THE GENE TINMAN IS REQUIRED FOR SPECIFICATION OF THE HEART AND VISCERAL MUSCLES IN DROSOPHILA

The homeobox-containing gene tinman (msh-2, Bodmer et al., 1990 Develo pment 110, 661-669) is expressed in the mesoderm primordium, and this expression requires the function of the mesoderm determinant twist. La ter in development, as the first mesodermal subdivisions are occurring , expression becomes limited to the visceral mesoderm and the heart. H ere, I show that the function of tinman is required for visceral muscl e and heart development. Embryos that are mutant for the tinman gene l ack the appearance of visceral mesoderm and of heart primordia, and th e fusion of the anterior and posterior endoderm is impaired. Even thou gh tinman mutant embryos do not have a heart or visceral muscles, many of the somatic body wall muscles appear to develop although abnormall y. When the tinman cDNA is ubiquitously expressed in tinman mutant emb ryos, via a heat-shock promoter, formation of heart cells and visceral mesoderm is partially restored. tinman seems to be one of the earlies t genes required for heart development and the first gene reported for which a crucial function in the early mesodermal subdivisions has bee n implicated.