DAVIDSON, B.J.*; LEVINE, M.S.; Univ. of California, Berkeley; Univ. of California, Berkeley: The way to a chordate�s heart is through its germ cells? Germ line silencing and other factors involved in Ciona intestinalis heart development.

We are investigating heart development in the basal chordate Ciona intestinalis. Several cardiac genes, including the sole Ciona ortholog of the Drosophila tinman gene (Ci-Nkx) and the orthologs of the vertebrate Hand genes, have been characterized. Currently, we are dissecting the regulation and function of these conserved cardiac genes. In Ciona, the cardiac lineage shares a common origin with the germline. Zygotic transcription is first detected in the heart progenitors only after its separation from the germline at the 64-cell stage. We are investigating the possibility that early transcriptional silencing by germline determinants influences the specification of the cardiac mesoderm. Eventually, these studies will provide a complete regulatory framework for cardiac mesoderm differentiation, beginning at the 110-cell stage and extending through the fusion of cardiac progenitors during tail elongation.