensory cells and motor neurons. We propose that the neural plate borders of the chordate ancestor already produced migratory peripheral neurons and pigment cells, and that the neural crest evolved through the acquisition of a multipotent progenitor regulatory state upstream of multiple, pre-existing neural plate border cell differentiation programs.

Gene regulatory networks direct the progressive determination of cell fate during embryogenesis, but how they control cell behavior during morphogenesis remains largely elusive. Cell sorting, microarrays, and targeted molecular manipulations were used to analyze cardiac cell migration in the ascidian Ciona intestinalis . The heart network regulates genes involved in most cellular activities required for migration, including adhesion, cell polarity, and membrane protrusions. We demonstrated that fibroblast growth factor signaling and the forkhead transcription factor FoxF directly upregulate the small guanosine triphosphatase RhoDF, which synergizes with Cdc42 to contribute to the protrusive activity of migrating cells. Moreover, RhoDF induces membrane protrusions independently of other cellular activities required for migration. We propose that transcription regulation of specific effector genes determines the coordinated deployment of discrete cellular modules underlying migration.

Heart development requires precise coordination of morphogenetic movements with progressive cell fate specification and differentiation. In ascidian embryos, FGF/MAPK-mediated activation of the transcription factor Ets1/2 is required for heart tissue specification and cell migration. We found that FoxF is one of the first genes to be activated in heart precursors in response to FGF signaling. We identified the FoxF minimal heart enhancer and used a cis-trans complementation test to show that Ets1/2 can interact with the FoxF enhancer in vivo. Next, we found that FoxF function is required downstream and in parallel to the FGF/MAPK/Ets cascade for cell migration. In addition, we demonstrated that targeted expression of a dominant-negative form of FoxF inhibits cell migration but not heart differentiation, resulting in a striking phenotype: a beating heart at an ectopic location within the body cavity of juveniles. Taken together, our results indicate that FoxF is a direct target of FGF signaling and is predominantly involved in the regulation of heart cell migration.