Davidson selected the sea squirt Ciona intestinalis for his study of heart development because, he says, it is “one step before becoming a vertebrate.” It has some of the advantages of the fruit fly—genetic manipulability, and relatively little genetic redundancy—but as a urochordate is in the last branch before the chordate lineage yielded vertebrates.
He used the genome sequence of Ciona to analyze possible homologues of fly and vertebrate heart genes. Many of the candidate genes had the expected expression in heart precursor cells.
The heart precursors can be traced all the way back to a pair of cells at the 32-cell stage—a pair of cells that also gives rise to germ cells. At this 32-cell stage, neighboring cells are already turning on mesoderm genes. Davidson suggests that the known repressive effect of germ plasm reserves some potential muscle cells for use in the heart rather than these other mesodermal tissues. In the next division, the heart precursors separate from the germ cell precursors, and both are free to follow their individual fates.
This temporary repression may be Ciona's way of coping with its rapid development—“a way of separating things spatially rather than temporally,” says Davidson. Despite difficulties with fate maps in more complicated organisms, he is looking for signs of linkage between germ cell and heart cell development in vertebrates. At the very least, he says, there are other instances where the time of separation from the germ line could be used as a way to determine cell fate. ▪